The Onyx Blade

This sword was made by Nath for reasons unknown, and upon being stricken by this sword, history itself changes. Few can wield such a weapon. It has the power to instantly kill almost anything it sets its gaze on, once per day, as long as the kill furthers Nath's sinister plans in some fashion.

Onyx Blade: +10 to hit, 3d8 slashing damage and 1d8 necrotic damage

Once per day, the Onyx Blade can be used like power word kill: on a hit, if the creature has less than 100 HP it dies instantly.

The blade must be used according to the will of Nath for this to happen.

If the blade has not been used in this way and it seems like it might miss the chance, it will compel the user to do so if they fail a DC 20 Charisma Saving Throw.