The omsomivae are subterranean creatures unique to the SisterWorlds. They are large, white, reptile-like creatures, some as long as fifteen feet in length, that move about through the underworld cave systems on six legs. They tend to be solitary, though adult females can have as many as half a dozen younglings with them. A single omsomif tends to be a skittish creature, moving away from any potential predators despite the size.

An omsomif can live off of many bacteria species, but tends to closely follow the movements of the black puddings, with specialized internal organs that digest and consume the cave-boring creatures with remarkable efficiency. Omsomivae fecal matter has been known to dissolve many rocks for some time after deposition. The blood of omsomivae is believed by many to be highly acidic, although this has been shown to be untrue. Their hide, on the other hand, is remarkably resistant to acid damage. Shed skins can fetch high prices; intact hides even higher prices.

The omsomif appears as a ghostly white lizard-like creature with large eyes and a segmented body design. The scales of the omsomif adjust to ambient conditions, whether thermal or visual, and the creature is believed to hunt as much by sight as by scent in the poorly lit phosphorescent caverns of its home. Particularly hungry omsomivae have been known to hunt humanoid prey, but such accounts are very rare. In rich subterranean areas, the omsomivae are quite skittish and will retreat at the first sign of detection.

Some scholars believe that the omsomivae are distantly related to behir or other multilegged reptilian monsters.