The Leviathan is an ancient sea monster from biblical times, usually portrayed as a giant whale or a sea serpent. It is the counterpart to the Behemoth, which is a giant land-based animal.
It has scales "packed so tightly together that no air can come between them", and its skin is indestructible. It spits fire and sparks from its mouth, and smoke comes from its nostrils. Its eyes shine bright like the sun; and it causes the sea to boil, leaving a foamy white trail in its wake. Some Christians believe its mouth to be the gate to Hell.
In Jewish legend, the Leviathan and Behemoth will have an epic battle at the end of the world, where they will kill each other and be eaten by the surviving people.