Derro

The Derro are a twisted and degenerate branch of the dwarven race, who live in the deepest, darkest bowels of the earth. They hate and shun all other living creatures, and are hated and shunned by them in turn. Though they retain the natural toughness and cleverness of dwarves, they long ago lost their martial fortitude and straightforward vitality. Instead they are creeping and half-blind, cowardly and vicious, hunting by ambush, fighting by trickery. They are said to be cannibals who will happily feed on their own kind or anyone or anything else that falls into their clutches. They are also masters of all manner of traps and pitfalls, and of many poisons, which they use frequently. Upright dwarves view them as pathetic and loathsome in equal parts, and if they find them, will kill them as much out of pity as anything else. The Derro, like the Duergar, are few in number and their reclusive habits mean that even other dwarves (let alone humans) will likely never encounter them. 
The origins of the Derro are not well-chronicled, but they are descended from dwarves who were captured and enslaved by the vile drow in ages past. Right-thinking dwarves will usually kill themselves before being taken prisoner by the drow, but the forefathers of the Derro did not, and as a result they and their offspring spent unhappy generations in captivity. How some or all of them came to escape is not known, but by the time it happened, they had been broken in both body and mind by countless torments. Some say the drow allowed them to scurry off into some forgotten tunnel when their usefulness as slaves was at an end; if so, this was likely done in the expectation that they would soon fall prey to the hunting horrors of the deeps. Others say that they broke the ultimate dwarven taboo and took up the worship of Korgul, which they learned from orcs who were likewise slaves of the drow. In this telling, Korgul promised to deliver them to freedom, only to laughingly lead them into some trackless maze of caverns and abandon them there, for of course the god of the orcs has no use for either the prayers of the weak or the offerings of dwarves. Yet another telling has it that the derro took up the worship of some other foul deity or spirit of the darkness, little known to the world, which truly did free them from drowish shackles in exchange for their veneration.
The root of the name "derro" is likely unclear, but it may well stem from some derivation of "drow", in which case the very name of these miserable creatures would be an unwelcome remnant of their ancestor's degradation by the sadistic dark elves.