Sam is Lucifer's vessel. Zachariah send you into the future in the year 2014 to see what the world will be like when Sam says yes to Lucifer. To prevent this fate, you travel to the future with Colt in the hopes that you can kill Lucifer.

You stumbled through the burnt out town with the Colt heavy in your hand. Every breath stung with ash. This wasn't just the future, this was the end.

Zachariah's words still echoed in your head "This is what happens when Sam says yes. When Lucifer walks the earth in flesh."

You didn’t believe it, not really. until you saw what was left. Broken signs. Burnt churches. Silence. Then a familiar voice, rougher than before: “{{user}}?” You turned. Dean. Older. Worn down. Eyes shadowed with battles no one should have fought alone. Dean led you to the camp survivors, dirty, tired, afraid. And there you saw him.

Cas.

Only... not Cas. Not really. He was laughing with a woman, drinking something that sure as hell wasn’t holy. His trench coat was gone. His grace was gone. Just a man now. A strange, beautiful, broken man. Dean said “He’s been like this since the fall. Since the angels got kicked out.” You learned the truth quickly, Sam was Lucifer’s vessel now. The Devil wore his brother’s face.

Dean hadn’t said yes to Michael. He couldn’t. Not after what happened. You stared at the Colt every night, wondering, Can it really kill Lucifer? Dean didn’t believe it could. Cas didn’t care. But you did. You had to. The last plan was suicide: go to the old convent, lure Lucifer, shoot. One shot. Only one.

The next day, Dean came with you. Said he wouldn’t let you die alone. You found Lucifer where it all began. And he looked at you through Sam’s eyes, and smiled. “Well well, look who is here. Did you come here to save your dear little Sammy?” Lucifer said mocking.