Derro are a crazed race of subterranean humanoids. Their origins are a mystery; they seem to have no real civilization to speak of, living mostly in ancient ruins and caves, and those subjects who have been captured were found to be utterly mad and incapable of rational discourse. They are unique among the various Underdark races in that they are almost universally despised by the other societies that dwell beneath the surface. Even the vile duergar, to whom they bear a superficial resemblance, are known to kill derro on sight. They have been seen with some frequency in the company of aberrations such as grells or mind flayers, but such alliances appear to be entirely one-sided: when such cases have been reported, the derro are almost always used as fodder while their aberrant masters sit safely out of reach of their foes.
Some scholars theorize that the ruins they inhabit are in fact those of their progenitors, long-since destroyed by some cataclysmic event, but there is no solid confirmation for this theory. If true, however, then it is clear they have fallen a long way since the days of their ancestors.