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At the beginning of your end step, bolster X, where X is the number of tapped creatures you control. (Choose a creature with the least toughness among creatures you control and put X +1/+1 counters on it.)
Bolster itself doesn’t target any creature, though some spells and abilities that bolster may have other effects that target creatures. For example, you could put counters on a creature with protection from white with Abzan Skycaptain’s bolster ability.
Count the number of tapped creatures you control as the ability resolves to determine the value of X.
The creature you put the counters on doesn’t have to be one of the tapped creatures.
You determine which creature to put counters on as the spell or ability that instructs you to bolster resolves.
