Leviathan

Leviathan is the enemy of Poseidon, and the demon of the seas, for he alone guards the entrance to Poseidon's kingdom, Atlantis. He can change himself into a terrible sea serpent and back again, plus he can use electricity like a morray eel. He is the most envious of his allies.

Origins
Leviathan, and Poseidon were once the best of friends in the seven seas. One, day during the battle of light and darkness Leviathan fought Poseidon for Control over the seas, and lost, now he is one of Lucifers Dark Kings. He plans to one day get his revenge on Poseidon for taking the seas away from him.

Trivia

 * Leviathan was once known as Leroy
 * Leroy swore revenge to Poseidon and became the Leviathan.