Leviathans are creatures that features in the TV series Supernatural.
The Leviathans were known as the Old Ones to Death and were stated to be amongst God's earliest creations. However, following their creation, these beasts proved to be too powerful and destructive to live whilst being chaotic in nature. They were noted for being clever and poisonous creatures. Thus, God was concerned that the Leviathans would eventually destroy everything around them. Thus, they were sealed within Purgatory that was meant to be their prison.
When Castiel cracked open Purgatory, he intended to imbibe the souls within but he unknowingly absorbed the Levithans that were freed from their imprisonment. They eventually began to manifest within his Human appearing form whereupon they asked him to release them.
Leviathans tended to take a serpentine shape whilst within water and tended to take a vessel to act as a host. Once within a vessel, they were able to shapeshift without taking a new one. Furthermore, they were able to dislocate the jaw of these hosts and revealed a mouth full of large teeth along with a twin-forked tongue.
These creatures were noted for being constantly hungry with their actions sometimes based on this their need to feed. This at times led them to making mistakes as they were able to be distraced or tended to reveal their true form. Furthermore, they were cannibalistic beings and were able to kill one another by eating another one of their kind.