
Commander creatures you ownCommander creatures you ownWhen a creature with undying dies, if it had no +1/+1 counters on it, return it to the battlefield under its owner's control with a +1/+1 counter on it. have "Whenever this creature becomes tapped, it and other creatures you control that share a creature type with it each get +2/+0 and gain undying until end of turn." (When a creature with undying dies, if it had no +1/+1 counters on it, return it to the battlefield under its owner's control with a +1/+1 counter on it.)
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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.
A Background can be a second commander alongside any creature with Choose a Background.