Ravener Ring

Wondrous Item - Rare - Ring - Cursed - Requires Attunement
A thin ring of black-stained metal, sculpted into geometric razor-lined shapes. Sculpted by ancient elven rebels dedicating themselves to the path of revenge, they have a dire reputation as being dangerous to the wearer.
When attuned to this ring, the wearer's critical hits deal one extra dice of damage.
Cursed: The ring tightens around the wearer's finger when attuned. To remove the ring, a DC15 Sleight of Hand check must be made by the wearer or another creature, at most once per day. The ring cannot be unattuned while it is worn.
Whenever the wearer scores a critical hit, the ring tightens - the DC to remove the ring increases by 1d8. The wearer may also elect to increase the critical hit damage by two dice of damage instead of one - if doing so, the DC to remove the ring increases by 2d8 instead. Once the removal DC reaches 30, the ring severs the finger it is worn on, and the removal DC resets to 15.