Leviathan is either a sea monster or a demon of darkness and chaos and leader of Hell referred to in the Tanakh and the Bible.
In demonology, Leviathan is one of the Seven Princes of Hell and its gatekeeper. Leviathan as the gatekeeper is known as the Hellmouth where the mighty demon's mouth is the gates of Hell itself. He is a fallen angel that joined Lucifer in the latter's failed rebellion against God. He is also the representation of the sin, Envy.
In the Book of Job, Leviathan is the primordial sea monster created by God and is one of the power interpretations of the Chaoskampf. She is an ancient beast of the seas, originating from Jewish folklore and different versions of Christian mythology. According to Judaic lore, the Behemoth and Leviathan will do battle at the world's end and kill each other, leaving their bodies to be feasted upon by the chosen peoples who survive.
The word "leviathan" has become synonymous with any large sea monster or creature. In modern literature, it refers to great whales, and in Modern Hebrew, it means simply "whale."
One of the four new bosses included in the PSP Version, Leviathan can only be fought after beating the game once. Its design is based on the Latter Dragon Type 2 form of Zeed in Devil Summoner: Soul Hackers.
"Cheating!? Unfair!? Silence! This is MY world! If you want a fair fight, then to hell with the game! I'll crush you by force. Fair and square, just as you like!"
—Shadow Sae, after transforming into Leviathan
Leviathan appears as the Shadow form of Sae Niijima to be fought as a boss in the Niijima's Palace. The Leviathan is this game however differs from the other variations, in that the Leviathan is a different origin design-wise. In the case for this Shadow, it follows more of the idea of the Leviathan as a political theory made by Thomas Hobbes. A evident reason to this is that the Shadow holds a sword and a gun (which is made to be the scepter in the original).