A wyrm, also called a darkling crawler, is a creature of Perimal Darkling. The only wyrm seen is described as grey and segmented, with antennae. It is about as thick as a man's upper arm, and long enough to wrap around a flying changer's neck to be carried; presumably it is light enough not to be an impossible burden in that circumstance. It leaves a slimy trail behind it as it crawls. Like changers, wyrms are able to change shape, and might well be the source from which the changers' ability was derived.
In To Ride a Rathorn we get a closer look at the same crawler when it lands on Jame after a pursuit through Tentir. It crawls not on legs but on human-looking, scarred fingers, although its appearance may have been caused by the Wyrm's proximity to Tori, who had returned to Tentir for the first time since having been been bitten by the crawler on the way to the Cataracts. Tori being a Shanir blood-binder, the wyrm ended up bound to the Highlord, and very confused, as it is still at least partially bound to the dead changer. The wyrm's 'scarred' fingers mimic the scar pattern on Tori's hands. It has a round mouth like a lamprey and has a caul that envelops its entire head except for the mouth. Jame sees its face become the dead changer and then other faces, but it is not certain whether it really does or whether the wyrm's presence is affecting her mind.
The wyrm leaves a venomous trail behind it which is strongly corrosive, burning through metal and stone. This venom can be used as a drug; in Dark of the Moon, Tirandys drugs Jame with wyrm's venom on Gerridon's orders so that she will be placid when Gerridon takes her to his bed. Jame recognizes the presence of the wyrm Beauty in the Knorth apartments at Tentir in To Ride a Rathorn because she is reminded by the smell of the taste of the wyrm's venom she drank. The wyrm appeared to have recuperated and shed its skin in those apartments.
The wyrms appear to have a somewhat insect-like lifecycle. The wyrm transforms into a spiky caterpillar with poison bristles; and then at some point cocoons itself, and after a hibernation, break out into a graceful, vaguely humanoid winged form with golden eyes.
Perhaps these were also the same shadows that by mating with Kencyr and Rathilien natives made them changers.