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Coloque um marcador +1/+1 na criatura alvo que você controla e, depois, cada criatura modificada que você controla causa dano igual ao próprio poder à criatura alvo que você não controla. (Os Equipamentos, as Auras que você controla e os marcadores são modificações.)
A creature that is equipped is considered modified no matter who controls the Equipment that's attached to it.
If the creature you control is an illegal target as Signature Slam tries to resolve but the creature you don't control is still a legal target, each modified creature you control (which may include the target creature you control if it's still on the battlefield) will still deal damage equal to its power to the creature you don't control.
An Aura controlled by another player does not cause a creature you control to be modified.
A creature with a counter on it is considered modified no matter what kind of counter it is or which player put it on that creature.
If the creature you don't control is an illegal target as Signature Slam tries to resolve but the creature you control is a legal target, you'll still put a +1/+1 counter on the target creature you control.