Derro were small, twisted Mortalkind of the Underdark who resembled stunted and thin Dwarves. Living to be only one hundred and fifty years old, their skin was a blue-gray hue, their hair often pale or white, and their eyes were uniformly blank, lacking pupils or irises. Though most closely resembling Dark Dwarves, they were uncommonly wiry and agile, able to see in complete darkness and even into ultraviolet light, though sunlight made them sick. Derro appeared haggard and pitiable at first glance, often dressed in ragged clothing, but beneath this appearance they were cunning and dangerous. They favored weapons that could strike from surprise, such as crossbows laced with poison or hooked blades suited to cruel attacks.
Derro were known for their instability, blending moments of craftiness with outbursts of sadistic madness. They were prone to obsessive pursuits, sometimes following bizarre personal quests or irrational missions set by their leaders. Some considered them cousins of dark dwarves driven by a similar ancient Curse, though one that affected their minds even further. The derro themselves all told oft-changing variations of this myth; all versions shared a common focus on a stealing of magic from a great evil to help the derro survive through their ideals of deceit and cruelty.
Derro often lived in close proximity to dark dwarves, though usually as a lower class or tolerated minority within their cities. This proximity created a complicated relationship, where mistrust and contempt were common but necessity kept them tied together in work, trade, and survival in the dark.