Leviathans

Leviathans aka the Old Ones, are beings that pre-date humans, monsters, angels and even souls. They were among the first, if not the original creations of God and were creatures of pure chaos. Death describes them as immensely powerful and hungry. Before God went on to create more creatures, however, the Leviathans threatened to consume and destroy everything "in the petri dish". For this reason God created Purgatory and sealed the Leviathans inside to contain their hunger.

7.01 Meet The New Boss

Whilst absorbing all the souls from Purgatory, Castiel also unwittingly took in the Leviathans and added their power to his own. However, he was unable to contain them and the souls for long and had no choice but to return the souls to Purgatory with the assistance of Dean, Sam and Bobby. But the Leviathans remained behind and, without the power of the souls to hold them back, assumed control of Castiel's vessel; declaring the angel dead and attacking Dean and Bobby with manic delight, remarking: "Oh this is going to be so much fun."