The planes have been engulfed in Chaos. Magic energy surges everywhere in random spurts, gusts, and vorticies, and the planeswalkers are trying their best to keep it under control. Controlling Chaos is very draining on the system, however, so mages must be careful about which Chaos effects they wish to prevent.
Before the game begins, the Chaos deck is shuffled and placed face down in the center of the playing area. If it is more convenient, each player may have his or her own Chaos deck, as long as they have been shuffled together beforehand and cut such that no player knows what Chaos cards his or her deck does or does not contain. The Global deck is also shuffled and placed in the center of the playing area.
After each player has finished making his or her mulligan decisions, reveal the top card of the Global deck and put it into play.
A card with the Global supertype comes into play under no player's control. As long as it is not controlled, any activated abilities it has may be played by any player any time he or she has priority. As a state-based effect, if there are no Global permanents in play, reveal the top card of the Global deck and put it into play. If a player somehow gains control of a Global permanent, its activated abilities can only be played by that player. If an effect causes a Global permanent to become tapped while it is not controlled by any player, it does not untap during any player's untap step.
After each player's upkeep but before his or her draw step, is that player's Chaos step. At the beginning of a player's Chaos step, he or she draws a Chaos card.
When a Chaos card is drawn, it goes on the stack face down. Players then have an opportunity to respond as usual. When each player has passed priority, if the top of the stack is a face-down Chaos card, turn it face up. If the Chaos card is a split card, the controller of the card chooses which side of the card to play. If there are any abilities that trigger from the card being turned face up, they are dealt with. Next, the controlling player chooses modes, targets, etc. and each player then has another chance to respond with spells and abilities. Chaos cards on the stack are not considered spells (effects that target spells can't target them, and they can target things that have shroud or similarly worded abilities). A Chaos card's colors are determined by the colors of its border, not its mana cost (Chaos cards typically have no mana cost and thus a converted mana cost of 0).
Chaos is a card type with no particular associated properties.
There is a new zone called "the middle" which is shared by all players (similar to the "in play" zone), and begins empty. Any player may play a card from the middle any time he or she has priority, as long as that card has been in the middle since the start of the current turn. The player must pay all costs normally involved in playing the card (such as mana cost), and must additionally respect the timing rules for the card (e.g. you can only play a land from the middle on one of your own main phases while the stack is empty and you haven't yet played a land this turn).
If a player would lose the game, if he or she hasn't spited so far this game, he or she gets to spite instead. To spite, the player looks at the top seven cards of the Chaos deck and puts one of them on the stack face up (abilities that trigger from Chaos cards being turned face up do not trigger). A Chaos card played as a spite can't be countered. A player can't lose the game during his or her spite. If, after the spite has finished resolving, the player no longer satisfies any condition for losing the game, he or she does not lose and remains in the game.