Fairy Arrow
Requires a complex spoken charm and the powder from a blue moth's wings.
Sprinkle the powder over an arrow. When you speak the charm the arrow becomes invisible. When it strikes its target, it will disappear entirely, leaving no visible mark on the target’s body. On a mishap, the arrow sings a warning to the target as it flies.
(this is for my WIP game To Places Lost, but wouldn't be hard to adapt into other systems I bet)