Leviathans

Leviathans are beasts created by God long before the creation of angels and humans. As they were too much to handle, God created Purgatory and locked them therein.

Background
When Castiel cracked open Purgatory and imbibed the souls found in there, he also unknowingly absorbed the Leviathans. After some time, they began manifesting in him and asked him to let them out. Due to their uncontainable power, Castiel's vessel began deteriorating, until he could no longer hold it together. Coming to Sam and Dean for help, he, with the help of Dean and Bobby, opened the door to Purgatory again and hurled the souls back in. Shortly after Castiel had got rid of the souls and his body was repaired, he realized the Leviathans were still inside him. Unable to hold up against them, he finally succumbed and the Leviathans surfaced, telling Dean that Castiel was now "gone". Relishing their new-found freedom, they inform Dean that now they are going to have fun.

Nature
According to Death, the Leviathans were God's earliest, if not his first, creations. Unfortunately the beasts proved to be too powerful and destructive and God was concerned they would destroy everything around them so he sealed them away in Purgatory. Death claims that the Leviathans are "clever and poisonous".

Trivia

 * They will presumably be the main antagonist(s) of Season 7.
 * They are possibly based upon the biblical sea monster mentioned in Job 41.
 * They are played by Misha Collins who plays Jimmy Novak and Castiel.
 * The Leviathans are unusual in the Supernatural universe in that they are the only creatures to possess another creature which is using a vessel, because they possess Castiel when he is possessing Jimmy Novak.
 * They were referred to as "The Old Ones" by Death. This may be a reference to the ancient creatures described by H.P. Lovecraft, who had previously created a portal to Purgatory, where the Leviathans were once imprisoned.

Appearances

 * Season 7
 * Meet The New Boss