I’d rather spend an eternity in Hell with you than a day in Heaven without you.
Season 10 - Twisted 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. They rule Hell together and wreak havoc on the people they once worked so hard to protect.