Azula's Sting

+1 Rapier, requires attunement:
You hear the whispers of Azula.
Once per day, as a reaction to taking damage from
a creature you can see within 60 feet, cast Azula's
Wrath. You point at the creature and it is momentarily struck by blue lightning. The
creature must make a Dexterity saving throw.
It takes 2d10 lightning damage on a failed save,
or half as much damage on a successful one.