Ring-Worm

A ring-worm passes for an ordinary gold band that can be worn normally and gives off faint magic. Cast identify on it and it seems to be a +1 protection ring.

Once on a finger, it devours its host's magic. A wizard wearing one loses spell levels at a growing rate: one the first day, two the second, and so on, forgetting randomly chosen spells as soon as they are memorized. After a wizard's spells are gone, or immediately if the wearer casts none, it feeds on carried magic items, each of which gains a cumulative 10% chance per day worn to fail.

When the wearer suspects the ring, a fresh detect magic shows a much stronger aura, swollen with stolen energy. By then it has fused to the skin. Heal, remove curse and cure spells will not free it; a dispel magic that succeeds is the one cure, with the worm resisting as if 6th level, gaining a level for each three spell levels or 'pluses' it has eaten. Once it is gone, magic items recover in 1-6 turns, and spells can be memorized normally the next day.

Fire makes it let go and try to escape, and any successful flame attack kills it outright. Out of ring form, it can be hurt only by bladed weapons of +2 or better.

It settles wherever food is plentiful, so it favors cities full of spellcasters, along with dungeons, taverns and other adventurers' haunts, often left behind by a former host who discovered the truth. A mated pair lays 10-50 eggs; the hatchlings scatter to live alone, so finding more than one at a time is extremely unlikely.

Its hunger for magic makes it easy to lure into traps, and rumor holds that some folk trap them deliberately to plant on spellcasting rivals.