Luke watched his father and uncle ignite their lightsabers and face each other. The deadly hum did not decrease, which meant neither one had lowered the setting. The blades were lethal.
Vader struck first from the left, but Piett swiftly parried, twisting away to let Vader past rather than try to stand against his brother. Vader turned and blocked Piett's own strike, pushing him back. The Sith wasted no time in pressed the advantage. Piett dodged and parried, backing up continually. He performed a backflip, dodging another attack, kicked off the wall, and sailed over Vader's head, landing behind him.
Luke shifted in his seat, watching eagerly.
Vader spun around to block his brother's next attack, and with that, Piett pushed him into defensive. The Admiral made use of his smaller frame, ducking under higher strikes, and proved to be faster than Vader, who was hampered by the weight of his armor and prosthetics. As Vader began using the Force to aid him in speed, Piett slipped into acrobatics, something Vader could not compensate for as easily as he could speed.
Ozone filled the air, the deadly hum of the lightsabers and their clashes rang in their ears. The Force flowed through the two combatants, and Luke was drawn in, unable to look away. A clash of Light and Dark, and for a moment he feared it would turn deadly.
A slip in concentration at a gasp from Luke at a too close for comfort block, a flash of blinding light, then fight stopped.
Vader clamped a hand over his exposed eye to shield it from the bright lights of the training room. Piett's strike had just missed his eye but had destroyed the right lens of his mask. In fact, a good chuck of the mask had been destroyed. His looked at his brother, who wore an expression of utter shock and horror at the fact that had he been any closer he would have carved right into Vader's face.
"I believe you win," the Dark Lord rumbled.
With that the trio departed, but rather than going back to his quarters, Piett led Luke after Vader and into the Sith's personal rooms. Vader disappeared into his meditation pod, and Piett removed the damaged mask from the machine, putting in one of the replacements Vader kept. Then he removed a tray from a workbench that was filled with parts to create a new mask or repair an old one.
Luke watched as his uncle selected the parts and tools he would need then return to his side, sitting down with the mask in his hand. The young Jedi flinched at the sight that met his eyes when Piett turned the mask over.
"It looks like a torture device."
"No one ever said the Emperor was a merciful man, Luke," Piett said as he began to repair the mask with an ease that said he'd done this before.
His nephew watched him work, seemingly aware of just how important and delicate the mask truly was. He worked with infinite care to repair the damage he had caused, chilled by how close he had come to seriously injuring his brother, possibly even killing him. He had drawn deep into the Force, as had Vader, and for that brief moment, their relationship had ceased. All he had been aware of was the evil presence he was fighting. As he'd made his attack, he'd snapped back to himself, remembering that he was sparring with his brother, not fighting to the death with an enemy. It had been close, far too close.
"I could have killed him."
Luke, having grown used to the quiet save for the sound of Piett working, started at his uncle's strained voice.
"Force, Luke, I nearly did kill him. For a moment I just... didn't recognize him. For a moment he was just an evil presence,... not even a person, just..."
"Evil you have to destroy," Luke finished for him.
Piett nodded, and Luke now understood even better why his uncle was white-faced. He knew the accident had rattled the Admiral, but to hear that he had been so drawn in the Force that he no longer recognized his own brother made it all the more horrifying. His uncle had reached so deep that the Darkside had, however briefly, taken hold, and because of it, Piett had almost killed Vader.
He finished the mask and lay it down, allowing himself to shudder at last.
"It was a horrible feeling, cold, dark,... vile. The Darkside. How does he do it? How does he willingly reach for that everyday? I feel sick from what I felt for only a few seconds."
Luke had no answer, and Piett wasn't expecting an answer, at least not from his nephew. He was shaken by what had happened, and he was now more aware than ever the danger of reaching deep into the Force in the presence of a Darkside user.
Vader sat in his meditation chamber, thinking. He had heard his brother's confession. He, too, was rattled by the accident, by close he had come to losing his eye or even his life. He, too, had been deep in the Force, deep in the Darkside, and it had traveled along their bond to his unprepared younger brother. This was something none of the Skywalker's would soon forget.