From publisher blurb:
The dwarven races are quite diverse, but perhaps the most deviant, both physically and mentally, is the derro. Though seen as corrupt and malignant by most of their kin, derro perceive themselves as superior to their brethren. They scoff at their brethren’s naïve notion of blind obedience to kin and kith. For derro, it is power that should be respected and nothing else. Guided by this precept, the derro follow the powerful not out of any sense of loyalty toward them but because it is from them that power can be derived. This lust for power drives derro from birth to death in a never-ending struggle for personal glory and honour.
Lurking just below the surface, derro have been, for most players and Games Masters, an unseen menace. This Slayer’s Guide shall remedy that situation. No longer shall derro be ignored as a race buried beneath the ground or an adversary caged in the caverns of the hidden world below. The sun can not keep the derro from what is rightfully theirs forever.