A Shadow Game (闇のゲーム Yami no Gēmu, literally Game of Darkness or Dark Game) is a contest in the Yu-Gi-Oh! series generally between two people that involves a harsh Penalty Game for the loser. Shadow Games are predominately mentioned in the original Yu-Gi-Oh! manga. Shadow Duels, which are Shadow Games of Duel Monsters, are also played in Yu-Gi-Oh! GX, Yu-Gi-Oh! 5D's, and Yu-Gi-Oh! ZEXAL, though not all Shadow Duels are Shadow Games as they are seen in the original anime.
Egyptians first allowed the Shadow Games. Using various means of sorcery, they send themselves into an alternate dimension where their duel would not be interrupted, and where cheating is strictly forbidden, punishable by death. In the Shadow Games, the sorcerers' skills would be tested by seeing how powerful the monsters they could summon, and how powerful a spell they could cast to power up their monsters.
The power of the monsters could best be harnessed by the Millennium Items. Created for the purpose of defending Egypt, they were put into the possession of the Pharaoh and his six priests. However, Dark Bakura wished to steal the Items all for himself, and tried to take the Items, both in Ancient Egypt and in the modern world. To do so, Bakura needed to win them in Shadow Games.
In the modern world, the possessor of a Millennium Item can challenge another person to a Shadow Game.
During his Shadow Game with Koji Nagumo in the manga, Dark Yugi mentions he is raising the "Shadow Mode" from 1 to 3. At 1, though there were magical effects (such as the players taking damage instead of the monsters), Koji was able to cheat by kicking Yugi in the ribs without automatically losing. At 3, the monsters they played with came alive, and Yugi mentions that Koji's soul is part of the game now.