Your party slew that hideous desiccated corpse that attacked you. Striking the killing blow, and after the blasted thaumaturge found there was only a 'mild magical emanation' from the ring said you could have it.
It is beautiful, but so dark and cold. You just can't seem to get the ring warmed up. Also, you don't seem to recall eating or drinking since acquiring the ring. No matter, surely it will fetch a pretty penny.
Onyx ring set with black opal will sell for 1300 gp to a jeweler. Slightly less on an open market.