
You may play Barce and any other legend with one color identitycolor identityA card's colours plus the colours of every mana symbol in its rules text. In Commander, every card in the deck must fit within the commander's colour identity. or less as your commanders as though they each had partner.
Whenever another legendary creature you control diesdiesA creature dies when it goes from the battlefield to a graveyard. Being exiled or returned to hand is not dying., you may put it into your command zonecommand zoneThe area where your commander waits before you cast it, and where it may return instead of going to the graveyard, exile, your hand or library.. It replaces one of your commanders.
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Read from the rules text. A card that enables a keyword makes or is the thing; a card that rewards it gets better when you have it. Two cards interact when one enables what the other rewards.