The Leviathan is a legendary beast of immense proportions, said to be one of the first beings created by the gods, along with the Behemoth and the Ziz. According to the oldest legends of the Secrets, the Leviathan was the first of all the beasts of the sea, created by Neptulos. Originally more than one was created, but the leviathans were too savage and destructive, leading to the other Deities killing all but one of them to stop them from decimating their fledgling creations. The same treatment was inflicted to the Behemoth and the Ziz, earning the survivors their common nickname; the "Lonely Three".
The remaining Leviathan is famous for a pact it made with Mephisto long ago, becoming the living doorway to his realm, Hades, in exchange for the ability to dream of the fellow creatures she lost. Ever since, Hades has had no actual gate or portal, and the only way to pass into it without dying is through the maw of the Leviathan.