Chaos is a casual multiplayer format for Magic: the Gathering where the players add a Chaos Deck to the game to create new and exciting challenges. Chaos Magic is a variant of Free-for-All and might be considered an early form of Planechase.
The Chaos deck consists of 20–30 cards selected by some of the players involved. The player who goes first is designated the Chaos player; before that player untaps at the beginning of their turn, that player flips a card from the Chaos Deck face up on the table. That card is considered "in effect" until the Chaos player flips a different card after a full series of turns. The active Chaos card isn't in play and can't be affected by spells or abilities. Any player may use any abilities granted by the card, or, if it has a specific effect, each player immediately resolves the effect.
If the Chaos player is eliminated from the game, the next time that player's turn would have started, the mantle of Chaos player is passed to the first player to the left that's still in the game.
Examples of cards that could be used for a Chaos decks include Armageddon (which destroys all lands when it's flipped over), Kumano, Master Yamabushi (which allows each player to repeatedly spend to deal 1 damage to a creature or player), and Howling Mine (which has each player to draw an additional card during their draw step).