The Great Leviathan also known as the "Great Beast" and known in Japan as The Orichalcos God is a gigantic serpent creature who is behind all the events in Season 4 of Yu-Gi-Oh! Waking the Dragons arc and thus is the overarching antagonist and final boss of the arc.
He was empowered by the dreaded Orichalcos magic which he is the sole master of and he employed the Orichalcos Stones to take over the world. It was these stones that made Dartz of Atlantis turn evil and use the Orichalcos Stones and cards to seek out victims to steal souls from to feed The Great Leviathan.
The souls of Gurimo, Pegasus J. Crawford, Yugi, Weevil Underwood, Rex Raptor, Alister, Valon, Jospeh Wheeler, Mai Valentine, Rafael and Seto Kaiba were among the many fed to the Orichalcos God. The final soul was that of Dartz himself after he lost his duel to Yami who avenged Kaiba's loss to Dartz and freed the chosen duelists.
Later the remaining duelists' Duel monsters battled The Great Leviathan but it was completely ineffective; The Great Leviathan sent numerous shards which easily dispatched all of them, while all the duelists were captured by him except Yami. Yami, however, converted the darkness inside people and Duel Spirits' souls into light, allowing the three Egyptian Gods to be released. It also greatly weakened The Great Leviathan, while the three Egyptian God's powers increased immensely. It required the souls and powers of millions to neutralize him; Yami finally caused him to disappear by removing all of his remaining darkness within his soul.
After The Great Leviathan's death, Dartz was released from his control and the souls of all his victims were returned during the battle.
- He was also dubbed the "Grand Dragon Leviathan" in English advertisement media.
- On the Yugioh.com site, the artwork displayed for this monster is mistakenly that of "Cave Dragon".
- It is considering to be one of the largest monsters that's ever seen in the anime, even larger all three Egyptian Gods. Dyson Sphere is one of the few larger monsters in the franchise.
- The Great Leviathan like Zorc due to goals of plunging Earth into Darkness is one of few villains to make death threats in the dub.