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When this creature enters, tap up to one target creature. You gain life equal to that creature's power.
Brightmare's ability can target a tapped creature. If the target creature is already tapped when the ability resolves, that creature just remains tapped and you gain life equal to its power.
If the target creature is an illegal target by the time Brightmare's ability tries to resolve, the ability won't resolve. You won't gain any life.
