The great beast Leviathan is a flying biological weapon created by the Devil-king. Sparda sealed it away with Temen-ni-gru, but it was released by Vergil and Arkham. It's body is a gateway to the Hell of Envy, it was brought back when Temen-ni-gru rose by Mundus's hand.
This is a gateway between the Human and Demon world. You can reach several other plains of Hell through it.
