In this casual, multiplayer format, you start with a life total of 40 (rather than the usual 20) and choose a legendary creature to serve as your Commander. You then choose cards to match your Commander’s color identity to build your deck. A card’s color identity is any mana symbol appearing on that card.
The commander deck contains 100 cards: 1 Commander and 99 others. Your deck may contain only one of any individual card, with the exception of basic lands. In the Magic Online version of this format, your Commander is the only card in your sideboard.
The Commander enters play in the command zone. You may cast your commander from the command zone for its normal costs, plus an additional two mana for each previous time it’s been cast from the command zone this game. If your Commander is ever headed to the graveyard or exiled, you may return it to its command zone instead.
In addition to the normal Magic loss conditions, you can be defeated in this format by taking 21 or more points of combat damage from a single Commander over the course of the game. The Commander is tracked across zone-changes for this purpose.