In season seven, the events take place right after the angel Castiel absorbed the souls of Purgatory declares himself as the new God. And the "mutated angel" goes around the planet to try and finish his "work". But eventually, thanks to a little bit of help from the Horseman Death, Winchesters convince the angel to return all the souls back to Purgatory. But Castiel manages to return all but the souls of some older creatures. Something in which the Winchesters will find themselves in an increasingly sinister, changing landscape, as they go up against this new foe, which is unlike anything they've ever fought before: the Leviathans, the very first beasts created by God Himself, but were then locked away in Purgatory for being too destructive. They brothers will find their old tricks, weapons and hiding places all rendered useless. All they'll have is each other. And the certainty that, like the last of the cowboy outlaws, whatever they face, they're not going down without a fight.