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When Skulking Killer enters the battlefield, target creature an opponent controls gets -2/-2 until end of turn if that opponent controls no other creatures.
Skulking Killer's ability triggers even if all your opponents control more than one creature. If the targeted creature's controller controls any other creatures as the ability resolves, the ability won't do anything. Notably, that player could cast a creature spell with flash or create a creature token in response to avoid the -2/-2 effect.
