In Jewish and Christian mythology, the sea monster often considered a demon of the abyss is Leviathan.
Leviathan is a gigantic sea serpent or dragon described in the Hebrew Bible, especially in Job, Psalms, and Isaiah. In these texts it represents overwhelming power and the untamed forces of the sea. Later Jewish traditions portray it as a cosmic monster dwelling in the deep, sometimes paired with the land monster Behemoth.
Christian interpreters increasingly connected Leviathan with the devil, chaos, and the deadly sin of envy. That identification is an interpretation rather than a universal definition: the biblical creature is primarily a sea monster, while later theology and demonology gave it a more explicitly infernal role.
The figure has older Near Eastern parallels, particularly the Ugaritic serpent Lotan and other chaos monsters defeated by a creator or storm deity. Leviathan is also sometimes interpreted naturalistically as a crocodile or another large aquatic animal. It should not be confused with the Kraken, a later Scandinavian sea monster, or with Behemoth, its traditional terrestrial counterpart.