The Devil's Stallion

"BRRRRAAAAARRPP!!!"
Twilight awoke to a very rude noise.  She wished she were too groggy to say it, but it was too close to dawn, and Spike's belch was too loud to even pretend to sleep through.  She groaned, "Spike, will you quit eating emeralds so close to bedtime?  You know what they do to you...!  It's too early in the morning for this!"
Spike groaned back, and just as groggily answered, "Twilight, I haven't had emeralds all week!  It's a letter!"
"A letter?!  At this hour?!" the words rolled off of Twilight's tongue as if they were as tired as she was.  "You can't tell me the Princess is sending us letters just before dawn!"
Spike blew fire onto a candle and looked the scroll over.  He immediately became more awake as he realized what it was.  "It's got the Princess's royal seal on it!  It's got to be from her!"
"Great.  She wouldn't send it at this hour if it wasn't urgent...ohhhhh, it's too early in the morning for urgent!"  Of course, Twilight didn't let that stop her from reading it.  Spike already had it unfurled by the time she dragged herself out of bed and put herself between the light and the scroll so she could read it.  It was very hastily horn-written, which wasn't like the Princess at all, but was still very legible:

My Dearest Twilight,
I may already be too late in sending you this letter.  I have seen a magical corruption spreading throughout Equestria, and you and your friends may be the only ones who can stop it.  Even I do not understand it, but anypony who comes under its effect becomes enraged, their eyes turn black and red, and they relentlessly attack anypony they see who isn't also under the corruption's effect.  And whenever a corrupted pony touches an uncorrupted one, the corruption spreads.  Soon, all of Equestria will be under this spell's influence, and I fear that nopony will ever feel anything other than anger ever again unless we stop this spell.
I have researched this, but I have only been able to find one document about this effect in my secret vaults.  It is an old ponytale, one that even I didn't believe was true before I saw the corruption spreading.  But it speaks of a demon alicorn who rules over the dark afterlife, reigning as king over all the ponies who wasted their lives in hatred, as they waste their eternities in hatred as well.  The ponytale spoke of his ambition to gain control over the lands of the living, his ability to create this corruption, and that the only thing that could protect against his influence was to be under the Spell of Unity.
I know that there is no magic in all Equestria stronger than the Elements of Harmony, so I am unlocking the elements and will have them to you as soon as I can.  But hide yourself and anyone else you can; I am writing this hoping that you haven't been corrupted, yourself, and that Ponyville has not yet been touched by the corruption.  If it has, all may well be lost.
I will be coming to you promptly to bring you and your friends the elements, and I will have the Spell of Unity prepared.  There is no time for pomp and ceremony:  the fate of Equestria and all ponykind hangs in the balance.
Your Princess,
Celestia

Spike and Twilight looked at each other in frightened bewilderment.  Spike gulped.  "What do we do?"
Pale, rainbow light filtered down the library's chimney, and soot rustled down the flue.  Twilight stiffened herself for battle and readied her magic, and blew out the candle to keep herself hidden.  Spike gulped, but readied himself for a fight, as well.  Twilight whispered, "If it's a corrupted pony, I'll hold him or her at bay with my magic.  Then you run to the basement and see what you can do to block the door so we can keep ourselves barricaded in."
"But...but there's no telling how long we'd have to stay down there, and there's no food in the basement!  We'll starve!"
Twilight's stomach gurgled at the thought of starving; she was beginning to get hungry for breakfast.  But she knew it was better to starve than to become corrupted, and she couldn't think of a solution, so she only whispered back, "Shh!"
The light got brighter, and the source of it quickly slipped down.  It was a glowing orb, flashing colors that reminded Twilight of the Princess's mane and coat.  Twilight gazed at it for a moment as it filled the room with beautiful light, confused about what this orb could be---she'd studied magic since she was a little filly, but hadn't seen or read or even heard of anything like this before.
"Oh, Twilight!  Thank heavens you aren't corrupted, yet!"  It was the Princess's voice, and it was coming from the orb.
"Princess?"
"I'm in spirit form, Twilight---it's a side effect of the Spell of Unity.  There's no time to explain how it works.  Just hold still."
Twilight did as she was told and held still as the orb floated towards her and a strange light enveloped her whole body.  She felt a strange sensation that she couldn't have described if someone had asked her what it was like.  But she knew that, in some strange way, she and the Princess had become one, even though they were still different beings and different minds.
"Are you okay, Twilight?" Spike asked.
"I think so," Twilight answered, still a little overwhelmed, and still pretty tired from the sudden awakening.
"There's no time to lose, Twilight.  Find your friends.  We'll need to join them to the Spell of Unity as well, to make them immune to the corruption.  They may have already succumbed to the corruption:  while I was flying in, I saw that many ponies in Ponyville have already been corrupted!"
Twilight gulped audibly.
"But even if they are corrupted, there is a way to reverse the corruption.  The Spell of Unity has many side effects, and because your special talent is magic, one of the side effects is that you will be able to fire arrows of pure light from your horn.  Anypony hit with those arrows will have the corruption removed from them, but be warned:  you mustn't use any magic other than these arrows of light, or the Spell of Unity will be disrupted."
Twilight nodded.  "I guess I'd better go out right now, and---"
Twilight's thoughts were thoroughly interrupted as the door burst open, and a pony was at the door---it was the Mayor.  Her eyes were black where they should have been white, and her irises and pupils were disturbingly small, glowing a fiery blood red.  The expression on her face was one of focused hate and rage.  The sight of the Mayor in this state sent shivers down Twilight's spine, but she knew she couldn't hesitate.
The Mayor charged at Twilight.  Her first instinct was to use magic to teleport behind the Mayor, but she caught herself before she tried that---literally five seconds ago, the Princess had warned her not to use any magic besides the arrows of light.  Instead, she summoned all her strength and leaped over the Mayor.  The Mayor was suddenly confused---no less enraged, but confused, not knowing whether she should keep charging to attack Spike, or turn around and attack Twilight.  After half a second of deliberating, she decided to try to turn around and charge Twilight.
Spike, for his part, wasn't sure what to do:  he didn't want to touch the Mayor and get corrupted, and he didn't want to use his fire breath on her and hurt her, either---he didn't seem to have any options for fighting back; he only stood still, half cowering, half wondering where to run.
But he didn't need to fight back:  Twilight fired an arrow straight at the Mayor, and her eyes went back to normal.  The Mayor seemed in a daze for a moment, but after blinking, she seemed to come back to her senses.  "Twilight Sparkle?  I...I apologize for what must seem to you to be---"
"There's no time," Celestia insisted.  "You and Spike must hide in the basement at once!  Twilight, find your friends."
Twilight bolted out the door and shouted over her shoulder, "Sorry, no time to explain!"  She herself wished there were more time to explain---for that matter, she'd like some explanations, herself.  But she'd seen problems spread exponentially before:  she'd seen what the Parasprites did, reproducing at an incredible rate, and if she and her friends had only listened to Pinkie Pie's warnings and asked a few questions, it wouldn't have taken months of work and incredible amounts of money to repair the whole town.  She felt like she was partly responsible for the damage, too, since it was her spell that had changed the Parasprites' appetite so that they consumed everything but the food, and she didn't want to be responsible for something like that again.  So she took the hint from the Princess's tone and hurried.
But no sooner had Twilight come out the door of the library than random, black-eyed ponies began charging her, determined to either spread the corruption to her or kill her.  So many of them were faces that she had passed a hundred times on the streets, in the shops or restaurants, or even at the library.  It was horribly unsettling to see all love, all joy, all normalcy drained from their faces, and hatred against her put in their place.  She kept running, not sure where she should go, first.  Nor was she sure how she could possibly fight so many ponies, there had to be dozens of them---and only one of her.  But she knew she'd be surrounded quickly if she didn't do something, so she fired a few arrows randomly at the crowd following her, hoping for the best.  The result shocked her:  even though only three or four ponies at most became uncorrupted, the entire herd of ponies stopped chasing her and began charging the ponies who had been uncorrupted.  One of the uncorrupted ponies screamed and ran away from the herd, leading them all away from Twilight.
It was horrifying to think of the fear that one uncorrupted pony must have been experiencing, having to run from her neighbors, friends, and family for dear life, but Twilight knew that the best thing she could do for that pony was to complete the mission Celestia had given her---she knew it, and she'd do it, but it still cut her up inside that she had to abandon that poor, screaming filly.
Twilight raced toward the closest place she could think of:  Sugarcube Corner.  The thought of Pinkie being taken over by the corruption was a sickening one; though Twilight didn't like to admit it, she had warmed up a lot to Pinkie over the months she'd known her---not just despite her hyperactivity and childish behavior, but in some ways, because of it.  She was fun to be around when you were looking for a good time, and beyond that, she was a very affectionate and loyal friend, however crazy she was.
Twilight had to push these heartwarming thoughts about her friend out of her mind as she came up on Sugarcube Corner.  She was worried that there might be corrupted ponies inside the bakery, but she didn't have time to find out:  the window to Pinkie's apartment flew open, and Pinkie flung a key out of the window into a pile of junk that was uncharacteristically sitting outside the building.
"HA!  You pony zombies will NEVER get me now!  Not even through the window!" Pinkie shouted, and before Twilight could shout up to her to come down, she slammed the window, and it locked tight with a "click".
"Pinkie, come down from there!" Twilight shouted.  "The Princess has a plan to save Equestria from the dark magic, but we need the elements of harmony to make it work!  Come on!"  Twilight listened intently for an answer, but she didn't hear a thing.
Twilight's mind raced:  the only way to get to Pinkie was to get the key.  But looking for the key in that pile of junk would literally be like looking for a needle in a haystack; there just plain wasn't time to look through it all.  Plus, Twilight couldn't lift the junk up with her magic, or else she'd disrupt the Spell of Unity:  she'd need to use her hooves and mouth to move it all.  That would take even longer!  If only there were a pony who was good at finding things---
Rarity!  Rarity's gem-finding ability might be able to find that key in that pile of junk!  And even if it couldn't, at least Rarity would be able to use her horn magic to lift the junk and look faster than Twilight could.  Twilight raced to Rarity's nearby boutique, only to find that a huge stone had been put in front of the entrance---it was Tom, the same boulder that she had mistaken for a giant diamond while under DisQord's illusion.  Twilight's first thought was to move it with magic; she'd moved things a lot bigger than that rock with magic, before.  But then she remembered the Princess's warnings and knew that option wasn't available.  She wished she were strong enough to move that rock with her muscles alone, but she was disappointingly physically weak, and she knew it.  If only she were strong like Applejack---!
Applejack!  Of course!  She was a hard worker and used to hauling and lifting heavy stuff; she'd be able to help move that stone!  Twilght turned around and raced toward Sweet Apple Acres, hoping that the herd that had followed her wasn't waiting in ambush.
Of course more ponies noticed her, and charged her again.  This time, she had some idea what to do:  she fired arrows at the ponies that were on the fringes of the herd---far to the left or far to the right, and then the whole herd broke off their pursuit of her and began pursuing the uncorrupted ones.  Some part of Twilight objected:  she was, after all, using those other ponies by uncorrupting them and letting them distract the others.  She remembered the ethics lessons she had learned in school, and how Immanuel Kanter had insisted that ponies should never be used as a means to an end.  But then again, what choice did she have?  And what would really happen?  The moment another pony touched them, they'd just be corrupted again, and then the terror would be over---only to be replaced by hate.  It was sickening to think of what was happening to all Ponyville and what she had to do to survive, so Twilight pushed the thoughts out of her mind and focused on the task at hoof.
She kept running to the farm---but she skidded to a stop as she saw that the irrigation ditch surrounding Sweet Apple Acres, while empty of water, was completely full of small animals---and each of them was corrupted.  Rabbits and squirrels who would normally ignore Twilight were swarming at her, teeth bared, eyes blazing red, all of them trying to jump up and snap at her.  They couldn't jump up the height of the ditch, but even if she was safe from them, the sight was a new flavor of disturbing:  the corruption had leeched everything good out of the ponies, and they were intelligent beings.  But the corruption had also leeched everything cute out of these unreasoning animals, as well, and made them into something that simultaneously scared Twilight and weirded her out.
She stared at the seething, shrieking balls of bristled fur and teeth for a few moments, pondering the horror and wondering whether there was a way over the ditch.  She looked up at the other side, and decided that she couldn't jump that distance, even with a running start.  Was there anything she could use to make a bridge?  ...If she could use her magic any way she liked, she could probably knock a tree over and use that for a bridge, but that wasn't an option.  She fired a few arrows at some of the animals on the other side of the ditch, hoping that that would be enough to distract them so she could climb down, get to the other side of the ditch, and climb back out.  But that didn't work, either; there were too many animals, and too few of them were actually distracted---it was as if Twilight were a more tempting target than their fellow uncorrupted little critters.  Twilight began to despair of ever getting past this obstacle; she knew she was running out of time!  If only Fluttershy were there; she'd know what to do with animals---
Fluttershy!  That's it!  Twilight began racing for Fluttershy's cottage.  She was on the outskirts of Ponyville, so there were fewer corrupted ponies that saw and attacked her---but when they did, Twilight knew what to do:  she fired arrows at the stragglers, and that distracted the mob long enough for her to escape.  But as she got closer and closer to Fluttershy's cottage, she saw fewer and fewer corrupted creatures.  She saw a squirrel in a tree that looked at her with curiosity, not hatred, and had normal, shiny, black eyes.  That gave Twilight a huge relief; maybe the corruption hadn't spread as far as Fluttershy's cottage!  Maybe she was safe!  A friendly face would've been like a drink of cool water in a desert, right about then, and Twilight was suddenly very much looking forward to seeing Fluttershy.  After all, she was always gentle and kind---the polar opposite of the rage, cruelty, and determined hate Twilight had seen in every other pony's face since she woke up.  And it was awful to be alone out there with only her horrifying thoughts to keep her company---yes, the Princess was with her, but she was being awfully quiet; it made Twilight wonder if she was in some kind of a trance---it might be a side effect of the Spell of Unity; the Princess did say there were many side effects.
The door to Fluttershy's cottage was wide open, but Fluttershy herself was nowhere to be seen.  Twilight knew that it was rude to just walk in without being invited, but time was so short---anything could bring the corruption into Fluttershy's house at any time:  a bird, a rabbit, another pony.  There was no time to be polite.  Twilight rushed in, hoping Fluttershy would be inside, and slammed the door behind her, hoping to at least slow down anypony or anything that might have followed her without her knowing.
Fluttershy was in the living room, at the opposite side of the house, looking into one of the many birdhouses she kept.  Her back was to Twilight, but she seemed normal enough.
Twilight was panting and agitated, not sure where to begin explaining what was going on.  She suddenly felt confused as the adrenaline began to fade from her system, and her stomach gurgled a little and her hunger caught up with her---the stress of the morning and of all the running and fighting clouded her mind for a moment.  She shook her head and tried to concentrate; there was an urgent need for her to quickly explain the situation to her friend; she couldn't be slowed down by stress, not now!  ...But where to begin...?
She decided to start with her rude entry:  "Sorry for coming in so fast, Fluttershy, but things are going crazy in town!" Twilight paused to catch her breath.  "There's this magical...corruption that's turning everypony into...I dunno...zombies or something!  The Princess wrote me a letter about it and somehow gave me immunity to it with something called the Spell of Unity.  I don't understand it myself, but we need to get to the others so we can use the Elements of Harmony again.
"Right now, I'm just glad you're safe.  You wouldn't believe how much I was looking forward to seeing a friendly face."  Twilight spoke too soon, and her heart sank:  Fluttershy turned around, and the gentle, shy, kind face Twilight was used to seeing---was twisted into a cruel, joyless scowl with black, burning eyes.  "NO!" Twilight screamed, horrified that Fluttershy's timid, sweet personality had been defiled and overwritten by this dark magic!  Fluttershy took off, and a flock of birds flew out of their houses and swarmed around her.  Twilight tried to fire an arrow of light up at Fluttershy to free her and restore her to her old self, but no matter how hard Twilight tried to aim the arrows upward, they always flew parallel to the ground.  Twilight thought for a moment that her inability to fire arrows at an angle was probably another side effect of the Spell of Unity, and she briefly wished that there were time for her to hear about all the side effects if the spell, but she had to focus on the battle at hoof:  Fluttershy sent the birds dive-bombing at Twilight, one by one.  They tried to scratch at her ears and eyes, but Twilight was too quick, and she was able to dodge them and even shoot a few of the birds with arrows.  That bought her respite from the birds for a few seconds.
She took that moment for the opportunity it was, and shouted up to Fluttershy, "I know you're in there somewhere, Fluttershy!  You're my friend!  This isn't you!  You're kind and gentle and...and you like cute animals!  You wouldn't attack anypony; it just isn't in you!  Come down from there!  I have the magic to make you better!"
Fluttershy didn't answer.  She snorted, a thing Twilight never thought she'd see her do.  She'd seen Rainbow Dash snort at her as she closed in to cure her of DisQord's spell, and that was a disturbing sight---she only smiled through it because she knew she was about to cure her; it wasn't clear whether she'd be able to cure Fluttershy or not.  And that uncertainty was terrifying; Twilight desperately wanted her friend back, and the thought that she might not be able to save her made Twilight feel sick to her stomach on top of being hungry---she again pushed the thought aside, determined to redouble her efforts and find a way to cure her friend, no matter what.
Fluttershy pointed at Twilight with her hoof, and now her chickens and her rabbit Angel rushed at Twilight, teeth bared and claws ready.  Twilight still knew what to do, though:  she fired arrows at a few of the chickens, and they began to scurry around aimlessly with corrupted creatures hot on their tail, distracting the whole swarm.
Fluttershy herself now rushed at Twilight.  Twilight fired an arrow at her, and the corruption began to dissipate---but it took more than one arrow to get the job done, and Fluttershy took off again before Twilight could fire another!  More birds swarmed around Fluttershy as she began the pattern of attack again.  Twilight thought to herself that now, she'd never be able to get Fluttershy to come back down to striking distance---she'd learned her lesson from that one arrow Twilight managed to land!  But Twilight kept fighting with the same tactics, shooting a few birds when they flew low enough, and then, when Fluttershy called the chickens and her rabbit again, Twilight routed those the same way---Twilight's only thought was to keep herself alive and uncorrupted until she could think of something, anything, that would let her use her arrows on Fluttershy herself.  She was panting and starting to get exhausted from the battle, but she was determined to finish this and get her dear friend back.  But to Twilight's surprise, Fluttershy charged her again, even though she should've known it would give Twilight the opportunity she needed to fire another arrow.  Twilight didn't let the surprise make her hesitate, though:  she fired another arrow into Fluttershy, and Fluttershy retreated back to the air.
The corruption was still on Fluttershy and still controlled her actions, but Twilight could tell that another arrow would finish the job and free her friend.  Twilight knew enough about how to finish the fight that she could spare some mental energy to ask herself just why Fluttershy was fighting in such a predictable way.  She realized that the hate that filled Fluttershy's mind (a thought that made her cringe inside!) was forcing Fluttershy to always do something to attack her.  If her animals couldn't do the job for her, she'd have to do it, herself, even if it put her in harm's way.  And Fluttershy couldn't think on a higher level or form strategies on the fly, because the dark magic was overwhelming those parts of her mind with blind rage:  the same magic that was making Fluttershy hostile was also making her stupid.  For a moment, this made Twilight feel elated, since it meant it would be easier to save her friend.  But then it made Twilight horrified, and even horrified that she'd been glad a quarter-second ago, since she realized that the corruption was doing another horrible thing to everyone it touched---worst of all, it was doing it to poor, timid, kind Fluttershy!  It wasn't bad enough that it made her hateful and relentless; it made her stupid, too!
Twilight pushed those thoughts and the internal conflicts they generated aside, and she kept shooting  birds and animals until she had opportunity to fire one more arrow at Fluttershy.  When she did, Fluttershy landed, her eyes restored to a bewildered, dazed white and blue.
Celestia broke her silence:  "Quickly, Twilight, touch her!  Before the corrupted animals do!"  Twilight knew better than to hestitate.  She rushed at Fluttershy and dove at her, as if to hug her---and then Twilight disappeared!  In her place, she found she was now a floating, purple and pink orb---she was a spirit, now, just like the Princess was!  But she could still see and hear and speak, and she saw that Fluttershy was looking around, tears in her eyes, whimpering a little.  "Fluttershy, please---say something!" Twilight pleaded.
"T-Twilight?" Fluttershy asked with a wavering voice.
"Please, tell me you're okay, Fluttershy!"
"No, I'm not okay.  I had the most horrible dream ever; it was even worse than the one where I started acting mean to everyone, and you and Applejack had to tie me down and cast a spell on me to free me!  And..." Fluttershy began looking around everywhere, everywhere except at the two orbs floating above her.  Another terrified, squeaking whimper rose from her throat.  "...And I'm scared I'm still dreaming, because I hear your voice, but I don't see you anywhere!  If I'm still dreaming, and you're awake and can hear me, then please wake me up.  This is scary!"
It was heartbreaking to hear that from Fluttershy.  Twilight wished with all her heart that everything was just a bad dream that they could wake up from, and then everything would be back to normal.  She wanted to see the smiling faces of the populace of Ponyville going about their daily business again.  She wanted to see fillies and colts playing in the streets without a care in the world.  She wanted to be with her friends again!  But all thanks to this corruption, those things could. not. happen.  The more she thought about what the corruption was doing, the more it disgusted and horrified her.
And there was no easy solution to what was happening; it wasn't clear exactly what had to be done to fix everything, but Twilight guessed that it was going to take a lot of work and a lot of fighting to repair the damage that the corruption was doing.  For a moment she wondered if she'd have to fire an arrow of light at every pony in Ponyville to get them all back to normal, one-by-one, but she pushed that thought aside---she didn't even want to think about how complicated, difficult, and dangerous a job like that would be.
Twilight stayed silent thinking about these things for a second too long, and Celestia again broke the silence that was obviously terrifying Fluttershy:  "Fluttershy, look up!"
She did, and she saw the orbs.  She let out a squeak of fear and dove behind the sofa, trembling.
Oh, no!  As if she needed something else to break her heart:  Fluttershy was now afraid of her!  Twilight sputtered, "Please, don't be afraid of us, Fluttershy!  It's me, Twilight, and the Princess!  I'm just in...some kind of spirit-form...  The Princess appeared in spirit form to me, too, and somehow merged with me so that she's following me around as a spirit, and...and it's all part of some kind of Spell of Unity.  I don't understand it, myself, and there's no time to explain it all."
Fluttershy nervously peeked around the sofa, still trembling.  Some of her animals, which were miraculously no longer corrupted, began edging towards her, some of them nuzzling her here and there, encouraging her to come out from behind the sofa.
Celestia spoke up again:  "The Spell of Unity allows one of us to be in bodily form at a time---anyone else who is bound by the Spell of Unity will be in spirit form, but we can switch out who is in physical form at any time, and instantly."
Fluttershy looked at Celestia, confused and still terrified.  It was clear she didn't understand.
Twilight sighed.  "Princess, I'm getting tired, but I think I should be in physical form again---Fluttershy doesn't understand, and there's no time to explain everything.  We have to find the others so that we can use the Elements of Harmony---but...I don't know what we're supposed to do with them, this time.  Can you please explain on the way back to Applejack's farm?"
"I'll explain what I can, Twilight."
"Okay.  Now...how do we switch out who's in physical form and who isn't?"
"Just will it, you and Fluttershy."
"Okay...here goes...you ready Fluttershy?"
Fluttershy snapped back behind the couch, and her sudden movement scared the animals back into their homes.
"Fluttershy, please be brave about this!  Just want me to appear, and I'll appear!"
"...okay..." she whispered, and in a flash, Twilight was behind the couch, and a pastel pink and yellow orb was in place of the purple and pink orb that Twilight once was.
Twilight found she had caught her breath, again, from switching out; she was feeling a lot less exhausted and was ready for more action.  She didn't waste any time wondering why she was no longer tired, but bolted for Sweet Apple Acres.
"So, Princess, what do we do with the Elements once we get everypony together?"
"There is a core to this corruption somewhere in Ponyville.  If you can use the Elements of Harmony to destroy that core, you will free all the ponies in Ponyville from the corruption."
"That's it?  ...Wow...for a while, I was worried I'd have to use my arrows on everypony one-at-a-time to free them all!  Thank goodness I don't have to do that!  But...what exactly are all the side effects of the Spell of Unity?"
"Even I don't know what all of the side effects are; the scroll I read it from was badly damaged, and most of it, except for the spell itself, was already destroyed and rotted away.  I know only a few select things about the spell:  I know that it allows one of us at a time to remain in physical form.  I also know that it allows a powerful magic-user bound to it to fire arrows of light that can cure the corruption, that it allows anyone bound to a pegasus to walk on clouds, that it allows those bound to it to absorb some types of crystals and have their wounds instantly healed, and that it sometimes interacts strangely with the special talents of some other ponies.  Beyond that, and the fact that nopony under the Spell of Unity can become corrupted, I'm afraid I don't know anything about it for sure.  Much about this is shrouded in mystery, and I'm not even sure how much of the ponytales we can trust to be accurate.  But for now, they're all we have to go on."
"You mentioned in your letter something about a demon unicorn who rules over the dark afterlife.  What's that about?"
"I don't know, myself---his name is Satallion, and I don't know whether he exists as a pony or is merely some kind of legendary symbol of evil.  But the ponytale says that he is able to inflict this corruption and forcibly turn other ponies into his thralls.  When they are corrupted, they feel the pure hatred that living ponies normally cannot feel---only the wicked dead are able to feel it without the influence of his magic."
Twilight shuddered.  "What do we do if we have to fight him?  He sounds like he's dangerously powerful!"
"There's no way of knowing how your power or mine measure up to his; we can only use the Elements of Harmony on him and hope for the best."
Twilight didn't like the sound of all this---there were so many unknowns; they weren't sure which parts of the ponytale were true, or whether they'd be able to do anything even if they were all true.  But, as she found herself doing a lot, she pushed the scary thoughts out of her mind and pressed on for Sweet Apple Acres, hoping that Fluttershy would somehow be able to help her get past those animals.
Twilight's thoughts began to dwell on Fluttershy---the poor mare was terrified, and clearly didn't understand what was going on.  On top of that, she'd just been freed from mind-altering magic:  there was no telling how scrambled Fluttershy's thoughts and emotions were on account of that magic.  The thought of her most vulnerable and timid friend suffering like that, and Twilight herself not knowing what to do about it, suddenly made Twilight want to cry.  But she was already at Sweet Apple Acres, so she again pushed these thoughts out of her head and skidded to a halt in front of the ditch that surrounded the farm.  The small animals in the ditch immediately began hissing and barking up at her, eyes flaming.
Fluttershy screamed, "Oh!" at the sight of the corrupted animals, filled with pity and horror.
Twilight gulped---now was the time when she had to ask for help from her confused, timid, terrified friend.  And she couldn't let Twilight down, either, because if she did, she'd never be able to join up with Applejack, the Elements would be incomplete, and---and Twilight tried to stop thinking about it.  She just thought to herself, please be with me enough to help me, Fluttershy!  Twilight gulped and tried to piece together a sentence explaining what she needed from her friend:  "I...I was hoping you'd be able to find a way around them, Fluttershy---everywhere I tried to go, there was always something in the way, until I got to your house."
Fluttershy's voice came back right away:  "I...I don't know what to do, Twilight; I've never seen animals act like this, before!"
She doesn't know what to do...no!  Twilight struggled to think of a creative solution, and she sputtered the most obvious thing she could think of.  "Well, if you can't calm them down, maybe you could at least fly over the ditch?"
"But I can't just leave these poor animals like this!" Fluttershy moaned, switching out into physical form again.  The sight of animals in distress was apparently enough to banish the fear and uncertainty that had kept her silent since they left her house.  Twilight was relieved that Fluttershy had forgotten her own suffering, so she could at least check one concern off her list.  Fluttershy knent down in front of the ditch, and the moment she got close to the animals, they immediately calmed down, and the corruption left them.
Twilight was startled as she saw birds that would've normally dive-bombed them turning back to normal and perching contentedly on Fluttershy's shoulders and hooves.  "What...?"  Her mind struggled to find an explanation  "...Well, the Princess did say that the Spell of Unity interacted strangely with ponies' special talents.  Maybe this is the side effect when you're around; the animals get uncorrupted just by you being around them, since you're just so good with animals?"
Just then, a thundering stampede of ponies rumbled towards them, each of them with a look of determined hate on their faces.  Fluttershy let out a frightened shriek, and the birds that had been perched on her flew toward the stampeding herd, each bird veering off in another direction as they got closer---and the herd dispersed in all directions, each following an uncorrupted bird, baited away from Fluttershy and the precious Spell of Unity.
Fluttershy stood there, not sure what had just happened and a little traumatized from having seen faces with those hideous, evil expressions.  Twilight explained, "That's what happens when someone or something uncorrupted gets near a corrupted pony---or really anything else that's corrupted:  the corrupted ponies...or animals, or whatever, chase after anything or anypony that's uncorrupted.  It distracts them; it's the way I've managed to stay alive."  There was another pause as Fluttershy tried to absorb the information and deal with the terror at hand.  Twilight's heart sank; just when her friend was beginning to forget her fear, it all came back to haunt her---confound this corruption; it just wouldn't give her a break!!  Twilight thought again of how to cure it, and shouted, "Come on, Fluttershy, we've got to get to Applejack and cure her---or maybe, if we're lucky, the ditch kept her safe, and she's not even corrupted!"
Fluttershy cautiously began flying towards the barn to see if Applejack was there.  Corrupted birds flew at her, but when they got close, their corruption faded, and they simply began circling her or flying in formation around her, as if they knew they might need to protect her.  She piped up, looking at Twilight and Celestia, "It's much too quiet, here..."
Since Fluttershy was too shy to shout, Twilight began calling out, "Applejack!  It's us, Twilight and Fluttershy!  Can you hear us?  We need your help!"
RIght on cue, Applejack leapt out of the barn, but their hopes that she had escaped the corruption were in vain:  her eyes, like every other corrupted pony's, were black and burning red.
Fluttershy let out a squeak of fear and began to fly away.
It burned Twilight's heart to keep her friend in the place that scared her, but she shouted, "Fluttershy, NO!  Don't fly away!  We need to cure Applejack; I've got magic that can cure her!  Land!  Land!!"
Fluttershy obediently landed some distance away, but Applejack was already charging.  Twilight popped out into material form, scattering the birds that had been circling Fluttershy.  Twilight ignored them and fired some arrows at Applejack.  One hit, but as soon as Applejack (or perhaps it was the corruption inside her) realized what was going on, she turned around and ran back toward the barn, hopping left and right and dodging randomly to make it impossible to hit her with another arrow.  She dove behind a bale of hay, and began launching apples at Twlight with her strong kicks.
Twilight did her best to dodge, but one smacked her on the shoulder---it hurt quite a bit, making Twilight scream and wonder if her bones were bruised.  But Twilight fought back the pain, kept dodging, and tried to think of a plan---then she saw the birds in the trees.  "Fluttershy, try getting some birds to fly at Applejack---see if that distracts her enough for me to cure her!"  Fluttershy popped into material form and flew up to the trees, collecting a handful of birds.  Then she sent them flying at Applejack, and Applejack immediately forgot about firing apples and began swatting at the birds with her hooves, jumping onto the hay bale to swat them better.  As soon as Applejack was distracted, Twilight popped back out and fired more arrows into her.  Applejack ducked back behind the hay and began launching apples again.
"Get some more birds, Fluttershy.  Trust me, it'll still work; corrupted ponies don't learn from their mistakes in battle!" Twilight shouted.  Fluttershy popped back out and sent more birds to distract Applejack, and they repeated the pattern a few more times.  Soon, Applejack was cured, and Twilight tried to rush at her to make her join the Spell of Unity---but her shoulder hurt too much from the impact for her to run.  "Ow...Fluttershy, fly up and touch Applejack!"
Fluttershy popped back out and flew up to Applejack, hesitating.
Applejack, for her part, was confused and ashamed.  She hesitantly asked, "Fluttershy?  What's goin' on, here?  What're those weird baloons followin' you around?"
"Just trust us," Twilight said.  "Touch her already, Fluttershy, before anything else happens!"
"Twilight?  What they hay?" Applejack sputtered.
Fluttershy timidly extended her hoof and touched Applejack on the nose, and then disappeared into spirit form, herself.  Applejack was left apparently alone, apart from the "weird baloons" floating around her head.  She started at Fluttershy's spirit, an orb with her mane and coat colors, in disbelief and confusion.
"It's us, Applejack---Twilight and Fluttershy.  And the Princess.  Something terrible is happening in Ponyville, and...and...well, we can explain on the way.  Just start running towards town."
Applejack did as she was told and began running towards town grabbing a rope off slung on a nearby fence as she passed it.  She ignored the flocks of animals that were beginning to chase her, while Twilight explained what she knew of the Spell of Unity and the corruption as they ran.  Applejack only stopped when she got to the ditch.
"Fluttershy, switch out and fly us over this," Twilight said.  "In fact, with your animal abilities and wings, you can probably get us to Rarity's boutique the safest."
Fluttershy did as she was told, and not a moment too soon---the animals from the farm were upon them!  They instantly calmed down and clustered around her as she flew over the ditch, following her as best they could.  Many couldn't climb up over the far side of the ditch, but Fluttershy trotted towards town with a large number of animals in tow.
It didn't take long at all for the corrupted ponies to start charging her.  She was visibly distrubed by the sight of them charging her, and closed her eyes and ducked her head with a terrified squeak.  But her animals scurried at them, and every last one of the corrupted ponies was thoroughly distracted---Fluttershy had a clear path to the boutique.
But she just stood there, eyes closed, trembling.  Twilight was getting tired of feeling sorry for Fluttershy; reasons just kept coming up for her to be terrified.  But Twilight again focused her mind on getting her friends together and dispelling the corruption.  She sputtered, "Fluttershy, quick!  Run!  Run to Rarity's boutique!  The animals you control will keep you safe, trust us!  The fate of Equestria hangs in the balance!  I've seen you be brave before; please be brave again!"
Fluttershy picked her head up and gulped audibly, nervously glancing around.  Already a few more birds were circling her, ready to protect her.  She cautiously started trotting towards Rarity's boutique.  "Better move it, Sugarcube!" Applejack shouted.  "There's no tellin' what'll happen if we drag our hooves now!"  Fluttershy bolted toward the boutique, and more corrupted ponies slowly began to scurry after her.  But a lot of squirrels and birds and rabbits were also following Fluttershy by the time she got to the boutique's entrance.  She hopped on top of the boulder to try to get away from the crowd, and with a squeak, sent the animals following her scattering in all directions---taking the crowd with them.
"Quick, Applejack!" Twilight shouted.  "While they're distracted, get Tom out of the way!"  Applejack popped out and shoved at the boulder with all her might.  It slowly rolled out of the way, and Applejack dived into the boutique and slammed the door behind her.  No one was in sight.
"You reckon Rarity put that boulder there to keep herself safe from the corruption?" Applejack asked, panting.
"I hope so, and I hope it worked," Twilight answered, "But that's the kind of luck I haven't been getting, today."
As if that were her cue, Rarity stepped out from around a corner with her cat Opalescence---and both of them were corrupted!
Applejack gulped.  "I see what you mean, Twilight!"
Unlike Twilight, Rarity had full use of her horn magic, and she used it to levitate the rolled-up bolts of fabric in her store to make platforms that hovered just out of reach of Twilight's arrows.  Rarity stood above them on these platforms, glaring down at them harshly.
Applejack tried to calm her down.  "Now, Rarity, don't go lookin' at us like that.  I know we don't see eye-to-eye on everything, but---"
"Talking doesn't work!" Twilight insisted.  "I need to use my magic!"
Rarity began using her horn magic to pick up sharp and heavy objects from around her boutique and hurl them down at Applejack.  Applejack dodged as best she could.  "What do we do, Twi?!" she shouted.
"I'm thinking, I'm thinking!" Twilight answered, trying to decide if there was a pattern to the attack.  Rarity was hurling things as fast as she could---an iron, a dozen needles held together with a ball of thread, a pincushion was the sharp parts out, a scissors, a ponniequin---but was hurling them one at a time, and was beginning to run out of objects to throw.
Then, it dawned on Twilight:  Fluttershy charged her when she ran out of animals to send at her.  Maybe when Rarity ran out of things to throw, she'd charge them, too, and that would give Twilight her chance!  "Keep dodging and get ready to switch out with me!" Twilight shouted to Applejack.
Twilight waited with bated breath as Rarity hurled the last object down at Applejack to see what she'd do.  ...But she didn't charge them!  Instead, she sent Opalescence to attack Applejack.  Opal clawed at Applejack's face, barely missing.  Unlike the hurled objects, Opal was able to chase them.
"Switch out with Fluttershy and let her cure Opal!" Twilight shouted.  Fluttershy popped out, and, just by being close to Opal, Opal was cured.
Twilight tried to think what to do next; Rarity wasn't charging them---she was just glaring at them and snorting, trying to pick up a very heavy ponnequin to throw at them, but unable to move it---Rarity was distracted enough to give Twilight a moment to think.  If only there were a way to yank the platforms down so Rarity would be low enough to hit with an arrow...!  Twilight's mind raced; what did they have to work with?  ...Applejack brought a rope, and she could easily lasso some of the bolts of cloth and yank them to the ground.  But Rarity could fight against the pull of the rope with her magic---was there a way around that?  ...Twilight remembered that, when a unicorn is distracted, it's hard for her to use her magic.  But the only thing that distracts a corrupted pony was an uncorrupted animal...like Opal!  That was it!
"Use Opal to distract Rarity, Fluttershy!" Twilight shouted, overjoyed that she had finally puzzled out a real solution, but still in suspense about whether it would work.  Fluttershy gave Opal a boost, and she charged at Rarity.  Rarity grabbed Opal between her hooves and let the corruption seep back into her cat, and Twilight took that for the opportunity it was.  "Quick, Applejack!  Lasso her platform and get her on the ground!"  Applejack popped out and did as she was told with alarming speed.  Twilight popped out and fired as many arrows as she could at Rarity---which turned out to be only one:  re-corrupting Opal had taken less time than Twilight had hoped, and Rarity again was able to focus and use her magic.  The platforms snapped back to their places with Rarity and Opal still on them, and Rarity's magic again drew the heavy and sharp objects back into the air, ready to be tossed.
Twilight began dodging them as they flew at her.  "Just rinse and repeat, girls!" Twilight shouted.
"I remember you said corrupted ponies don't learn from their mistakes, back when you were fighting---" Applejack hesitated to finish the sentence.
Twilight knew she was feeling guilty about fighting them, so she shouted, "Don't feel bad about fighting us, Applejack!  It wasn't your fault!"  Twilight let her feelings and the conversation distract her just a little too much, and the iron Rarity was hurling at her clobbered her on the shoulder---the exact same shoulder Applejack had hit with an apple in the last battle, no less!  Twilight screamed in pain and collapsed.
Applejack popped out again to take her place.  "Leave the heavy fightin' to me, Twi!  I'm a tough ol' girl; I can take it!"  A ponniequin slammed into her head, and she shook the blow off.  "See?"
Rarity was out of ammunition, and again, she sent Opal to attack Applejack.  Fluttershy knew what to do, and without a word, she picked Opal up and put her back on Rarity's platform.  When that distracted Rarity, it gave Applejack the opening she needed to bring Rarity to the ground, and that gave Twilight the opening she needed to fire an arrow at Rarity.  Twilight popped out, fought back the pain, propped herself up, and fired another arrow into her friend.
They repeated the cycle another two times, and finally, Rarity was cured.  She looked terribly sheepish, and almost immediately began to shed tears.  "I...I can't believe I was attacking my own dearest friends!!" she sobbed.
"Don't feel bad about it, Rarity!" Twilight insisted.  "We're just glad that we're all okay, right now."  Twilight began to walk toward Rarity, but her shoulder reminded her that she wasn't okay the second she put her hoof on the ground.  She screamed in pain.
Rarity began blubbering again.  "Look what I've done!!" her voice got harder and harder to understand as she began blubbering.  "I don't deserve to have such close frie-he-hends!"
Twilight tried to concentrate, but there was just too much pain for her to think clearly.  Rarity needed to be quiet so they could explain the situation to her, how she needs to be---wait, what did they need to do with her?  OW!  Twilight couldn't think to save her life; it felt like her brain was completely cramping up; she couldn't even remember what was going on!
Suddenly, Rarity went quiet.  Twilight looked at her, and saw that Opal had walked up under her owner and was using her paws to clamp Rarity's mouth shut.  Thank goodness that cat knew how to handle her!
Applejack and Fluttershy began talking while Twilight tried to regain her concentration.  They quickly explained to Rarity what the Spell of Unity was, and what the corruption was doing to everyone, and how they needed to assemble everyone to use the Elements of Harmony.
"This isn't good at all," Rarity reflected.  "So...I guess the first thing to do is to find Pinkie and Rainbow Dash?  Where would they be?"
"Pinkie would be at Sugarcube Corner fer sure," Applejack thought aloud.  "No tellin' where Rainbow Dash is; I sure hope she's fast enough to not be corrupted, so she can find us."
Twilight's mind raced again; she was in no condition to fight---and there's no telling what would happen if she were seriously injured or even died while under the influence of the Spell of Unity.  Whatever happened, it wouldn't be good, and she wasn't going to get far in fixing the situation, as injured as she was.  If only there were a way to heal wounds quickly with magic---wait, the Princess said that the Spell of Unity could absorb crystals to heal wounds!  And Rarity had a lot of gems in her boutique:  maybe she could use those to heal herself!
"Let's just do what we know is right for now and head for Sugarcube Corner," Twilight said, glad she could finally concentrate enough to think up another solution.  "But...before we go, Rarity, can I have a few of your gems?"
Rarity was a bit reluctant.  "What do you need them for?  It's not Spike, is it?"
"No-no; the Spell of Unity lets us absorb crystals and get healed when we touch them and absorb them, and I got pretty banged up in the fight with Applejack and..." Twilight was reluctant to say anything that sounded even a little accusatory; she needed her friends more than ever before.  And Rarity was feeling even worse about fighting them than Applejack and Fluttershy had felt.  But before she could stop herself, the words came out:  "...and with you..."
Rarity didn't seem too shaken up; she just said, "Then of course!  Take as many as you need!  I'm just glad there's a way you can heal quickly."  Thankful she had one less concern for the moment, Twilight limped toward Rarity's gem chest.  She tried to reach in with her good hoof, but she couldn't put weight on the bad one.  She yelped in pain.
"Oh, Dearie, let me get that for you!" Rarity insisted.  She pulled an orange, heart-shaped gem out of the chest and touched Twilight's shoulder with it.  Immediately, the gem disappeared, and the pain was gone.
"Wow," Twilight said, moving her shoulder to make sure she was okay.  "Rarity, we might need you to find more gems for us if we start to get injured out there." Twilight paused and sadly reflected, "This is probably going to be our most dangerous adventure yet..."
"You can count on me!" Rarity slung a saddle bag over her shoulder and filled it with gems from her chest, trying to be prepared for whatever was coming in the adventure.
Twilight smiled.  Between the two times she'd managed to reason out a solution since she got into the boutique and the fact that the pain was gone immediately, she was starting to feel some hope that things would turn out all right in the end, after all.  "Thanks, Rarity!  Now, let's get you joined to the Spell of Unity!"  Twilight touched Rarity's shoulder with her hoof, and Twilight disappeared as Rarity joined the spell.
Just as Rarity was about to move to the door, Celestia again broke her silence:  "Your special talent involves gems, Rarity?"
"Y---yes it does, your Majesty."
"Then there is a chance you have a special power that the scroll spoke of.  The particular part of the scroll that mentioned this was damaged, but I could make out two words:  'crystal prison'."
"I'm not sure I like the sound of that..."
"Still, if we can, we should try to find out what that phrase meant.  It could be helpful if---"
At that moment, the door to Rarity's boutique burst open, and there was a gray, snorting, cross-eyed pegasus at the door:  Derpy.  Her crossed eyes, which were normally comical to look at, were now even more horrifying than properly aligned ponies' eyes were, thanks to what the corruption had done to them.  Rarity let out a startled whimper:  Derpy was a walking disaster at the best of times, but now---what would she be like?
"Hurl a gem at her!  See what happens!" Celestia commanded.  Rarity obeyed, and struck the charging mare in the chest with a small sapphire.
Something that amazed Rarity happened:  the sapphire instantly grew around Derpy and trapped her body in crystal.  This was, in effect, a "crystal prison", just as the Princess had said.
Derpy wasn't completely entombed in crystal, however:  her chest, her face, and her tail and hind legs were open to the air.  She could still breathe, and was snorting at them and wriggling to try to free herself---to no avail.
Rarity found the sight of Derpy snorting at her and struggling against her own magic unsettling.  However accident-prone Derpy was, it could always be said of her that she meant well---except for today.  It felt like Derpy didn't deserve to be trapped like this, and yet, the way she was looking at Rarity was disturbing.  And then there was the question of how long Derpy would be stuck in the Carosel Boutique.  Then it occurred to Rarity that Derpy's rear end was clear of the crystal...
"Uh, Princess, how long is this Crystal Prison supposed to last?"
"I don't know.  That part of the scroll was damaged.  It could be for hours."
Rarity's face took on a horrified expression, and she rushed out of the room.  "I've got to find the commode right now!"
"We don't have time for this, Rarity!" Applejack insisted.
"This will save far more time than it costs, trust me!" Rarity found the commode in another room and carried it to Derpy, setting it down just behind her.  "There."
"...Just how did  that save us more time than it cost?"
"Do you know how much time and effort it would take to remove the stains from the carpet in here?!  I've saved hours of work---just by taking thirty seconds to fetch the commode!"
If Applejack still had a hoof and a face, she'd have facehoofed.  With a groan, she shouted, "You may have saved your carpets, but you ain't saved Equestria!  Get a move on to Sugarcube Corner, already!"
"All right, I'm going!" Rarity shouted back, irritated that Applejack didn't care about what could've happened if she hadn't put the commode where she did.  For a moment, she thought to ask Applejack how much she would like manure on her rugs, but she thought better of it.  After all, the last thing they needed right then was more arguing, and besides:  Applejack was so uncouth that the answer might not be what Rarity would've expected.
Just as Rarity was stepping out the door, she heard a relieved sigh from Derpy, and a stench wafted out of the boutique.  "See what I mean?!" Rarity shouted.
"Aw, fer Pete's sake!  Just get a move on!" Applejack moaned.
"I'll take point," Twilight groaned, hoping that both of them being in spirit form would end the argument.
As she sprinted toward Sugarcube Corner, her stomach gurgled louder than before, giving her a painful reminder that it was still early morning (or would've been if Celestia had had a chance to raise the sun), and she'd still not had any breakfast, yet.  As if she needed something else to stress her out!  It had been straight out of bed and into the adventure for her.  No hay, no toast, no eggs, just get up and run and fight---and think and worry.  Of course she wouldn't say "no" to saving Equestria just so she wouldn't miss breakfast, but she secretly hoped that she'd have time to grab at least a cupcake or two before she left Sugarcube Corner.  She'd need her energy for this adventure, if nothing else.
Again, she had to fight her way past crowds of corrupted ponies, but by now, she'd gotten the hang of it---she just had to stay aware and watch her back.  Beyond that, the same strategies as before applied:  shoot the stragglers, and they'll distract the whole crowd.
By the time they got to Sugarcube Corner, there were fewer corrupted ponies than she had expected.  "Okay," Twilight began, "Pinkie's locked herself in her room, and even locked the window.  She threw the key to the window into this pile of junk, and because of the way the Spell of Unity works, I can't use my magic to search through the pile.  Can you find the key, Rarity?"
Rarity popped out, and her horn wasn't glowing for two seconds before the key levitated up out of the pile.  "You mean this key?"
"Yes!  That's the one!  Fluttershy, can you fly us up to the window of Pinkie's room?  If she's not been corrupted, then she's probably still up there."
Fluttershy popped out and cautiously flew up to the window, the key between her hooves.  "But...what if she is corrupted?"
Twilight's heart sank at that thought, and she tried to drown out the horrific thought of seeing bubbly, fun-loving Pinkie Pie's personality overwritten by the dark magic by focusing on strategy:  "Then we might be able to get the jump on her if we come in through the window."
"Ooo," Fluttershy moaned, "This doesn't feel right, barging in like this..."
"There's a lot of things that aren't right about this, darling," Rarity lamented, "But we have to just keep doing whatever we can think of to save Equestria.  And if barging in or sneaking through the window helps save Equestria, then like it or not, that's what we have to do."
It was comforting to hear such words of plain, simple wisdom from her friends.  Everything about the corruption had felt horrible and complicated to Twilight, and it got worse the more she thought about it.  But hearing her friends say what she had been half-thinking all along felt soothing---as if her friends' words were massaging the tension out of her cramped, knotted thoughts.  Twilight hoped for more of this easing of her mind, and wished she could put the stress of the morning into words and ask for more help from her friends in dealing with it.  But then, it dawned on her that they might be walking into a trap, and she had to keep her wits about her.  Twilight was reluctant to push such warm thoughts out of her mind to focus on the task at hoof, but she did so, knowing that too much was at stake to dwell on anything---even something as wonderful as friendship.
Fluttershy peeked in the window and saw no one in Pinkie's room.  Not even Gummy.  She opened the window slowly, and it make a soft creak---Fluttershy gasped softly in fear that she'd been heard, but there was no reaction---still no one in the room, not a sound from the rest of the house.  She kept opening the window and flew inside.  She looked around nervously, trembling a little, expecting danger to leap out at her at any moment.  The tension in the air could've been cut with a knife.
Twilight broke the silence and whispered, "Maybe---"
Fluttershy panicked and rolled over on her back, frozen, and a sound that reminded Twilight of a goat's bleat drifted up out of her throat.
Twilight finished the thought, trying to keep her emotions in check by focusing on the task at hoof:  "Maybe I'd better do this; if we find any corrupted ponies, only I can cure them."
Fluttershy was all too glad to let Twilight switch out with her.  Twilight looked around the room and spotted a bit of trembling, curly, pink tail-hair sticking out from under Pinkie's bed.  "Pinkie, is that you?  Are you..." Twilight hesitated; Pinkie probably wouldn't understand "corrupted" if Twilight used the word.
Pinkie's tail zipped under the bed and the whole bed started to shake.  "No!  No, it's not me!  And I'm not---whatever it is you were gonna say!" Pinkie answered, her voice quivering with fear.
"Pinkie, it's all right, it's me, Twilight!  I'm not corrupted!  You don't have to be scared of us!  The Princess is here, too, and she has a plan to stop the dark magic that's making everypony hostile!  ...I'm just glad that, for once, I found a friend who isn't corrupted.  I had to fight to cure Fluttershy, Applejack, and Rarity to make it this far!"  Twilight began to feel a little bit better again as she realized that luck had finally favored her:  she would have a friend back easily, with no fighting or injury required!  Maybe things would get better, after all...!
Pinkie peeked out from under the bed.  Her eyes were still their natural, filly-ish blue, but full of worry and uncertainty.  She saw Twilight with four orbs floating around her.  Noticing that Pinkie's eyes were fixed on the orbs, Twilight motioned to them and explained, "Believe it or not, these are Applejack, Rarity, Fluttershy, and the Princess in spirit form---only one of us can be in physical form at a time, but the spell that keeps things that way protects us from the corruption."
Pinkie tried to understand, "So...you can't turn into...zombies?"
"Exactly!  Just touch my hoof, and it'll protect you, too!"  Twilight extended her hoof for Pinkie to touch.
Pinkie eagerly touched Twilight's hoof---only to find Twilight disappeared!  Pinkie screamed and hid under the bed again, causing the whole frame to shake.  Twilight's heart sank.  Pinkie squealed, "You didn't say it'd make you disappear, Twilight!"
Twilight tried to insist to herself that things could still get better; she just needed to explain...  "I haven't disappeared, you're just part of the Spell of Unity, now!" Twilight insisted.  "I'm in spirit form, just like Applejack and Rarity and Fluttershy!  Don't worry about me, I'm fine, and---and so are you!"
Pinkie opened her eyes and looked at the orbs keeping her company under the bed.  "Don't worry, Sugarcube," Applejack assured her, "We're all right here, and we're just fine.  Just come on out and help us find Rainbow Dash so we can use the Elements of Harmony again to fix this mess."
Pinkie zipped out from under the bed, and the fear was instantly gone from her---she stood at attention and saluted with a cheery, "Okie-dokie-lokie!"  She bounded up to her door and unlocked it, but her sudden happiness and eagerness was instantly shattered as a loud pounding came up to the door.  Pinkie backed up to her bed, shivering, and was about ready to dive under it again.  The door flew open, and a corrupted Mr. and Mrs. Cake burst in, glaring straight at Pinkie Pie with that same dark, horrifying, hateful look that Twilight had seen all too often that morning.  Pinkie screamed and pulled a cannon seemingly out from nowhere.  "Stay back!  Stay back!" she screamed.
"Pinkie, that's your party cannon!" Twilight shouted.  "It's not going to---"
Twilight didn't have time to finish that sentence before Pinkie fired the cannon at Mr. and Mrs. Cake.  What looked for a moment like streamers flew out at them, but the streamers seemed to stick to the middle-aged ponies' fur and keep them from moving.
"...What did you just do, Pinkie?"
"Oops!" Pinkie giggled.  "Looks like I put fly strips in my party cannon instead of streamers---again!"
"Worst birthday party ever," Rarity mumbled under her breath.
Pinkie's ears flopped down in sorrow.  "I said I was sorry!" Pinkie whimpered.
The Cakes had un-stuck themselves by now, and they snorted at Pinkie.  Pinkie gulped loudly.  Twilight expected them to attack, but instead, they just glared at Pinkie and spoke harshy to her, their voices twisted by the dark magic into horrible roars, and there were hints of another, darker voice being blended with theirs:  "Pinkie!  You are always eating our wares and wrecking our profits!  We shouldn't even be paying you a salary; you eat more than you're worth!  You shirk responsibility and play like a little filly and never act your age!  It's time you learned your lesson and paid for the things you've done:  YOU!  ARE!  FIRED!!!"
Pinkie's face went white, and she disappeared from her physical form.  Twilight popped out and wished she could spare more time to think about why Mr. and Mrs. Cake were saying things instead of fighting them directly, or to sympathize with how horrible poor Pinkie must've felt at that moment---she was a very sensative soul under all that joy and randomness.  But Twilight didn't have the mental bandwidth  or time to think about these things:  the Cakes charged her.  She tried firing some arrows at Mrs. Cake, but the moment she cured her, Mr. Cake touched her and corrupted her again.
Twilight kept dodging their charges and trying to cure them, but the moment she cured one, the other just re-corrupted him or her.  It didn't matter which one Twilight tried to cure, it was always the same---the only reason the battle was bearable was because she was fighting unarmed earth ponies; they had no magic or things to kick back at her.  But it was certainly a challenging fight.
Finally, Applejack shouted, "Let me in, Twi!  I'll separate 'em!"  Twilight let Applejack out, and Applejack kicked Mr. Cake solidly---it wasn't a kick to injure, but a kick to push, and it knocked him off his feet.  Mrs. Cake slammed into Applejack, but she didn't flinch; she just pushed Mrs. Cake in the other direction.  "All yours, Twi!"
Twilight popped out and fired arrows at a charging Mrs. Cake, then turned around and fired again at Mr. Cake, just as he was getting up.  They were both uncorrupted, and a bit dazed.  She walked up to Mr. Cake and asked, "Are you all right, Mr. Cake?"
"I'm fine," he assured her, dusting himself off.  "But what about Pinkie?  Where is she?  What happened to her?"
Twilight looked down at the pure, pink orb floating near the floor beside her---but something wasn't quite right about it; the orb seemed like it was shrouded in red, almost as if it were somehow...blocked, for lack of a better term.  "That's her," Twilight said, "In spirit form.  It's a side-effect of the spell that keeps us immune to the dark magic.  She can hear everything we're saying...I think..."
Mrs. Cake said to Pinkie, "Pinkie, please---you're not fired.  That wasn't us talking, it was the dark magic.  We like you for who you are; we like the fact that you're so much like a filly---please don't be hurt that we said those things!"
Twilight cut in, "Pinkie, I think you should be in physical form to have this conversation."
Only silence answered her.  Celestia added, "As long as Pinkie doesn't want to use her physical form, she can't, and we need her help in physical form to destroy the core.  Please, Pinkie Pie---listen to them.  Make up with them.  I don't like putting pressure on another pony's heart, but the fate of Equestria depends on this."
Twilight felt like she was in a daze; part of her wanted to help Pinkie and apply her wisdom, knowledge, and lessons about friendship---but her head hurt from worrying and thinking and pushing thoughts aside.  She couldn't think what she should say!
Mr. Cake broke the silence first:  "Pinkie, you really don't eat away at our profits that much!"  Twilight raised an eyebrow at that, finally able to think a clear thought, as she distinctly recalled seeing Pinkie eat an impossible amount of dessert the day the Parasprites appeared, when the Cakes were preparing for Celestia's arrival.  The clear thought became a coherent train of thought as she also recalled that there were days when she found it inconceivable that Sugarcube Corner could possibly make a profit if Pinkie was allowed anywhere near the place, let alone allowed to work there.  Twilight also noted that Pinkie didn't seem to believe it, either; she remained completely silent, and her orb hovered solemnly near the floor.
Rarity's orb flew at Twilight and smacked her across the cheek.  "Ow!"  Twilight looked at Rarity's orb, and while there was no facial expression, she could almost feel Rarity glaring at her for giving Mr. Cake that look of disbelief.
Mrs. Cake's resolve to stick to that polite lie broke:  "...Okay, maybe there are days where you eat more than your salary would buy, but they're few and far between!  But even though there are days like that, we still like having you around, Pinkie!"
Twilight began to think that maybe she didn't need to say anything as the moment grew more and more touching between Pinkie and her employers, and both Cakes knelt down by Pinkie's orb.  "Pinkie, every pony in Ponyville knows you, and they know you throw the best parties!" Mr. Cake insisted.  "If Sugarcube Corner lost you, nopony would want to have a party here ever again!  You're a vital part of this place---it wouldn't be the same without you at all!"
The red slowly faded from Pinkie's orb, and she popped out, sitting dejectedly and staring at the floor.  "Oh, Mr. and Mrs. Cake, I'm sorry for being such a bad employee!  From now on, I'll pay for everything I ever eat!"
They chuckled, "You'd go broke if you did that, Pinkie!  And the change doesn't have to happen all at once.  Just---try to control yourself a little.  We don't want to lose the fun-loving Pinkie we know and love!"
"Awww...you guys are the best bosses I could ask for!"
They closed in for a hug, but the Princess shouted, "STOP!  Don't touch them; there's only enough room in the Spell of Unity for one more pony, and we need that room for Rainbow Dash!"
"It's true," Twilight admitted, saddened that the heartwarming moment had to be shattered.  She stifled a thought that this emotional roller-coaster might never stop, and she continued, "You get more ponies into the Spell of Unity by touching them once they're no longer corrupted.  ...I'm sorry, Pinkie; I'd want to hug if I were in your place, too.  ...I guess there are downsides to the Spell of Unity..."
Pinkie looked wistfully at Celestia's orb and asked, "What happens once the Spell of Unity is full, and then I touch another non-zombie pony?"
"Nothing happens."
Pinkie immediately perked up.  "Perfect!  Then I'll go find Rainbow Dash and THEN we can have our hug!"  She beamed at her bosses.  "Don't worry, it won't take long; I'm the BEST at finding Rainbow Dash!"
She was about to trot down the stairs, when Twilight suddenly remembered her hunger and asked, "Uh, before we go, Mr. and Mrs. Cake---do you mind if I have a cupcake or two?  It's just that this whole mess started before I could get any breakfast, and I'll need my strength for this---"
"Say no more, Twilight.  Help yourself."
"Thanks---I'll be sure to pay you back for it, later---"
"Don't worry about it," Mrs. Cake insisted.
Twilight was about to argue that she could and should pay for the cupcakes; it was only a few bits, after all.  But she knew there was no time.  She just thanked them and popped out, quickly moving down the stairs and picking a few cupcakes out of the display window before sneaking to the door.  She swallowed them quickly with no time to enjoy them---no time for anything, really.  Twilight reflected that, in some ways, it was a problem that they had to stop for a few minutes so that Pinkie and the Cakes could patch things up so that Pinkie would be psychologically capable of helping them; lost time was a major issue because of how fast the corruption could spread.  But being able to stop fighting for few minutes felt very refreshing to Twilight, now that she thought about it.  She wished she had more time to think about what was going on, maybe to see some patterns to it all, or even invent a spell or potion that would cure it faster, or at least carry a train of thought to some conclusion that might give her some hope in this dreary crisis---but there was no time to think, and she and her friends had already spent too much time at Sugarcube Corner; they needed to find Rainbow Dash before things got any worse.
She opened the front door, only to find that the town square was covered with corrupted ponies, all staring darkly at her.  Pinkie Pie, still in spirit form, spinning around wildly, screamed, "EEEEEE!!  Where's my bed when I need to hide under it?!"
"I can handle this!" Twilight shouted, half to Pinkie and half to herself.  She fired a few arrows into the crowd, and they immediately lost interest in her and scattered in all directions, chasing after the uncorrupted, terrified few.
Twilight did her best to smother the thoughts that were resurfacing in her mind about how terrified those few uncorrupted must be, and said, "Okay, Pinkie---come back out and help us find Rainbow Dash.  I know you have a sixth sense about these things, and I still don't understand it, but I trust it."
Pinkie popped out, and immediately she pointed to the Town Hall.  "She's on the roof!  Maybe she's not corrupted!"
"How can you tell?" Rarity asked.
"Trust me on this one," Twilight groaned, "With Pinkie, you never ask how.  Just don't do it.
"Fluttershy, you're the only one of us who can fly.  Can you get us there?"
Fluttershy gulped---how she could gulp in spirit form, where she had no mouth and no stomach to swallow down into, Twilight didn't know, but there was no time to wonder about it.  "Okay," she feebly whispered, and popped out.  She quickly gathered a flock of birds around her and flew up to the town hall.  Only a few pegasi bothered her, and her birds quickly distracted them.  But when they got to the roof of the Town Hall, Rainbow Dash was waiting for them.  Her eyes were black and red, but instead of the raging hate, there was a sneer of cruelty on her face.  It was almost more disturbing than the hate that most of the ponies had, and it certainly made Fluttershy want to cower.
Rainbow chuckled darkly at Fluttershy.  "So...come to make me see the error of my ways, huh?"  Rainbow seemed to have the same dark presence behind it that Cake's voices had, only rather than aggressive rage, this was cool, calm, tormenting cruelty.  "My 'friends' trying to make me see the light, make me a better pony?  You're a bunch of naive idiots, just like you were when DisQord set me free to be who I really am!  Then you worked your mind-control magic and turned me back into one of you..."  Rainbow Dash said "you" as if the word stained her tongue, and started pacing back and forth, hamming up her dark, horrifying speech.  "You think I wanna be one of you?!  Ha!  You lie to yourselves and each other---you don't really want to help each other; you want to do your own things!  Twilight just wants to read, Pinkie just wants to play, Applejack just wants to work, Rarity just wants to roll in gems, Fluttershy just wants to be alone---and instead, you just waste time with each other and pretend to care about each other.  What for?  You're just pretending to be somepony you're not---all of you!  This 'corruption', as you keep calling it, is really just setting everypony free to be who they really are, inside!  You should be grateful we're all taking the time to let you join us in freedom---but even if we didn't want to invite you to join us, it's not like you left us any choice.  You're the worst possible slaves to so-called 'society and civility'; it's not bad enough that you pretend to care so much about each other; you also try to force the rest of ponykind to pretend with mind-control magic, too!  Honestly, how wrong can a bunch of ponies be!?"
Some of Twilight's reading about psychology popped into her mind.  One of the major theories about a pony's mind, originally put forward by Sigmund Fjord, was that the mind was divided into three parts:  the id, which consists of basic desires; the ego, which seeks balance; and the superego, which consists of principles and is basically the conscience.  It seemed like Rainbow Dash was saying the id was all there really was to a pony's mind, and that what a pony wants or is tempted to do is who they really are---that was a scary thought, to Twilight.  It meant that raw desires---the desire to have your friends do things exactly the way you want, the desire to have the best food or know everything or win a contest---was all there was to the mind, and that everything your conscience taught you, everything you did to be polite, like sharing or waiting in line---it was all some kind of lie, and shouldn't be there.  But wouldn't a world without politeness and kindness be utter chaos and total war?
No---Twilight couldn't stomach a world like that, even if she had to lie to herself and the world to avoid it.  She popped out and shouted back, "Something is wrong with you, and if there's anything of the real Rainbow Dash left in what's talking to us now, then hold still so I can cure you!"
"They should've sent you back to magical kindergarten!" the corruption chuckled through Rainbow Dash's mouth.  Twilight winced inside; this corruption certainly knew exactly which nerves to pluck---even if she wasn't going to let it slow her down, no matter how much she ached inside.  But the corruption didn't slow down, either; it continued, "I am the real Rainbow Dash, you dunce!  ...But nothing I say will make you believe that, will it?  You pathetic bookworm---dead-stuck in your ways, you just won't see the truth...!"  She paused, narrowed her eyes, and hissed darkly, "I pity you."
That was enough.  Twilight began firing arrows at Rainbow Dash, but she was too quick to hit---in fact, Rainbow spun around in a circle and created some kind of whirlwind that smashed the arrows to pieces before they could touch her.  Then, Rainbow Dash charged.  It wasn't easy to dodge her swoops and dives, and Twilight just couldn't land an arrow no matter how hard she tried.
She tried to think as she kept dodging this way and that, even taking a few hits to her sides---then she thought about how Fluttershy's birds would distract other ponies, and that their drive to spread the corruption would take over whenever they got close enough to anything that wasn't corrupted.  She shouted for Fluttershy to pop out, and she did, though very nervously, knowing that she wasn't nearly as fast or strong as Rainbow Dash.  Fluttershy sent a few birds at Rainbow, but as soon as one bird got close enough, Rainbow grabbed it between her hooves and, with a horrifyingly twisted grin, let the corruption spread slowly into the horrified little creature.  That only distracted Rainbow for a second, but that was all the opening Twilight needed to fire arrows at her.  "STOP IT!" Rainbow shouted in deperation, flying around again.  "Stop taking away my freedom!  You don't realize what you're doing, you morons!"
The desperation in Rainbow's voice would have made Twilight hesitate, but Fluttershy popped back out before she had the chance to do so, and kept the fight going.  They repeated this pattern, and surprisingly for how intelligently Rainbow Dash was speaking to them, the intelligence wasn't applied to the battle itself---Dash didn't run away when the corruption was nearly gone from her, nor did she change her patterns of diving and charging at them and throwing windwinds at them.  Nor did she even ignore the birds when Fluttershy sent them at her; the corruption's behavior really didn't make any sense at all to Twilight.  As if she didn't have enough on her mind already, between what she had had to do to make it this far, how many things were horrifyingly wrong with the corruption, and the words the corrupted Rainbow Dash had said to her!  She again pushed the tidal wave of unwanted thoughts out of her mind, into the steadily-growing pile of thoughts that were being discarded to the benefit of the immediate and the urgent, and she kept fighting.
It wasn't long before Rainbow Dash was freed from the corruption.  With her eyes back to their normal purple, Rainbow quickly sputtered, "That wasn't the real me, guys!  Twilight, you're the smartest pony I know; please don't believe a word that I said---I mean, that it said---"
Twilight kept pushing away the growing doubts in her mind and heart and limped up to her.  "Don't think about it," she said flatly, as much to herself as to Rainbow.  "We just need to get you into the Spell of Unity so you'll be immune to the corruption, and then we need to find this---core-thing that's holding the corruption together, and destroy it with the Elements of Harmony."  Her answer was short---she wanted to get the job done as quickly as possible; she was getting tired fast---not just physically from the beatings she'd been taking in these fights, but emotionally.  She didn't know how much longer she could keep pushing these thoughts out of her mind like this.  "When I touch you, you'll be joined to the spell, and when ponies are joined together, only one can be in physical form at once, so I'll turn into a spirit orb like the rest of them---I'm just giving you fair warning."
Rainbow Dash knew that Twilight was agitated, so she ignored what Twilight was saying and kept insisting, "Twilight, I mean it, whatever it was that said those things was just trying to mess with your head; please don't let it get to you!  I can't stand that thought that something I said would hurt my---"
"Rainbow!" Twilight insisted, not wanting to think about it any more.  She touched her shoulder with her hoof and vanished into spirit form.  "Don't think about it!  We have a job to do, here, and the fate of Equestria hangs in the balance!  We need to find that core, already!  Princess, do you know where we can find the core of all this corruption?  You said if we use the Elements of Harmony to destroy the core, we could free all of Ponyville.  So...where is it?"
"I'm not sure, myself," Celestia answered.  "But Rainbow Dash, the fact that you were speaking intelligently for a time while under the corruption's influence is particularly interesting; most corrupted ponies can't do that.  It implies that the core has some intelligence of its own, and that it has taken an interest in us---it's unsettling to think that it might know what we're up to and be prepared, but at the same time, your mind and it were connected for a time, and that might provide us with some clues.  Can you think where it might be?"
Rainbow stopped pushing the subject of messing with Twilight's mind and answered her Princess, "I'm not sure what this 'core' thing is you're talking about, but I felt like whatever was making me talk was really closeby---like, probably in this building.  In fact, I think I remember seeing this dark cloud-crystal-energy thing inside the town hall while I was flying up here..."
"That's it!  That's the core!" Celestia declared.  "Get into the Town Hall and let's end this nightmare!"
Rainbow Dash didn't waste any time; she actually broke a window rushing into the Town Hall to get to the core.  When she burst in, she saw the core again---it was difficult to describe; it reminded her of a large, pulsating, swirling dark cloud, but it was glued to the floor by bizarre-looking crystal structures that also seemed to be pulsating---growing here, shrinking there, breaking off and swirling into the cloud, dissolving, and growing back all in a second.  Describing it would've been like trying to describe fire to somepony that had never seen fire.  But it gave off a very creepy vibe, as if just being around it without having the Spell of Unity on you could corrupt you.  She gulped and stared at the monstrosity for a few seconds.  It was not hard to believe that it was the cause of all the misery in Ponyville.
While she was staring at it, Fluttershy asked, "What do we do to destroy this thing?"
Celestia explained, "The Spell of Unity will allow all of you to enter physical form together for a short time, long enough for you to use the Elements of Harmony to destory it.  But it takes a great deal of energy and all of you will be able to remain in physical form for only a short while; we shouldn't try it if there are any enemies who could interfere with---"
She didn't get a chance to complete the thought, but it was pretty obvious where she was going with it.  Especially because an enemy did, in fact, show up:  Trixie.  Her eyes were black where they should've been white, but instead of glowing red, they were glowing an otherworldly purple.  Twilight wondered if this was a side effect of being so close to the core, but pushed the thought aside as the battle began.
Trixie threw all kinds of magic at them, and was surprisingly good at using their own abilities against them.  For instance, when she fired a magic wave at them that surged along the ground, and Rainbow Dash tried to reflect it with a tornado, Trixie just made the tornado swing back at her.  Twilight popped out to try to even the odds, but Trixie was too good at reflecting the arrows of light back at Twilight for direct curing to work, try as she might.  Rarity shouted above the battle-clamor, "Fluttershy!  Try distracting her with some birds!  That's these corrupted ponies' only weakness; their lack of attention!"
Fluttershy was very reluctant to pop out.  "I'm no good at fighting!" she protested, "I hate violence!"
"This isn't violence," Twilight insisted, "It's the only way to cure her and free Ponyville!  And we all know you can be brave, Fluttershy.  We all believe in you!"
That was enough pressuring.  Fluttershy popped out, and began to call birds in through the broken window.  They swarmed around her, and she sent them flying at Trixie.  Trixie began firing her magic at the birds instead of at Fluttershy, and Twilight took that as the opportunity it was, and fired arrows at her.  But there was some kind of shield around Trixie that kept the arrows from touching her.  And it wasn't long before the birds were re-corrupted and flew back at Twilight, either.
Fluttershy popped out, and before the birds could injure Twilight, their corruption faded, and they circled Fluttershy again.  She sent them back at Trixie to keep her distracted, and tried to think of something that would break through that shield.
Applejack shouted, "Let me at her!  If magic won't work, let's see how hooves work!"  Applejack popped out and charged at the distracted Trixie, giving her a solid kick to the shoulder.  Twilight popped out and shot her with arrows at point-blank range.  Trixie unleashed a burst of magic that sent Twilight  skidding across the room, and began firing more and more magic at them---different spells from before.  It was as if she were learning from her mistakes, this time, the thought of which made Twilight shudder inside:  it would be more difficult to cure Trixie than it was to cure any of the other ponies.
"Let me at her; I have an idea!" Rainbow shouted.  She popped out and tried to reflect Trixie's magic with another tornado, but Trixie spelled her tornado to fly back at her.  But Rainbow was clever, and while Trixie was spelling the tornado, she flew over the tornado and was right above Trixie.  "NOW, Applejack!" Rainbow shouted, and Applejack popped out as they were dropping right on top of Trixie.  Applejack kicked her again before she could fire off another spell, and Twilight popped out and shot as many arrows as she could at Trixie.  It was enough to cure her, but only just barely.
Trixie stared at Twilight for a moment---Twilight was exhausted and injured from the fight, and she couldn't quite think what to say to Trixie.  In fact, she was feeling very overwhelmed, and finally getting a chance to catch her breath practically made everything else in her mind shut down.  She could hardly think or feel, let alone speak.  Trixie didn't say a word; she merely fled.  Twilight didn't try to stop her.
After a few seconds, Twilight shook off her exhaustion and grimly coughed, "Let's destroy this core before anything else happens."
Celestia's spirit-orb glowed and filled the whole town hall with blinding light.  When the light faded, all five of Twilight's friends were standing beside her.  Celestia herself was standing there, too, with a saddle bag over her back.  She used her horn magic to open the bag, and the Elements of Harmony were inside.  She quickly slipped the necklaces on each pony's neck, and the magic tiara onto Twilight's head.  "Quickly, before reinforcements arrive," Celestia commanded.
They wasted no time; the energy of the elements flowed through them, and a wave of rainbow-colored magic flew at the core and made it collapse and vanish.  The crystals that glued it to the ground stopped their unnatural movements and clattered on the ground.
Rarity walked up to one of the crytsals and looked at it.  "These are just ordinary gems!  Twilight, what kind of magic would leave these behind?"
Twilight was still staring off into space, still overwhelmed, exhausted, and battered, not paying attention to anything that was said.
Celestia broke the silence:  "Twilight, you, Applejack, and Rainbow Dash are injured.  Touch those gems to heal yourselves, quickly!"
Rainbow Dash and Applejack immediately collected a few heart-shaped gems, and their bruises were gone in an instant.  But Twilight didn't move.  "Twilight, what are you waiting for?"
"She must be really torn-up, poor thing," Rarity thought aloud.  She used her horn magic to pick up several gems and touched them to Twilight's wounds.  The crystals immediately disappeared, and took Twilight's wounds with them.  Twilight shook herself a little as she realized she was feeling better, and tried to come to her senses.
"Are you alright, Twilight?" Celestia asked.
Twilight's thoughts were horribly fuzzy; she still felt overwhelmed from the fight, and from having to push thought after thought out of her head all throughout her journey through Ponyville.  How could she even put into words what she was feeling?  Would her friends and her mentor understand?  ...She had to say something in any case.  She willed her voice to answer, hoping that she'd be able to explain some of her stress, but all that came out was, "Y-yeah, I'm alright, I'm just...tired...from the fight..."
"Is it safe for us to take a rest, now, yer Majesty?" Applejack asked.  "I mean, if you're right, then all of Ponyville is safe, now, right?"
Celestia shook her head.  "There are more of these cores, throughout Equestria, and one of them is dangerously close to Ponyville.  I can sense its general location; it's in Everfree Forest."
The very name of that forest wiped the smiles off of all the ponies' faces and replaced them with frightened and worried expressions:  Everfree Forest wasn't a natural place at all.  Even if the clouds and animals and seasons didn't take care of themselves, the woods were full of dangerous beasts, and needed to be avoided at the best of times.  With those beasts corrupted, the forest would be an even more dangerous place.
There was a pause as they waited for someone to say something in response; Twilight thought to herself that they ought to head straight for the core, wherever it was, destroy it, and then get out of the woods as quickly as possible---she thought this for the briefest instant, but she felt too tired to suggest it.  And the corrupted Rainbow Dash's words cropped up again in her mind and haunted her---she really did care about her friends, right?
Applejack broke the silence and said exactly what Twilight had begun to think:  "D'we know where to look first?  I, fer one, think it best we get in an' outta there as quick as we can."
"A good place to start looking is the ruins of our old castle; even if the core isn't there, the castle should at least give us a clear view of the rest of the forest, so we might be able to figure out where to look next from there."
The Spell of Unity once again kicked in, and they were all drawn back into spirit form, except for Rainbow Dash.  Rainbow floated there for a moment, shocked that her friends had disappeared so quickly.  She blinked, then after realizing what had happened, she said, "You weren't kidding when you said we could only all be in physical form for a little while..."  Silence hung in the air for a moment, but then, without another word, Rainbow Dash bolted out the door for Everfree, not wanting to waste any more time.