Who/What is Leviathan

Leviathan (/lɪˈvaɪ.əθən/; Hebrew: לִוְיָתָן, Livyatan) is a creature with the form of a sea serpent from Jewish belief, referenced in the Hebrew Bible in the Book of Job, Psalms, the Book of Isaiah, the Book of Amos; it is also mentioned in the apocryphal First Book of Enoch.

The Leviathan of the Book of Job is a reflection of the older Canaanite Lotan, a primeval monster defeated by the god Baal Hadad. Parallels to the role of Mesopotamian Tiamat defeated by Marduk have long been drawn in comparative mythology, as have been wider comparisons to dragon and world serpent narratives such as Indra slaying Vrtra or Thor slaying Jörmungandr,but Leviathan already figures in the Hebrew Bible as a metaphor for a powerful enemy, notably Babylon (Isaiah 27:1), and some 19th century scholars have pragmatically interpreted it as referring to large aquatic creatures, such as the crocodile. The word later came to be used as a term for "great whale", as well as for sea monsters in general. 

Also our massive friend has been featured in many famous pieces of literature ,such as Lovecraft's Novel "Call Of Cthulhu" where Leviathan is the main antagonist. It is a Star-Spawn worshipped by the inhabitants of Darkwater Island since the 14th century. It was mistaken for a giant whale and slain by the crew of the Scylla, becoming known as the "Miraculous Catch". Consuming its eldritch flesh enabled it to curse the town's inhabitants from beyond death, gathering those who succumbed to its will into a cult dedicated to unleashing the Great Old One Cthulhu. And is known as one of Cthulhu's many offspring.