Leviathan is a creature with the form of a sea serpent from Jewish belief, referenced in the biblical 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.
Leviathan is often considered to be a demon or natural monster associated with Satan or the Devil, and held by some to be the same monster as Rahab. Leviathan can also be used as an image of Satan, endangering both God's creatures—by attempting to eat them—and God's creation—by threatening it with upheaval in the waters of Chaos.
Immortality - Death reveals that the Leviathans were God's first creations, being before angels and humans.
Shapeshifting - Upon physical contact, they can transform into the person they are in contact with. They can also use hair instead of physical contact to assume a person's identity. Soon after changing, they eat the original person. They are also able to take on the memories of not only those they possess, but those they shapeshift into.
Healing Factor - Leviathans can heal from injury quickly.
Superhuman Strength - They are stronger than guardian angels and can overpower them with ease. The Leviathan's have proven to be extremely stronger then humans as they can over power them while feeding on their flesh.
Invulnerability - So far, it hasn't been revealed what can kill a Leviathan. However they can be weakened with Borax.
Possession - They can take over any one by a touch.
Resistances - It is slightly resistant to Magic overall. It's also resistant to Ice, and greatly resistant to Fire.
Leviathan is weak only to Lightning