From publisher blurb:
For whatever reason, magic rings have become even more popular with players as of late (hmm). Don't let them down. Give them a new ring every game.
Magic Rings I features 50 new magic rings for use with D20 fantasy settings.
Ring of Allegiance (Good)
This ring can only be worn by a creature of good alignment. This ring functions as a ring of protection +2 with a couple of extra bonuses. The wearer becomes protected as though under the effects of a permanent protection from evil spell (as described in the Dungeons & Dragons? Player's Handbook, Third Edition). In addition to this the ring keeps track of evil aligned creatures slain by the wearer. Once the wearer has dealt the killing stroke to a total of 20 evil aligned creatures the ring will bestow a one-time bonus of 3000 experience points. This ring will bestow 1d4 negative levels upon any non-good creature that attempts to wear it.