Lovecraft's Horrors: "Call of the Cthulhu" World.

An arrow of slaying is a magic weapon meant to slay a particular kind of creature. Some are more focused than others; for example, there are both arrows of dragon slaying and arrows of blue dragon slaying. If a creature belonging to the type, race, or group associated with an arrow of slaying takes damage from the arrow, the creature must make a DC 17 Constitution saving throw, 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. Other types of magic ammunition of this kind exist, such as bolts of slaying meant for a crossbow, though arrows are most common.