Zorc Necrophades, (called Zorc the Dark One in the English dub), is a "Great Evil God" who was created out of the darkness of people's hearts, and his dark power is what is later used to create the Millennium Items and their powers. He serves as the main antagonist of the Millennium World arc and the prime mover of the original series.
Zorc appears as a large, purple-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 Aknadin, who brought all seven Millennium Items together in an underground crypt in Kul Elna village 3000 years previous (5000 years in the English dub). He then infused a shard of his soul within Aknadin, creating the Great Shadow Magus. Thief King Bakura then managed to obtain all of the seven Millennium Items, and sacrificed himself to grant Zorc a physical form on Earth.
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. Also, during an unspecified point in time, Thief King Bakura also seals his soul in the Millennium Ring, with both souls merging together to form Yami Bakura. Zorc later resurfaces when he possesses Bakura Ryou 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, known as the Dark RPG, played in the world of the Pharaoh's memories, attempting to release Zorc's full self from the Millennium Puzzle, giving him a second chance to destroy the world.
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. 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.
Having reclaimed the Millennium Puzzle, the Pharaoh summoned the three Egyptian Gods to battle 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 delivered 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 Muto, Joey Wheeler, Tea 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.
Yami Bakura and Yami Yugi reentered the RPG at this point, Yami Bakura having revealed to Yami Yugi that he was Zorc.
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 of Light. Zorc was horrified that Atem had summoned the "ultimate God", and he was consumed by the Creator of Light's attack, destroying him along with Yami Bakura for good.
Zorc, having been born from the darkness of people's hearts, has power exceeding that of the Millennium Items, which were created using his powers (In the dub, he is the creator of the Shadow Realm). Because his powers stem from darkness itself, there is virtually no limit to his powers, which he stated multiple times.
A testament to his powers have been how Zorc had single handedly defeated Exodia the Forbidden One, the three Egyptian Gods, and the Blue Eyes White Dragon. He had also fared equally against the Blue Eyes Ultimate Dragon, and withstood a devastating attack from the Dragon Master Knight, the attack having destroyed the Dragon Master Knight himself. There were only 2 times that Zorc was actually stopped.
3,000 years ago (5,000 in the dub), Zorc was only stopped when Atem used a spell to seal Zorc away, at the cost of having his own soul sealed into the Millennium Pendent, before it shattered and became the Millennium Puzzle. When he was revived in the Memory World, he was finally destroyed when Atem remembered his name and then combined the 3 Egyptian Gods to summon the Creator of Light. The Creator of Light was Zorc's only weakness.
Aside from its immense durability and power, Zorc is able to make attacks made of darkness, capable of cutting powerful beings like Exodia in half, petrify enemies, create a solar eclipse, and shooting fire created from his hands or fired from the dragon/snake appendage. Also, when he destroyed the monsters summoned by Yugi and his friends, the damage to their Life Points was equal to the monsters' ATK (i.e. When Yugi's Dark Magician was destroyed, he lost 2500 Life Points, while Joey lost 2400 Life Points when Red Eyes Black Dragon was destroyed).