"Out of the way, small fry," Dean roughly pushed Cas aside.
For a moment Cas wasn't sure what he should do. He wanted to hold Dean and fix whatever it was that was inside of him. On the other hand, as Dean passed, the aura he gave off was most unlike his. It was almost like when Sam had Lucifer infiltrating his head. Of course, it had been Dean's body that had pushed Cas aside, but Sean wasn't fully there. Cas had felt Sean's soul pulsing like another heartbeat inside of his body. His soul was weak though. His body and his mind where overrun by evil. Cas guess that it was the Mark of Cain that had done this.
Dean was still in there somewhere, Cas knew, but the Mark was taking over. Cas was relieve that Dean was still alive, but he thought it was a selfish happiness that he had. At what cost did Dean live?
"Dean!" Sam yelled from the next room over. "You're alive!" Sam started stuttering with glee. The things that Crowley had said made it seem like Dean would never be back, but there he was, right in front of Sam. He hardly knew what to do, much less knew what to say.
"Sorry Sammy, the Dean you knew is gone. He's mine now." Demon Dean growled.
Sam instantly seemed defeated. Sam slumped down back into his chair. He wasted to embrace Dean and keep him stuck on the spot, so he couldn't leave, but his energy was drained. He was empty. "Dean..." he said feebly.
The demon's eyes flickered to green. Dean seemed to be trying to fight the demon he had become. The beast intertwined with him was too strong. Dean's eyes went back to black. Dean scowled, "Goodbye brother," he turned to Cas, "Angel."
Dean made his way out of the bunker and disappeared in a puff of black smoke.
"How do we save him?" Sam asked Cas desperately.
Cas looked down at his shoes, avoiding Sam's depressed look. "I don't know," he said softly.