Dartz, the king of Atlantis, was brainwashed 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. He is by far one of the largest monsters ever seen in the any of the Yu-Gi-Oh! animes, dwarfing the Egyptian Gods, Sacred Beasts and King of the Netherworld in size. The only 2 monsters that could be said to rival him in size is "Number 9: Dyson Sphere", shown to be barely smaller than a star in its sealed form alone, and"Number 92: Heart-eartH Dragon". In order to resurrect the Leviathan, Dartz would need the souls of the three Duelists that hold the Legendary Dragons cards: Yami Yugi, Seto Kaiba, and Joey Wheeler.
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 Dragons 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 10000 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 his fellow Atlanteans alone. Also it could be due to the Leviathan being weakened from several battles (it is implied there was more than one battle during the Atlantis conflict). As Critias and Hermos aren't seen during the flashback, it is possible they were defeated prior to the battle.