Heart-Piercer Bow
Artifact — Equipment
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.
Franz Vohwinkel
Khans of Tarkir (KTK)
• #221Rulings
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.
(wotc • 2023-07-28)
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.
(wotc • 2019-07-12)
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Rulings
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.
(wotc • 2023-07-28)
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.
(wotc • 2019-07-12)