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 red thread has taken the lives of many a target. Once per turn, when you make a ranged attack and you have advantage against the target you can add an additional 2d6 piercing damage to the damage roll.