The powers of the ring of justice are unimportant (except that they be almost munchkinlike). My ring of justice (in a D&D setting) had detect magic at will, fly 3/day, mage armor 3/day, disintigrate (2/day, only affects evil creatures), and some more. It’s not important—it merely has to be so powerful and useful that the PC’s will be loath to part with it.
Enter twist number one: It is intelligent, highly moral, and very powerful (in D&D terms, it has a very high ego). It can only be worn by those who are good, and wont permit itself to be used for evil. It cannot communicate with the bearer directly, only empathically, so they cannot ask about its origins.
Enter twist number two: It is the phylactery of an evil lich-lord (I’m not sure that this translates into other systems, but liches in D&D can’t truly be killed unless their phylactery, or the storage-locker of their soul, is destroyed). So obviously the PC’s must destroy the ring in order to destroy the lich.
Don’t bother telling me it’s like LotR, the question in my mind is it too much like LotR?
—Verul may attack the PC’s mercilessly, and will always know exactly what they are up to and what they are thinking, seemingly inexplicably (shielding oneself from scrying has no effect).
—The bearer is unwilling to part with the ring simply because it is so powerful (you might have to scale its powers up for a higher level party). This would create an interesting tension within the party itself.
—The PC’s know of Verul and are attempting to destroy him, but first they must find his phylactery. How suprised they might be…?
—The ring will not permit itself to be destroyed (granted, very Tolkien-esque), requiring all sorts of mental fortitude and luck to destroy it.
—Obviously, they are now tied to Verul. He is, in a sense, their problem. You now have another villian at your disposal, but one who doesn’t have to mercilessly hunt the PC’s, but can do so only when their ends diverge.
—The party’s paladin can start to lose his powers because his protection of the ring constitutes an evil act. He would have to find out why and then find some way to remedy it.
Verul knows full well what his phylactery is. He gains clairvoyance/clairaudiance (or in none D&D terms, the power to see and hear the wearer) as well as limited powers of telepathy (he knows what the ring knows). He might well choose to hunt the PC’s down and destroy them so as to rescue his phylactery, or may choose to leave it in their hands so long as they don’t catch on. The applications for this are pretty numerous.