Lucifer's possession came as swift and overwhelming as Ungoliant's invasion of Valinor millennia ago. Sam felt a shocked recognition, then deep malice coming from him. "Ah, so you call yourself Sam now. But I remember you... Maedhros Fëanorion. You will not escape me so easily. There is no Fingon to come help you this time. But let it not be said that I do not pay my debts. Always your deeds have aided me, unwitting or no. Your reward will be to watch as I rule."
Sam did his best to hide his disgust, shame and (he would not admit it even to himself) doubt. It would not help matters. He knew the final battle would begin very soon and he did not know if he would have the strength necessary to wrest control back.
The hours passed with endless tiresome gloating from Lucifer about how he would defeat the Valar as well as mankind, but finally the time came. Lucifer teleported to a field in Kansas where he knew Michael was. Sam watched as Michael and Lucifer talked. He watched the Impala pull up and Dean, Cas and Bobby step out. He watched as Cas banished Michael with the Molotov of holy oil.
But when Lucifer turned to his Team Free Will, as they had affectionately named themselves, he could not just watch. His struggles grew louder and stronger as Castiel and Bobby were killed. And when Lucifer attacked his brother, memories of all their times together in both lives ran together until they became a continuous stream. He could not allow this to happen. He opened his eyes and they were his own again, soft instead of burning. He looked at Dean for the last time.
The portal to the Cage opened. Michael appeared and tried to stop him. But before he could, Sam let himself fall. Michael grabbed him desperately but all he did was drag himself down as well. A flash. The portal closed as if it had never existed.
Dean will not speak of these events, so I* do so. What happened in the aftermath? No one but One knows why Castiel and Bobby were sent back to life, but they were. Castiel went back to Heaven to sort out the chaos. Bobby kept hunting. That was the life he knew.
And Dean? Dean had promised Sam that he would live a normal life and not spend his life in the search for ways to bring him back. He knocked on Lisa's door. Nothing was said. She took one look at his face and let him enter.
THE END
*Author's note: The bottom part of the chapter is narrated by Chuck Shurley.