A Shadow Game, known as a Death Game in the Japanese version (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!. Shadow Duels, which are basically Shadow Games of Duel Monsters, are also played in Yu-Gi-Oh! GX and Yu-Gi-Oh! 5D's.
In a normal Shadow Game for the players, all of the damage done by their monsters is real (their monsters a maintained by a duelist's spiritual energy), and if one loses the game, he loses his soul to the Shadow Realm.
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 of monsters they can summon, and how powerful a spell they can 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 must play Shadow Games.
In the modern world, the possessor of a Millennium Item can challenge another person to a Shadow Game.