5.04 "The End"

Sam tells Dean he wants to rejoin him in the battle of the Apocalypse. Dean tells Sam that they are better off apart in fighting evil on their own terms. Later, Dean awakens five years in the future in an abandoned city. There he is attacked by humans who have been infected with a demonic virus that turns humans into mindless, flesh-eating zombies. Zachariah appears to Dean and explains that this is the world that exists as a result of Dean refusing to help the angels fighting Lucifer. Dean meets up with a future Dean, who tells him that the virus is the Devil's endgame for destroying mankind.