Soul fists

While wearing this item you gain the following:

Soul Gathering.
When you kill a creature above cr 0, this item gains 2(+2) charge up to a maximum of 6 charges. This stacks with other instances of soul gathering, each item gaining the combined charges.
Relentless combo.
When you hit with this weapon, your next attack deals an additional +1d4 necrotic damage Soul critical. When you attack, you can spend 3 souls to increase your critical hit range by 1 or 6 souls to increase your criticalhit range by 2.
Soul critical. 
When you attack, you can spend 3 souls to increase your critical hit range by 1 or 6 souls to increase your criticalhit range by 2.

Rarity

Very rare Dungeon Gauntlets (requires attunement)