For fans of the TV Show Supernatural they will recognise Leviathans as the rulers of Purgatory. Seen as an ancient race of primordial monsters that served as God's first beasts they are hungry and destructive before they were released from Purgatory during the shows sixth season. But in (mythological) reality, Leviathans aren't just a small, powerful beast but in reality a keeper of Hell itself.
Leviathan is often referenced in the Jewish faith in the Hebrew Bible in Psalm, Isaiah, and the Book of Job as one of the Seven Princes of Hell and one of its gatekeepers of the Hellmouth which represents the gates of Hell itself. The Hellmouth is actually often envisioned as Leviathan's mouth itself and its mouth filled with the damned appears in many forms of Anglo-Saxon art from about 800AD.
The demon of darkness and chaos himself, Leviathan was once an angel who joined Lucifer's rebellion and turned into the form of a sea serpent, in which it is often depicted. In later Judiasm reference, Leviathan is also depicted as a dragon, or as a rather large whale, and has come to be the go-to term to reference any kind of sea monster, including Hermen Melville's Moby Dick and even through interpretations of Cthulhu and the Kraken.
Leviathan represents the element of Water and the direction of the west and is listed in the demonic book The Book of the Sacred Magic of Abra-Melin the Mage as one of the princes of Hell. The Church of Satan uses the Hebrew letters at each of the points of the Sigil of Baphomet to represent Leviathan in their infamous statue, further approving to Leviathan's place in the demonic kingdom.