It took two more weeks of intense negotiation, tracking demon omens, and liberal use of Jesse’s puppy eyes and pleading face before Sam finally caved and figured out a plan to combat Dean that involved all four teenagers.
It failed rather spectacularly when they met up with Dean, with Jacob unconscious, Claire bleeding, and Ben trying to shield them both, with dead and mutilated bodies all around.
Dean raised the First Blade against the boy he would have once called son and it was only Jesse and Sam screaming “Dean STOP!" that froze him in his tracks.
His muscles tensed for a moment, as if he was trying to move but couldn't, and then he snarled and disappeared.
No one was sure if it was Sam or Jesse who actually got through to him.
It took another two weeks of regrouping, burying themselves in lore, and Sam honing their ability to watch each others’ backs before they decided to track down Cain himself.
Jesse proved why he was the Antichrist that Heaven feared by turning Cain’s blood to holy water with a blink of his eyes and smiled when he finally starts screaming as Jesse sets it to boil.
Sam curbed Jesse’s bloodthirst with touch of his hand on his shoulder and reached into five thousand years of knowledge gained from archangels he didn't want to remember - but he would, for Dean, there was no suffering he would not endure for Dean - and an hour later, they were leaving with an arsenal of new spells and sigils and plan.
Jesse spent the next week unable to sleep unless Sam held him, soothing him and murmuring lowly into his hair that he wasn't a monster, he wasn't, that his choices were good, that he was good.
The next time they met Dean, Jacob and Ben set on him with wild abandon, all violence and years of fury and hatred and grief coming to the fore. Ben shot him three times with Devil’s trap bullets, and Jacob left long poisoned gashes up and down his back.
Bastard had an angel on call, after all, he’d be fine.
They kept Dean distracted long enough for Claire, Sam, and Jesse to lay down sigils and start up a ritual. Ben and Jacob forced Dean to his knees as Jesse gripped the Mark of Cain tightly with one hand and the wrest the First Blade from Dean with the other.
Claire chanted a spell in Latin while Sam chanted in a language only archangels knew, one no human was meant to speak that left his throat bloody and raw. He felt his energy and traces of Lucifer’s grace fill the air and tried not to let bile join the mix.
Jesse reached for the demonic taint of the Mark of Cain and pulled, forcing all the energy corrupting Dean’s soul into the First Blade itself. It was wild and powerful and bucked at his psychic touch, but he was a cambion, the fucking Antichrist, and Sam was counting on him.
The First Blade glowed, shuddering in Jesse’s grasp and disintegrated into ash. Half a country away, Cain closed his eyes for the last time.
Dean dropped unconscious at Ben and Jacob’s feet.
Sam coughed up blood and sank to his knees, smiling tiredly at his teens, strong and victorious and brilliant.
And when Dean woke up a day later, fully healed, confused, disoriented, and still vaguely furious, Sam just laughed in relief.