Wrap of Death
Wondrous item, legendary (requires attunement)
—
A strange blood-stained leather wrap for a weapon. Strange runes are running along the wrap and blood seems to drip from it.
—
When it is wrapped around the handle of a weapon a permanent bond is made. When the bond is complete, teeth erupt all over the weapon. If the weapon the wrap is attached to didn’t require attunement, attaching the wrap makes it require attunement. If the weapon already required attunement, there is no change. This bond can only be broken by a wish.
—
Whenever the wielder attacks a creature made of flesh, the target takes an additional 1d4 necrotic damage and the wielder is healed half the necrotic damage dealt (rounded down to a minimum of 1).
—
When a critical hit is scored, the teeth grow larger and more wicked. This growth brings the necrotic damage die up to a d6 if another critical hit is scored it increases to a d8 and so on to a maximum of a d12. When the wielder takes a long rest, the damage die is reset back to a d4.
—
However, if the wielder attacks a creature not made of flesh, the necrotic damage is instead dealt to the wielder and the creature regains half the necrotic damage received (rounded down to a minimum of 1).

(Credits: the_scrying_eye)