Spellcasters and Thieves only. A particularly nasty item, this appears as an ornate ring with a magical aura. If tested, it will seem to be a ring of great power and utility. The Ring Of Severing will always have a spell function such as Fireballs, Flying, Teleportation, etc.
The ring can be removed easily, and no amount of magical testing will reveal its true nature. It does, in fact have the identified power, whatever it is, but upon activation in a crisis, or when otherwise needed (not just when the user is checking it out), the ring will perform properly, then immediately constrict around the wearer's finger, severing the digit. This causes 1d4 HP of damage, and it then immediately teleports to another of the wearer's fingers at random. No amount of tugging will get the ring off at this point, nor will Dispel Magic, Remove Curse, or any other spell short of a Wish.
Fingers severed, remain so. Neither magical Healing nor regeneration will reattach or bring them back (again, a Wish may do so, but only for one finger at a time). In addition to all the inconvenience and disability of losing fingers, spellcasters must roll d100 every time they cast a spell with a gestural component, with a cumulative 10% chance of failing for every finger lost. Thieves do the same for any Thief skills that requires hand dexterity (Pick Pockets, Pick Locks, Climb Walls, etc.).
The magical power of the ring, whatever it happens to be (Flight, Invisibility, etc.), remains active. Adventuring characters may well find themselves in emergency situations where they need to use the ring, despite knowing of the curse, making it a constant temptation in a crisis. Once all the wearer's fingers have been severed, the ring drops off, ending the curse; one way to be rid of it, therefore, is to deliberately chop off all of one's fingers (or hands), and later have them magically reattached or regenerated.
Optional: A diabolical version of this ring might be one of Protection, be it +1, +2, or what have you, since, every time an enemy attacks, it is automatically activated and the wearer loses another finger!