Derro are a species of small, monstrous humanoids which inhabit the Underdark.
At first glance, Derro appear as a degenerate hybrid of dwarves and humans. The creatures are slim of build, and stand about 4 feet tall. Their rough skin is a ghastly pale-blue with coarse yellow hair. The eyes of the Derro were a dead-white, without any visible pupils.
Society
Derro are widespread and most likely to be found in small bands, throughout the Underdark. Derro enclaves also exist in many drow and duergar cities; independent Derro strongholds lay in the deepest reaches of the Underdark. Derro scouts and marauders often search the areas around their settlements for victims to enslave and torment.
The Derro are a strange and sadistic race. While clever and stealthy; many are also murderously insane. It is common for Derro to devote themselves to some strange quest, such as collecting a certain types of gemstone or slaying as many members of a particular race as possible. Some Derro are assigned specific missions by powerful savants, with the possibility that they might even join a company of comrades to fulfill some irrational goal. It should be noted that Derro, despite their tendency toward insanity, still retain the natural knack for inventiveness shown in dwarves. An example of this is the repeater crossbow, which the creatures may have invented.

History
Legend has it that the Derro originally lived on the surface of Corwyn. They opened portals to Charos; "the Far Realm," which caused many of their race to be driven mad. Their most-hated enemies are dwarves and Svirneblin.