Withering Bowstring
Wondrous Item, rare
This bowstring can be strung to any bow or crossbow. Once attached, the string forms to fit the weapon and becomes tight. You can only have one bowstring per weapon and it requires a short rest to replace the bowstring of your weapon.
This deep black string leaves the users fingertips slightly numb after each use. 2 times per day you can speak the command word to transform your next attack into a Sickening Shot. You add an additional 1d8 necrotic damage to the damage roll and the target can't regain any hit points until the start of your next turn. You regain all uses of this item at the start of the next dawn.
The ammunition used in the Sickening Shot is destroyed as the necrotic energy quickly rots the arrow after impact. You must declare that you are using a Sickening Shot before you attack for it to take effect.
https://www.dndbeyond.com/magic-items/119657-withering-bowstring