Future!Castiel is a version of Castiel 5 years into the future of 2009. In 2012, Lucifer's minions re-released the Croatoan demon virus - in this post-apocalyptic world, most humans have been reduced to murderous zombies. Castiel is at Camp Chitaqua with Future!Dean, Future!Castiel and other survivors.
Future!Castiel informs Dean, that sometime after the Croatoan virus was released, the Angels left, and so did most of his angelic powers so that Cas is now effectively human. He can however detect that Dean from 2009 is different from Future!Dean. He is embittered at his situation and has immersed himself in "women and decadence". When Dean first encounters him, he is organsing an orgy, and when Dean asks if he is stoned he says "Generally - yeah".
He is presumably killed during the attack on Lucifer.