Vorpal Arrow

You touch a piece of ammunition or weapon with the thrown property, imbuing it with the ultimate edge. The first time you attack with the empowered item, it adds +3 to attack and damage rolls, and pieces all resistances and immunities to the weapon's damage type. On a hit, the creature takes 6d8 additional damage. If this damage reduces the target to zero hit points, it is killed by a mortal wound.

Once an attack has been made with the piece of ammunition or weapon, the spell ends.

Source: Kibble's Casting Compendium