Ring of Retributive Fireball (3.5e Equipment)
From D&D Wiki
Ring of Retributive Fireball: A silver band with a golden pearl set in a plain fitting.
The ring's pearl is actually a condensed fireball. When the wearer reaches −10 hit points, the ring activates and the golden pearl explodes after a fuse time of 1d3−1 rounds, dealing 10d6 fire damage with a DC 14 Reflex save for half damage. If the ring is removed from the wearer's finger during the fuse time, the the perl will not explode and the ring can be used by another person.
Each time the ring is triggered, but does not explode, there is a 25% chance that the hit point limit for the trigger is reduced by 1d3 hit points, so there may be rings with a trigger level of −7 hit points or even less! The minimum trigger level is −1 hit points.
Moderate Evocation;CL 12; Forge Ring, fireball, contingency; Cost 1650 gp, 132 xp, 4 days; Market Price: 3300 gp