The mark of Cain drove Dean into isolation. While he was grappling with guilt, anger and depression, Castiel showed up on his doorstep. But the peace he brought couldn't last. He was followed by Crowley, who is desperate to have his personal killing machine back.
After talking to Dean proved... ineffective, he tried a different tactic and after Dean and Castiel had a fight and he got a few drinks in Dean, it paid off. But before Dean can get his hands on the blade again, Castiel takes it and organised a welcome party of angels and his brother.
Castiel meets Dean in the trees alone and after a scene that was very similar to the scene that broke Naomi's hold on Cas, the two finally admit the feelings they've had since the first moment they laid eyes on each other.
Dean then forced the mark onto Crowley and Hannah took him to Heaven's lock up where he is to remain for the rest of time.
At least he has Metatron for company...
What could possibly go wrong?