Weapon (Arrow), Very Rare, 650 gp
An arrow of slaying is a piece of magic ammunition effective against a particular kind of creature. If a creature belonging to the type associated with an arrow of slaying takes damage from the arrow, the creature must make a DC 17 CON save, taking an extra 6d10 piercing damage on a failed save, or half as much extra damage on a successful one.
Once an arrow of slaying deals its extra damage to a creature, it becomes a nonmagical arrow.
Ammunition of this type exists for other weapons, such as bolts of slaying meant for a crossbow, though arrows are most common.