Lucifer grabbed Dean at the convent instead of Sam. Sam wanted him back and figured out how to do it. Now he's the Anti-Christ with the powers of the Four Horsemen and Lucifer is defeated which means Dean's back in his own body but he's kind of not, but he's got some ideas on how to fix him. Inspired by the movie Subject Two and a Grimm fairytale about a determined sibling.
This is Supernatural post-apocalypse fiction but it's so well written and strangely compelling that I'm going to recommend it to anyone who likes to read at the twisty end of the pool. You don't even have to know much about Supernatural to enjoy this story. All you have to know is that Doc Benton was real. [Weird New England] Whether he was obsessed with immortality is up for debate but that's how he was presented in the show. Sam tried once before to convince Dean that being immortal would solve their current problem but Dean said no. I guess Sam never forgot it though.
Warning: VERY DARK (but also very, very good).
We Are Legend written by tru_faith_lost and chocca2