(gif set) Season 10 AU: "...in which Dean acts like a true demon. The normal demon cure does not work, and there is no cure for the Mark. This new Demon!Dean is permanent. When Castiel’s stolen grace burns out, he becomes human again with a soul. Knowing that he will eventually die and never see Dean again, he sells his soul to be with Dean in Hell, because he can’t imagine a life in Heaven without him. Eventually Castiel’s soul breaks and he becomes a Knight of Hell under Dean..."