Leviathans

 Leviathan are creatures described to be "dangerous beyond control". Death gave a description that they are older than the souls or even Purgatory itself. They are the reasons that God created Purgatory, to lock the Leviathans up. When Castiel open the door to Purgatory, he absorb the power of the Leviathans. Even Castiel couldn't control their powers as he became the new "God". Not much else is known about them