Source: Tal'Dorei Campaign Setting Reborn

Weapon (dagger), very rare (requires attunement)

This dark metal blade bends in a wicked, hooked curve. You gain a +1 bonus to attack and damage rolls made with this magic weapon. When you hit with an attack using this weapon, you can spend 1 Hit Die to deal an extra 3d6 necrotic damage.

Curse. This weapon is cursed, a fact that is
revealed only when an Identify spell is cast on the weapon or you attune to it. Attuning to the dagger curses you until you are targeted by the Remove Curse spell or similar magic; disposing of the dagger fails to end the curse. While cursed, whenever you receive magical healing, you must spend 1 Hit Die to regain any hit points. If you spend all your Hit Dice at any point, you die.