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.

Nature
According to Death, the Leviathans were among 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".

It is revealed that the Leviathans operate on a form of hierarchy, with several of them taking orders from a yet unknown leader. The Leader orders several Leviathans to hunt down those in their group who have been feeding indiscriminately and to deal with the Winchesters and their allies.

Though their true form is yet to be shown, they appear under the water to be serpents. In a vessel, they can dislocate the jaw and reveal a mouth full of large teeth and a twin-forked tongue.

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.

However, they are soon forced to leave Castiel's body due to the vessel being unable to handle them. So using Castiel, they head to a water reservoir and, using the water, spread to the population, taking new vessels, thus possessing multiple people. They then break into two distinct groups of those who can control their hunger and proceed to observe the human world and those who act careless in their feeding; drawing attention.

Two Leviathans meet in a park to discuss the current situation. The Leviathan posessing Edgar reveals to the Leviathan in the girl's body that he is here because the "Boss" heard about the careless feeding of some of the others and is angry that it has drawn human attention. He tells her to get the Leviathans in the area in line because their leader "hates bad news".

They realize they can't leave Sam, Dean and Bobby alone, and decide to get rid of them. This leads to them burning Bobby's house and the Leviathan possessing Edgar attacks Sam and Dean.

Powers & Abilities

 * Shapeshifting - Upon touch, they can transform into the person they are in contact with. Soon after changing, they eat the original person.
 * Regeneration
 * Impervious to silver
 * Superhuman strength
 * Immortality
 * Invulnerability
 * Possession
 * Connection to other Leviathans - This is proven when the Leviathan possesing Dr.Gaines recognized Bobby Singer telling him that he saw him througth the angel's (Castiel) eyes

Trivia

 * They are the main antagonists of Season 7.
 * They are possibly based upon the biblical sea monster mentioned in Job 41.
 * 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 while in the possession of 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
 * Hello, Cruel World