Überwaldean land eels
From Discworld & Terry Pratchett Wiki
These are black, slippery eels which slither on land. They are from the dark and mysterious Überwald. They live in deep underground caves, where there is not sunshine, not the lack of light, but the substance of darkness itself. An Überwaldean land eel will emit black light when surprised, much as a salamander will emit light. One vampire iconographer in Ankh-Morpork, Otto Chriek, once attempted to use black light for iconography (in The Truth). The results were unreliable in the traditional iconographical sense. Black light seems to affect the psyche of humans and members of other humanoid species. When used in iconography, black light seems to illuminate, if such word can be used, a sort of truth, such as who really was in a room a few hours ago, or a father who's constantly watching over a son's shoulder. Although not fully understood, it is believed that dark light is light without time. It can illuminate the past, future or stranger places. The dwarfs are very superstitious about black light. Considering the wide spectrum of effects, the dwarves' choice would seem to be fully justified.