Orichalcos God

The Orichalcos God, or the Great Leviathan (4Kids dub), is a gigantic serpent creature who is behind all the events in Season 4 of Yu-Gi-Oh, Waking the Dragons, and thus is the true and final antagonist of the arc.

He was empowered by the dreaded Orichalcos magic and he employed the Orichalcos Stones to take over the world. It was these stones that made Dartz of Atlantis turn evil and use the Orichalcos Stones and cards to seek out victims to steal souls from to feed the Leviathan.

The souls of Gurimo, Maximillion Pegasus, Yugi, Insector Haga, Dinosaur Ryuzaki, Amelda, Valon, Katsuya Jonouchi, Mai Kujaku, Rafael, and Seto Kaiba were among the many fed to the Orichalcos God. The final soul was that of Dartz himself, after he lost a duel to Yami. Yami battled the Orichalcos God, and eventually used his power to destroy him, causing him to shrink down to nothing. Dartz was released from his control, and the souls of all his victims were returned during the battle.

Trivia

 * He was also dubbed the "Grand Dragon Leviathan" in English advertisement media.
 * On the Yugioh.com site, the artwork displayed for this monster is mistakenly that of "Cave Dragon".
 * It is considering to be one of the largest monster that ever seen in the anime, being larger then all 3 Egyptian Gods.