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.
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.
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 extermination 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 Duellists 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 and calling out 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 itself. Yami Yugi greatly powered his Egyptian Gods through the light of the captured souls, which proved to be the Leviathan's weakness. Hence, they were able to neutralize the creature, for the light that fuelled 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. He did this by using his own light representing by the vessel of memory container to destroy all of the hatred and resentment from the world that powered the monster and vanquish it for good. In the Japanese version, Dark Magician Girl mentions that the creature was sent by her to live in the Duel Monsters world, now that the curse of the Orichalcos has now been extinguished. Atlantis then returned to the bottom of the sea.
The Great Leviathan is an extremely ancient and powerful entity, and it is the one responsible for the creation of Orichalcos Stones, which in turn will grant wielders immense knowledge at the expense of emotional intelligence. Hence, it is responsible for the Atlanteans to completely degenerate emotionally, and corrupt their personalities, which can be reflected as when the citizens turned into hideous monsters. It is also capable of removing souls from both humans and Duel Spirits on a global scale, and absorbing them physically to absorb their souls using tentacles from its substance.
Much like Zorc Necrophades, the Great Leviathan is said to be born through the hatred and rage of humanity, and it has even been considered to not be related to the Duel Spirits, such as the Egyptian Gods (in the Japanese version, it would later be spirited to the world of Duel Spirits). It is capable of usurping control of the Egyptian God cards as well as draining the power away from them, similar to how Zorc created a solar eclipse to do so. His powers stem from the darkness of human souls and captured souls themselves.
Via this source, the Great Leviathan is shown to have immense offensive and defensive powers during the final battle. His mere presence is capable of creating a storm of global proportions. The Great Leviathan had singlehandedly and simultaneously defeated up to twelve powerful Duel Monsters fighting together, including Blue-Eyes Ultimate Dragon and all three Legendary Knights, while remaining unharmed from all their attacks. It is capable blasting numerous shards that can defeat extremely powerful of the Duel Monsters in a single hit.
The Great Leviathan's body can be subdued or destroyed, but its spirit cannot be defeated conventionally; its physical body was defeated through Yami Yugi's conversion of the 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 good memories and friendship.
- 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 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, it is not related to the Duel Monsters.