Corecut Dagger

The  was one of Avantika's weapons until Fjord killed her undead form and retrieved the dagger from the sea.

Description
The dagger has a hooked, dark metal blade that is cold to the touch. A faint black energy drifts off the edge of the blade.

The dagger has a +1 bonus to attack and damage rolls. The user can also spend one of their hit dice to deal an additional 3d6 necrotic damage on a hit.

The dagger is cursed, so that the wielder bound to it wants to remain bound to it.

Avantika stabbed Nott with the dagger at the Darktow docks, which is the first time any of the Nein noticed the necrotic magic of the blade.

The undead Avantika used the dagger again when she and her allies tracked the last Cloven Crystal to the Midnight Hammer in the Frigid Depths. This time, when Fjord killed her, he recovered the weapon and had Caleb identify it.

Veth used the dagger for the first time on Gestaf the yeti, where it was revealed to be cursed. Although not all the details of the curse are known, attuned wielders do not want to part with the dagger.