The last angels in the universe are hiding in Purgatory.

Hell is too nasty. Earth has the rebel-leader-turned-mad-god Castiel on it, and plenty of angels have died on Earth in the last five years. Heaven is a graveyard. Purgatory is bleak and irrelevant and still holds a few stray Leviathan that didn't make it out the portal, but they're easy enough to avoid, and Purgatory is the one place no one would ever look for an angel.

They don't take kindly to Castiel showing up.