Yu-Gi-Oh! Episode 147: "Legend of the Dragons"
|Leviathan, The Great|
Dartz, the king of Atlantis, was brainwashed and manipulated by the Orichalcos Stone and told to resurrect their godhead, 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, to the point that it was almost fully conscious after Seto Kaiba's soul was captured, able to let out a roar. After the capture of Joey Wheeler's soul, a manifestation of the Leviathan began to appear in the sky across the world.
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 rebirth and is unlikely to have been able to claim as many from the Atlantean populace alone. Also it could be due to the Leviathan being weakened from several battles (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 probable they were defeated prior to the battle.