The Great Leviathan, or the Orichalcos God in the Japanese Version (also dubbed the Grand Dragon Leviathan in English advertisement media) was the deity of the Orichalcos and the main antagonist of the Waking the Dragons arc.
In the flashback of the Battle of Atlantis, the Great Leviathan appeared to be smaller than it was when it was revived 10,000 years later. If so, it could have been due to not having as many souls to empower it the first time, as Dartz stated that 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 was noted that there was more than one battle during the Atlantis conflict). As Critias and Hermos were not present during the flashback, it was assumed that they were defeated and sealed prior to the final battle.
Dartz, the king of Atlantis, was brainwashed and manipulated by the Orichalcos Stone and told to resurrect their god, the Great Leviathan, by using human souls. It was defeated by the three Legendary Dragons, though Atlantis was sunk in the process. Like Zorc Necrophades, the Great Leviathan was pure evil and wanted nothing more than the extermination of all life on Earth. In order to resurrect the Leviathan, Dartz needed the souls of the three Duelists who wielded 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's soul, a manifestation of the Leviathan began to appear in the sky across the world, and by the time that Kaiba's soul was captured, it was conscious enough to roar from behind its astral prison.
Ultimately, Dartz gave up his own soul in order to resurrect the Leviathan after he was unable to acquire Yami Yugi's soul (Dartz said 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 that the Leviathan absorbed, used that light, released, and called out the three Egyptian Gods. In a battle in the skies over the ocean, Yami Yugi and the Egyptian Gods faced off against the Leviathan. While the Leviathan was seemingly stronger than even the Gods itself, Yami Yugi greatly powered the Egyptian Gods through the light of the captured souls, which proved to be the Leviathan's weakness. Hence, they were able to neutralize the Leviathan for the light that fuelled their power was greater than the darkness that coursed through the Leviathan.
Later, Yami Yugi was able to release Dartz from the Leviathan's malignant influence: He did that by using his own light that was represented by the vessel of a memory container to destroy all of the hatred and resentment from the world that powered the Leviathan and vanquished it for good. In the Japanese Version, Dark Magician Girl mentioned that the Leviathan was sent by her to live in the Duel Monsters Spirit World, when the curse of the Orichalcos was extinguished. Atlantis then returned to the bottom of the sea.
The Great Leviathan was an extremely ancient and powerful entity, and was the one responsible for the creation of the Orichalcos Stones, which in turn, granted wielders immense knowledge at the expense of emotional intelligence. Hence, it was responsible for the Atlanteans to completely degenerate emotionally, and corrupted their personalities, which could be reflected as when the citizens turned into hideous monsters. It was also capable of removing souls from both humans and Duel Spirits on a global scale, and absorbed them physically in order to absorb their souls and used tentacles from its substance.
Much like Zorc, the Leviathan was said to have been born through the hatred and rage of humanity, and it was even considered to not be related to the Duel Spirits, such as the Egyptian Gods (In the Japanese Version, it was later spirited to the Duel Monsters Spirit World). It was capable of usurping control of the Egyptian God Cards as well as drained the power away from them, similar to how Zorc created a solar. The Leviathan's powers stemmed from the darkness of human souls and captured souls themselves.
Via that source, the Great Leviathan was shown to have immense offensive and defensive powers during the final battle. His mere presence was capable of creating a storm of global proportions. The Great Leviathan had singlehandedly and simultaneously defeated up to twelve powerful Duel Monsters who fought together, including Blue-Eyes Ultimate Dragon and all three Legendary Knights, while it remained unharmed from all of their attacks. It was capable of blasting numerous shards that could defeat extremely powerful Duel Monsters in a single hit.
The Great Leviathan's body could be subdued or destroyed, but its spirit could not be defeated conventionally; its physical body was defeated through Yami Yugi's conversion of humanity's darkness into light which was channeled to the attacks of the Egyptian Gods. It was finally destroyed when Yami Yugi vanquished the last of his negative emotions through his good memories and friendships.
- On the Yugioh.com site, the artwork displayed for that monster was mistakenly that of "Cave Dragon".
- It was considered to be one of the largest monsters that were ever been seen in the anime and was far larger than all three Egyptian Gods combined. It was the largest seen until the appearance of " Dyson Sphere".
- However, according to the English dub, the Leviathan was not related to Duel Monsters.