So I read this story a couple of weeks ago, closed my browser window and now I can't find it again. I can't remember the title, and I believe it was on FF.net. The story took place when Dead was still a demon. He beat Sam up, and then he killed Cain instead of Sam, or maybe he killed Death. I think towards the end Castiel is there. And by killing which ever one he did, it took away the mark of cain.