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Derro were a species of monstrous humanoids that inhabited the Underdark.
Derro appeared to be some degenerate hybrid of dwarves and humans. Their rough skin was white with blue undertones and had sporadic tufts of coarse tan or pale blond hair. The eyes of the derro lacked any visible pupils.
Derro were degenerate and evil creatures of the underground, created from dwarf and human stock by some nameless deity of darkness and madness. Incredibly cruel and murderously insane, they enjoyed taking slaves and torturing surface dwellers (especially humans) to death.
Derro were afflicted by a form of racial madness, which most often manifested as delusions of grandeur coupled with an overpowering urge to inflict torment on other creatures. Derro were capable of holding their murderous impulses in check for short periods of time in order to cooperate with creatures of other races, but such arrangements rarely lasted more than a few weeks. Of course, no derro was capable of recognizing that he was out of his mind.
Derro were stealthy and bloodthirsty. They liked to carefully arrange cruel traps and deadly ambushes, and struck savagely from hiding. They delighted in taking captives who could be tortured to death later, and favored traps and poisons that disabled without killing.
Derro were widespread and were likely to be found in small bands throughout the Underdark. Derro warrens existed in many drow and duergar cities; independent derro strongholds lay in the deepest reaches of the Underdark. Derro scouts and marauders searched the areas around their settlements for victims to enslave and torment.
The derro were a strange and sadistic race. While clever and stealthy they were also murderously insane. It was very common for derro to devote themselves to some strange quest, such as collecting only certain types of gemstones or slaying as many members of a particular race as possible. Some derro were assigned specific missions by powerful derro savants, with the possibility that they might even join a company of comrades to fulfill some irrational goal.
Most derro revered the god Diirinka, but very few became actual clerics.
Legend has it that the derro originally lived on the surface of Faerûn. They opened portals to the Far Realm which offended the World Serpent who cast the derro into the Underdark.