Zorc Necrophades

Zorc (Zorc Necrophades in the manga, Zorc the Dark One in the anime) is an evil demon and the Dark God of the Shadow Realm. He is by far the most powerful and most evil villain in the whole Yu-Gi-Oh! franchise. He feeds and gains strenght from the darkness in human hearts and he wants to merge the world with the Shadow Realm.

The Anime
Zorc is an evil demon who created the Shadow Realm (English version). Pharaoh Atem sealed himself and Zorc inside the Millennium Puzzle to stop him, but Zorc sealed a portion of his soul in the Millennium Ring. This portion of his soul became Yami Bakura, the evil spirit of the Millennium Ring. Yami Bakura set in motion his evil plans to resurrect Zorc, and though he is clearly up to something starting from Season 1, his true objectives are never revealed until Season 5. Yami Bakura and Yami Yugi entered an RPG, which played out exactly as it did in reality, except with modifications from Yami Bakura. In the end, Yami Bakura was able to summon Zorc, who defeated most of the Pharaoh's men and destroyed the three Egyptian Gods, Obelisk the Tormentor, Slifer the Sky Dragon, and the Winged Dragon of Ra, as well as Exodia. Yami Bakura recombined with Zorc at some point during the battle. In the end, Zorc was destroyed when the Pharaoh learned his true name, Atem, and fused all three Egyptian Gods together into the Creator of Light, whose rays of light alone were enough to completely destroy him.