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 dealing 10d6 fire damage with a DC 14 Reflex save.
Moderate Evocation;CL 12; Forge Ring, fireball, contingency; Market Price: 3,300 gp