1) The ring makes the character function as undead regarding healing. He takes damage from cure spells and heals from inflicts. Once per day, the character can cast inflict X wounds (something higher than character levels should allow them to cast) targeted at someone else AND heal the amount inflicted. Effectively draining life from someone. This special inflict X wounds must always be inflicted at someone else. The ring is cursed and can't be removed easily. The longer he wears it, the more undead traits he starts accumulating. At some point forgetting to breath, heart no longer beats, not needing food etc. At that point, if the ring is removed, the character dies.
2) The ring is a dimensional link to an outerplanar creature. That creature starts talking to the fighter through it. Of course, it claims to be an angel imprisoned by the Big Baddie and wants the characters to free it. It can offer aid in the form of various divination spells, like contact other plane. Naturally, the creature is really a demon or devil or something who wants to enter the real world. The Big Baddie was really doing Awful Things to keep the Even Worse Thing locked away.
3) Oh, it is the Ring of Truth! The wearer can hear the thoughts of everyone closeby. He as well broadcasts his thoughts constantly, whether he wants it or not. Sounds cool? Take a second thought... "Mmmm, nice ass" *slap* and so forth. If worn longer, it might grant other abilities like True Sight, but the social problems it causes are vast. Plus of course, enemies always hear the fighters thoughts before they see him.
4)
It is the ring of optimism, as in Oglaf! Oglaf is usually NSFW, by the way, although these examples are clean.
Oh yeah, the ring makes a return. Edit: 5) Sentimental option - it is the Big Bad Guys daughter's wedding ring. The ring is the last memory he has of the daughter, who is now dead. All the Bad Things he is doing are a misguided revenge for her death. The magic is a link to her memories. Wearing the ring and sleeping while wearing it lets the character have lucid dreams featuring the daughter, where he can interact with her. (Alternatively replace daughter with wife, mother, wife or whatever.)