Heart-Piercer Bow
Whenever equipped creature attacks, this Equipment deals 1 damage to target creature defending player controls.
Equip 
(: Attach to target creature you control. Equip only as a sorcery.)
Designed by an ancient artificer, the finest Mardu bows are carved from dragon bone and strung with the wind itself.
Illustrated by Franz Vohwinkel
This printing
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Heart-Piercer Bow (not the equipped creature) is the source of the triggered ability and the source of the damage. This means that a creature with protection from green may be targeted even if Heart-Piercer Bow is equipped to a green creature, but a creature with protection from artifacts may not.
If a creature is attacking a planeswalker, the controller of that planeswalker is the defending player. If a creature is attacking a battle, the protector of that battle is the defending player.
