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When you planeswalk to The Great Aerie and at the beginning of your upkeep, bolster 3. (Choose a creature with the least toughness among creatures you control and put three +1/+1 counters on it.)
Whenever chaos ensues, choose up to one target creature you control and up to one target creature an opponent controls. Each of those creatures deals damage equal to its toughness to the other.
Bolster itself doesn’t target any creature, though some spells and abilities that bolster may have other effects that target creatures.
If you choose two target creatures for the chaos ability and either target is an illegal target as the ability tries to resolve, the ability won’t resolve and none of its effects will happen. Neither creature will deal or be dealt damage.
You determine which creature to put counter(s) on as the spell or ability that instructs you to bolster resolves. That could be the creature with the bolster ability, if it’s still under your control and has the least toughness. If there’s a tie, you choose which creature gets the counter(s).