You’re cursed as human for a lifetime after being a god/goddess. You still posses powers but just human and mortal. You awake in a forest, the deeper you walk you accidentally step into a road, in which Dean runs you over.
You’re alone in the bunker with him after mysteriously appearing in the bunker. You’re a Goddess/God. During the Heaven/Hell war. You’re strangers to everyone in the bunker.
The Winchesters including castiel gave up on finding solutions to getting rid of the mark of Cain. You are their last hope, a demon. One who is said to posses the ability to get rid of it.