Dartz is the main enemy of the third season of Yu-Gi-Oh!, known as the Doma arc, or in the English dub, the Waking the Dragons arc. Dartz is the head of his own company, Paradias (a pun on "Paradise"), and is the supposedly-immortal king of Atlantis.
In the Yu-Gi-Oh universe (and 10,000 years in the past), Atlantis was an island of great peace and prosperity, separated from the rest of the world, which was corrupted with darkness and greed. However, one year meteorites rained down on Atlantis, meteorites which were either formed from or contained the mysterious Orichalchos stone, which glowed bright cerulean (a greenish blue). Thanks to the Orichalchos, the Atlanteans made advancements in technology that put them thousands of years ahead of the rest of the world.
However, there was a drawback-- the Orichalchos slowly began to corrupt the people of Atlantis. They became greedy and "dark-hearted," no longer living in peace with the nature that surrounded them, and no longer thankful for the bounty bestowed upon them by the earth. People lost their imaginations and their emotions, and the Orichalchos "revealed them for their true natures," their dark hearts turning into monsters that attacked anyone freely.
This transformation hit Dartz particularly close to home when his own wife, Iona, started showing signs of a monster living within her heart. She ran screaming from Dartz, and when he caught up with her, he found a hideous beast clutching her lifeless body. The beast bore a strange resemblance to Iona (red hair, a crown, and a tattered dress). Dartz killed the beast, and later told Dark Yuugi and Seto Kaiba that the day his wife died, he realized what the true purpose of the Orichalchos was...
Dartz believed that the Orichalchos came to the Earth to test humans-- or more accurately, test their hearts. Those that couldn't stand up in the face of their own darkness became monsters, and those that could remained (supposedly) untouched. If humans succumbed to the darkness within them, they would destroy themselves.
Dartz genuinely believed that there was no hope for the Atlantean society at that point, despite the protests of his daughter Chris and his father, the former king Ironheart. Dartz claimed that he heard the voice of the planet from within the Orichalchos, and that he wouldn't try and stop the stones from corrupting more people. Dartz continues to say that the Orichalchos gave him the abilities to watch as the stone tested humans, weeding out the corrupt, dark-hearted ones, and leaving behind a humanity destined for more, destined for true peace.
He uses a chunk of Orichalchos to summon several soldiers (monster-soldiers) from the sky, which surround Chris and Ironheart, who quite vehemently oppose Dartz's views. Dartz explains that in order to summon the Orichalchos God (called The Great Beast or The Great Leviathan in the English dub) many souls are needed. The Orichalchos God is the one who will purge the planet of corrupt souls, leaving a new mission for Dartz and a fresh planet behind.
Dartz and his Orichalchos soldiers went to war with Ironheart and the remaining Atlantean refugees, who managed to summon the spirits of ancient monsters (Duel Monsters). However, neither side was victorious, and so Atlantis sunk to the bottom of the sea, while Ironheart sealed his monster army was sealed away and scattered their spirits around the world (which would explain why they showed up in Ancient Egypt 5000-7000 years ago). Ironheart later died, leaving Dartz to try and continue his mission anew, observing Earth and humanity all the while. The corruption of the Orichalcos made Dartz believe the world to be a wicked place, and that man had an inherent wickedness in his heart. The transformation the Orichalcos had inflicted on his fellow Atlanteans was, he believed, the punishment for being wicked, as the outside of man became as twisted as their inside. Dartz was driven to revive the Great Leviathan to raze the world, so that he may remake it in the image of glorious Atlantis. And to do so, Dartz required human souls. He proceeded to wander the Earth for the next 10,000 years, presumably kept alive by the power of the Orichalcos, and knew he had found one of the powerful souls he required when he encountered the Pharaoh Atem in ancient Egypt, witnessing his control of the powers of darkness and his ability to command Duel Monsters. He witnessed the Pharaoh, with Slifer the Sky Dragon pursuing Bakura and his Diabound (episode 206). It is unclear precisely when Dartz began to collect souls, but eventually, Dartz established a multinational conglomerate named Paradius, which became a well-known, yet mysterious force in the business world. But this was not the only organization Dartz had formed. He also controlled the Doma Organization (unnamed in the English version), a group formed to gather souls to feed the Great Leviathan by challenging unsuspecting players to games of Duel Monsters only to have their souls taken by The Seal of Orichalcos, a card he had suffused with the Orichalcos' power. For ten thousand years, Dartz collected the souls of his adversaries in order to fuel the return of the Great Leviathan into this world. With time, the number of souls Dartz had stolen stretched into the millions, thanks in part to his minions, his most recent being Rafael, Valon (Varon), and Alister (Amelda), a gang of Duel Monsters players called Doma's Three Swordsmen whose lives Dartz manipulated to make them feel only hatred, molding them to be ideal servants to his cause. He gave them each a small fragment of the Orichalcos, which can be used to break the Seal of Orichalcos from outside in case of emergency. Once aware of the fact that the Pharaoh had been reincarnated and his spirit revived in the present day, Dartz began his plan to take his soul by stealing the Egyptian God Cards, using their power along with human souls to revitalise the Leviathan. Subsequently, Yugi Muto, Seto Kaiba and Joey Wheeler freed and unite forces with the Legendary Dragons as the souls of both them and their friends come under threat. Events eventually converge on a duel between Yami Yugi, Kaiba, and Dartz, in which Yami Yugi claims victory by unlocking the power of the Legendary Dragons and returning them to their true forms. Kaiba, Joey and Yugi's souls are freed from the clutches of the Leviathan. However, Dartz sacrificed his own soul, with his body reduced to dust, to give Leviathan enough power to fully resurrect and raise Atlantis once more. In the ensuing battle, Yami Yugi freed the spirits of the Egyptian Gods from within the Leviathan, and together, they destroy it, freeing all the captive souls. However, a trace of its darkness remains, along with Dartz's soul, which Yami Yugi is able to banish, freeing Dartz from the influence of the Orichalcos and allowing him to be reunited with the spirits of his father and daughter. As Atlantis sank beneath the waves, Dartz's spirit was finally freed to be with his family.
Dartz plays an Orichalcos Deck, with his most important Spell Cards designed to either summon or enhance the power of the "The Seal of Orichalcos", and many of his monsters named after and presumably linked to the evil stone. In keeping with this theme, his cards, like the Orichalcos, become stronger and more influential as the Duel progresses. "Kyutora" and "Gigas" both get stronger when destroyed, "Deuteros" and "Tritos" both increase his Life Points each turn.