It's my old item from Dungeons and Dragons, second edition.

Here's the old text from twenty years back!  Not having armor the item proved rather helpful.

The ring of regeneration restores one point of damage per turn (and will eventually replace lost limbs or organs). It will bring its wearer back from death. (If death was caused by poison, however, a saving throw must be successfully rolled or the wearer dies again from the poison still in his system.) Only total destruction of all living tissue by fire or acid or similar means will prevent regeneration. Of course, the ring must be worn, and its removal stops the regeneration processes.

Addendum: It isn't explicit but there are many ways around the regeneration.  Being eaten for example, being subject to constant damage effects like drowning, being crushed into paste,having the limb chopped off that wears the ring negates the effects, instant death spells, disintegration… soul destruction...

Then there's being impaled... Taking an arrow to the eye socket or stuck with a stake vampire style would out someone in suspended animation.  (Where you would still age of course)


Also in a later errata it also stipulates the wound had to be inflicted on the person while the ring was worn. You couldn't put the ring on after you took an arrow to the chest.  Otherwise people would share the thing around.