Supernatural - Season 05 - Episode 04 - The End

Sam tells Dean he wants to rejoin with him in the battle of the Apocalypse, but Dean tells Sam that they are better off apart. Later, Dean awakens five years in the future in an abandoned city and is attacked by humans who have been infected with a demonic virus that turns humans into Zombies. Zachariah (guest star KURT FULLER) appears to Dean and explains that this is the world that exists as a result of Dean saying no to helping the angels fight Lucifer. Dean meets up with the Future Dean, who tells him that the virus is the Devil's endgame for destroying mankind.