Leviathan (Supernatural)

The Leviathan (also referred to as "Leviathans") is the main antagonist of the seventh season of Supernatural.

In the show's universe, the Leviathan was created by God long before He created angels and humans. However, God became concerned that the Leviathan would "chop the entire petri dish" (as Death put it while he told the Winchesters and Bobby Singer about the Leviathan), so He locked them all away in Purgatory, which He created for the express purpose of imprisoning the Leviathan.

Thousands of years later, the angel Castiel opened Purgatory and absorbed all the souls that resided there to increase his power and become the new God; by doing this, he also absorbed the Leviathan, which began taking control of Castiel's body. Castiel, with the help of the Winchesters and Bobby, returned all the souls he absorbed back to Purgatory; unfortunately, some of the Leviathans managed to remain inside him and quickly seized control of Castiel's vessel. When Castiel's body was unable to contain them for long, the Leviathans made their way to a reservoir, where they left Castiel's body and proceeded to possess numerous people.

In the episode "Out with the Old," a Leviathan named George stated to Dean and Sam that they were "here to help" and intended to develop a cure for cancer. In "The Girl with the Dungeons & Dragons Tattoo," Bobby confirmed that the Leviathan was setting out to cure cancer and other illnesses, but only to turn humans into the perfect food.

Following Dick Roman's death, Crowley commented that the Leviathan is now just another monster, albeit a hard-to-kill one.