In short, Brawl is a variant of multiplayer Commander that uses exactly 60 cards from the Standard pool, 30 life points, and no Commander damage. As with Commander, only one copy of each card is allowed, except Basic Lands, and all cards must be within the colour identity of the Commander.
The game is explained by Wizards here, and its rules are published in Magic: the Gathering’s Comprehensive Rules as follows:
- 903.12. Brawl Option
- 903.12a Brawl is an option for a different style of Commander game. Brawl games use the normal rules for the Commander variant with the following modifications.
- 903.12b Brawl decks are usually constructed using cards from the Standard format.
- 903.12c A player designates either a legendary planeswalker or a legendary creature as their commander.
- 903.12d A player’s deck must contain exactly 60 cards, including its commander. In other words, the minimum deck size and the maximum deck size are both 60.
- 903.12e If a player’s commander has no colors in its color identity, that player’s deck may contain any number of basic lands of one basic land type of their choice. This is an exception to rule 903.5d.
- 903.12f In a two-player Brawl game, each player’s starting life total is 25. In a multiplayer Brawl game, each player’s starting life total is 30.
- 903.12g In any Brawl game, the first mulligan a player takes doesn’t count toward the number of cards that player will put on the bottom of their library or the number of mulligans that player may take. Subsequent mulligans are counted toward these numbers as normal.
- 903.12h Brawl games do not use the state-based action described in rule 704.6c, which causes a player to lose the game if they’ve been dealt 21 or more combat damage by a commander.