The Great Leviathan, or the Orichalcos God in the Japanese version (also dubbed the Grand Dragon Leviathan in English advertisement media) is the deity of the Orichalcos and the main antagonist of the "Waking the Dragons" story arc.
Dartz, the king of Atlantis, was brainwashed and manipulated by the Orichalcos Stone and told to resurrect their god, the Great Leviathan, using human souls. It was defeated by the three Legendary Dragons, though Atlantis was sunk in the process. Like Zorc Necrophades, the Leviathan is pure evil and wants nothing more than the destruction of all life on Earth. In order to resurrect the Leviathan, Dartz would need the souls of the three Duelists that wield the Legendary Dragons cards: Yami Yugi, Seto Kaiba, and Joey Wheeler.
As more souls were collected, the Leviathan became more and more active, especially as the souls of the Chosen Duelists were collected. After the capture of Joey Wheeler's soul, a manifestation of the Leviathan began to appear in the sky across the world, and by the time Seto Kaiba's soul was captured, it was conscious enough to roar from behind its astral prison.
Ultimately, Dartz gave up his own soul to resurrect the Great Leviathan after he was unable to acquire Yami Yugi's soul (saying that his soul was one of "equal" value). Although it overwhelmed the Legendary Knights and an army of monster spirits, Yami managed to draw in the light in the souls the Leviathan absorbed and used that light to release the three Egyptian Gods. In a battle in the skies over the ocean, Yami Yugi and his Egyptian Gods faced off against the great beast. While the Leviathan was seemingly stronger than even the Gods, they were able to destroy the creature, for the light that fueled their power was greater than the darkness that coursed through the Great Leviathan.
Later, Yami Yugi was able to release Dartz from the Leviathan's malignant influence. Atlantis then returned to the bottom of the sea.
It should be noted in the flashback of the Battle of Atlantis, the Great Leviathan appears to be smaller than it was when it was revived 10,000 years later. If so, it could be due to not having as many souls to empower it the first time, as Dartz states he had collected millions of souls by the time of the Leviathan's resuscitation and could not have been able to claim as many from the Atlantean populace alone. Also it could be due to the Leviathan being weakened from the prolonged hostilities (it is noted there was more than one battle during the Atlantis conflict). As Critias and Hermos are not present during the flashback, it is assumed they were defeated and sealed prior to the final battle.
- On the Yugioh.com site, the artwork displayed for this monster is mistakenly that of "Cave Dragon".
- It is considered to be one of the largest monsters that have ever been seen in the anime, being larger than all three Egyptian Gods.
- However, according to the English dub, it is not related to the Duel Monsters.