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Dean waits for his brother by the old railroad trestle. After returning from the hellscape of his possible future, he'd had time to think. To mull over his experience. To make choices. He'd called Sam a week later saying that if he wanted back in, to fight beside Dean, then they should meet and discuss it in person. He'd prayed to Cas after that, and asked him to ward their meeting place from both Heaven and Hell. This conversation was not something that should be overheard by either.
Sam pulls up in an older car, parking parallel to the Impala. He gets out, and strolls nervously over to where Dean leans on Baby, hands in his pockets. He nods at his brother.
"Dean. " Dean returns the nod. Tears fill his verdant eyes.
"Sammy. I'm sorry." Sam looks confused at Deans apology. He steps closer to the black muscle car, and goes to reach for Dean.
"Sorry for wh-" Dean straightens and draws the Colt, retrieved from his future, out of his pocket. Before Sam has time to react, he levels it and shoots Sam between the eyes. Sam slumps to the ground, dead.
"We can't end up...there." Dean whispers. He lets the Colt fall from his grip. Once again he prays to Castiel. In a rush of wings the angel appears. His eyes go wide in horror when he sees Sam's body, and the smoking Colt on the ground at Deans feet. He turns on the elder Winchester, eyes now narrowed.
"What have you done? " He growls, angrily.
" What I had to do, as much as it hurt." He sighs and continues. "In the time that Zachariah zapped me to, I saw my future self try to kill Lucifer with the Colt. Lucifer just shrugged it off like it was nothing. Apparently it doesn't work on Archangels, even fallen ones. I retrieved the gun before I confronted Lucifer, and brought it back with me, hoping I didn't need to use it. But there was no other way. Sam was going to say yes, eventually. I used the Colt to keep that from happening. It may not work on the Devil, but it sure as fuck works on his True Vessel. I've saved humanity. I've stopped the Apocalypse. Now help me clean this up."
Cas nods, now realizing why Dean had wanted the area so heavily warded. Had they sensed Deans plan, both Heaven and Hell would have tried to stop him. He was right, of course. This was the only way to keep Sam from saying yes to Lucifer. To halt Heavens plan for mankind. Only...
He quickly turns Sam's body to ash, putting it beyond Heavens reach forever. He sends the stolen car back to its owner, then returns Deans side. He sighs,
"You're right, Dean. Without his True Vessel, Lucifer loses the war. This was difficult, but necessary." He pauses, turning to Dean. "So is this." Dean looks to the angel, puzzled. With a palm to Deans forehead, Cas smites him to ash, as well. The Winchesters are now two piles of dust that will remain in this place for as long as Earth continues to exist.
"The thing is, Dean, you would have given in too and Michaels plans for humanity are just as bad." He whispers. Cas picks up the Colt, the most dangerous weapon in existence, and sends it to the Void. No hand can return it, not even his own. After shielding the area from man's eyes as well, he vanishes, leaving the muscle car to rust where she sits, in a place where Heaven and Hell can never tread again.
The world is safe from Heavens machinations. At least for a while.