Arrow of Slaying

From Dungeon Master's Guide, page 152. Found on Magic Item Table E.

Description
Weapon (arrow), very rare.

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.