Imbued Onyx Stone

Equipped creature has "At the beginning of your upkeep, put a -0/-1 counter on every other
creature on the battlefield, equipped creature
gains {x} +0/+1 counters for every counter added
this way. When equipped creature leaves the battlefield, exile Imbued Onyx Stone.
Deal {x} damage to target opponent, you gain {x} life, where {x} is the number of counters on equipped creature."
Equip {3}