Diminishing Returns

You limit the attacking capabilities of a creature within range. Choose a specific weapon attack, spell attack, or damaging ability that the target used during its last turn. The target must then make a Charisma saving throw. On a failed save, the target takes a cumulative -3 penalty to damage each time it uses that attack. This penalty lasts for the duration, and only effects damage done by that particular attack. If this penalty reduces the damage of the attack to 0, and the attack is made with a non-magical weapon, the weapon is transported to the Ethereal Plane for the rest of the duration.