Source: The Book of Many Things

Weapon (dagger), very rare

You gain a +1 bonus to attack and damage rolls made with this hooked, obsidian dagger.

When you hit a creature with this magic weapon and roll a 19 or 20 on the attack roll, the creature must make a DC 15 Constitution saving throw as the dagger flares with sickly light. The creature takes 6d6 necrotic damage on a failed save, or half as much on a successful one. If this damage reduces the creature to 0 hit points, the creature disintegrates into dust.