Yu-Gi-Oh! episode 204: "Makings of a Magician"
Zorc Necrophades, (called Zorc the Dark One in the English dub), is a demon god who was created out of the darkness of peoples hearts, and his dark power is what is later used to create the Millennium Items and their powers. He serves as the main antagonist in the Millennium World Arc, the final villain of the original series and the central antagonist of the Yu-Gi-Oh! manga. Though he only physically appears in the final season it is revealed that Yami Bakura is an incarnation of Zorc, meaning he has been present for the whole series.
Zorc appears as a large, black-skinned demon with huge, curved horns, bat-like wings, and a large dragon/snake appendage between his legs. The dub changes this appendage to protrude from his chest instead of between his legs, due to its likeness to the phallus. In the bunkoban of the manga, it is made spiral around him, rather than point out suggestively. He is very similar in appearance to Diabound (albeit much larger and more demonic), which is possibly due to his connection to Thief King Bakura.
Zorc Necrophades was born eons before the Millennium Items were originally created, by his own claim, from the darkness in the hearts of people (in the English anime, he is the creator of the Shadow Realm, and therefore predates the Items, though they were made using his power). He was summoned by Akhenaden who brought all seven Millennium Items together in an underground crypt in Kul Elna village 3000 years previous (5000 years in the English dub) in order to be resurrected with the power of the newly made Millennium Items.
Zorc attempted to destroy the world, but was stopped by Atem. Atem sacrificed his life by sealing himself and Zorc (or at least, the memories of Zorc) within the Millennium Puzzle. Zorc managed to live on by sealing a fragment of his soul within the Millennium Ring. Zorc later resurfaces when he possesses Ryo Bakura as the villain Yami Bakura wishing to obtain all seven Millennium Items in order to resurrect himself. Eventually, when Atem wished to regain his memories, Yami Bakura turned the event into a Shadow Game played in the world of the Pharaoh's memories, giving him a second chance to destroy the world.
Zorc was once again resurrected, by Thief Bakura following the events of the past Unbeknownst to Zorc, his awakening had triggered that of Hassan, who protected the Pharaoh. Zorc had put so much power into the attack, he capacity to stop time ended, and he was beheld by the Priests. They were transported above ground to battle.
In this second face-off, though his legion of spirits was matched, Zorc easily decimated the forces of Egypt, resisting the attempts to stop him. In the anime only, Isis regathered the scattered Millennium Items, then attacked Zorc with a powered-up "Winged Dragon, Guardian of the Fortress." Zorc dispatched the monster with ease, killing Isis. Shimon next stood in Zorc's way, calling forth "Exodia" to battle Zorc. In sheer power, even Zorc was matched by the "Forbidden One," and the fight initially seemed even, with both combatants losing and regrowing arms. But Zorc's power was drawn from darkness itself, while "Exodia" drew its strength from Shimon, and Zorc ultimately dispatched "Exodia," killing Shimon as well. He also slew both Shada and Hasan as they protected the Pharaoh.
Following this, the anime expanded upon Zorc's clash with the Priests. Having reclaimed the Millennium Puzzle, the Pharaoh Summoned the three Egyptian Gods to battle Zorc. Yami Bakura and Yami Yugi reentered the RPG at this point, Yami Bakura having revealed to Yami Yugi that he was Zorc. The Egyptian Gods clashed with the Great Evil God, and initially appeared to have defeated him. Despite the assault, including the Egyptian God Phoenix attack of "The Winged Dragon of Ra," Zorc survived and created a solar eclipse, robbing the Egyptian Gods of strength, and he petrified all three. Seto then opposed Zorc with the "White Dragon," and again, appeared to match Zorc, but Zorc petrified the Dragon as well, then destroyed it, taking Seto out of the fight. As he began to destroy the city, Seto Kaiba, Seto's modern reincarnation, stepped in, refusing to tolerate Zorc's atrocities any longer. Kaiba called out three "Blue-Eyes White Dragons, from himself, Seto and Kisara, and fused them into the "Blue-Eyes Ultimate Dragon." The Ultimate Dragon clashed with Zorc, and for the first time, the battle was even. The ultimate clash between light and darkness began to break down the barrier between worlds. The Pharaoh then appeared to merge with Kaiba and the Ultimate Dragon using the Millennium Scale, becoming "Master of Dragon Soldier," and delievered an attack so powerful that it destroyed the Dragon Soldier itself. It still wasn't enough to defeat Zorc, and he prepared to destroy the Pharaoh. Shadi proceeded to intervene using the body of Hasan, saving the Pharaoh.
At that point, Yugi Mutou, Joey Wheeler, Téa Gardner and Tristan Taylor arrived, having discovered the Pharaoh's true name. They Summoned monsters to attack Zorc, including Mahad's spirit alongside Mana's ka, but he resisted even the most powerful of Mahad's magic and dispatched them with ease.
Téa then got the idea to cause the Pharaoh's name to appear on a cartouche she'd given him. As they did, Zorc tried to destroy them but Kaiba had recovered and used his "Ring of Defense" to protect them. Zorc broke through, but it was too late, and the Pharaoh re-learned his name - Atem. Atem Summoned all three Egyptian Gods, and fused them together into "The Creator God of Light, Horakhty." Zorc was horrified that Atem had Summoned the "ultimate God", and he was consumed by Horakhty's attack, destroying both him and Yami Bakura for good.