The name Lucifer sent another jolt of adrenaline through Sam's system. But he wasn't entirely awake, he couldn't be, because he couldn't make sense of what Dean was saying. Lucifer was here - he knew that, but no, it was the other one. Theirs. The one that wanted... Cas, apparently.
"Why would he possess Cas?" he said. "Why would-"
Even as he said it aloud, the pieces came together. The Darkness, and the Mark, which had once been Lucifer's responsibility before it was given to Cain, and then to Dean. The Darkness, which they'd set loose. And then - what? Cas had gone down to the cage to let Lucifer out?
He lifted his hands and ran them over his face. "Shit."