"27 In that day the Lord with his sore and great and strong sword shall punish Leviathan the piercing serpent, even Leviathan that crooked serpent; and he shall slay the dragon that is in the sea."
- Isaiah 27:1 (King James Version.)
Leviathan is an enormous Demon that is associated with the seas, and is referred to in the Tanakh and the Bible. Leviathan is also one of the Seven Princes of Hell, representing the sin Envy, and acts as the gatekeeper of Hell. Before Falling, Leviathan was a Throne in Heaven, and was partnered with Satan. She still shares her connection to Satan, though the two seems to have grown resentful of each other since the Rebellion.
Prior to the Rebellion, Leviathan was God's chief enforcer of his authority, and was known to keep an open mind for both sides of a story, a trait that would cause both er and Satan's fall. Lucifer's initial speeches intrigued her, and she suggested to Satan that the two of them consider his opinions. After hearing him speak, Satan immediately sided with him, with Leviathan following suit.
The three of them allying together made for a force that Heaven's Council could no longer ignore. As the Council began debates with the self proclaimed Princes, more and more Angels started flocking to their side. Ultimately, a third of the Heavenly Host would align themselves with Lucifer's cause, many of whom came from the warrior class. Tensions began to escalate between the two factions, culminating in the death of an Angel that was still loyal to the Council. Both sides demanded the killer come forward, but they never did. The Council accused the Princes of the killing, with the Princes blaming the Council. Debates turned into arguments, and rallies became riots, eventually resulting in the beginning of the Rebellion, that Lucifer's rebels would ultimately lose.
When Leviathan was cast into Hell, her brilliant beryl wheels melded and grew, morphing into a colossal serpent with wings that could create hurricanes with a single beat. Her massive form can be seen guarding coiled along the docks of the River Styx, watching for anything crossing the river that may mean Hell harm. Her opinion of humanity is not as scornful as her former partner, but she still looks down on them as lesser beings.