Leviathan is a fictional criminal organization in DC Comics, later revealed to be a schism of the League of Assassins under the leadership of Talia al Ghul, the daughter of Ra's al Ghul.

The organization appears in a different form in the fourth season finale and fifth season of Supergirl.

Leviathan was introduced in Batman: The Return (Jan. 2011) and was created by Grant Morrison and David Finch.

Leviathan is an organization founded by Talia al Ghul upon leaving her father, Ra's al Ghul's League of Assassins. Leviathan's liturgy is staunchly anti-capitalist and seeks to dismantle society and impose itself as the leader of a new way.

Leviathan first became known to the heroes of Earth as a terror group who kidnapped the child of a Yemeni sheik. However, Batman discovered that Leviathan was working with the sheik and was planning to attack the world with mind-controlled children and engineered metahumans.

Batman immediately connected the organization to an apocalyptic vision he had received while coming back from the dead and positioned his Batman Incorporated organization to oppose them. After some moves, Leviathan seemed to form up behind Doctor Dedalus, a Nazi master spy imprisoned on the Falkland Islands, with a predicted lifespan of months due to Alzheimer's disease. In the aftermath of Dedalus' breakout, further investigation revealed that a paper trail showed a flow of child soldiers from training camps in the central African republic of Mtamba to locations around the world for use by Leviathan in their campaign.

In Batman Incorporated's struggle against Doctor Dedalus' master plan, they struck and hit empty locations which were in fact pre-prepared traps by Leviathan. Recovering, they realized that Dedalus was a feint, designed to make them show their hand on the eve of the war. Via a telephone, the leader of Leviathan, Talia al Ghul, spoke from her new headquarters opposite the cinema on Crime Alley. Talia stated Leviathan's goal was war with Batman's followers and the destruction of civilization.

Talia took control of the League of Assassins and added its agents to the plan. The plan began by seeding mind-control chemicals in the Gotham food supply through the "Dark Tower" fast-food chain while taking control of the Brothers Grimm Syndicate, which controlled the club on Gotham's West Side. Using a combination of agents in the school system, the police, and the civil service, Leviathan worked to control Gotham's children.