Duergar (pronounced: DEW-ur-gar or: DWAIR-gar), also known as gray dwarves, or underdwarves were a subterranean subrace of dwarves. They carved out an existence in the Underdark, often near volcanoes. Their kinship to other dwarven subraces could be compared to that of a Drow to a surface Elf.
Consider that the duergar began as homeless outcasts, and today their fortresses are some of the most impregnable strong points in the Underdark. The question might not be if they will conquer the realm below, but when.
— Mordenkainen
Description
Like other dwarves, duergar were typically stocky figures, though beyond this there were many differences. Both male and female duergar were typically bald, with females also lacking the capacity to grow facial hair. Many were also thinner than their dwarven brethren. Most obvious, however, was their dull gray skin and hair, often matched with an equally stolid expression. Regular dwarves said they had flat heads, possibly as an insult.
Because many duergar found on the surface world were criminal exiles, a surface dweller who encountered one of the gray dwarves was likely to notice facial and arm tattoos that marked the duergar as a traitor to his or her people.
Personality
Tyrannical, grim, industrious and pessimistic, the lives of the gray dwarves were bleak and brutal. Rather than a flaw, however, they viewed their lack of happiness as their greatest strength, the defining feature of duergar pride. The duergar saw themselves as the true paragons of dwarvish ideals compared to their weak and pampered kin, but in truth, their ways were a dark reflection of those found in normal dwarves.
While they did display the redeeming virtues of determination and bravery, duergar took dwarven flaws to their logical extremes. They were violent and hateful, sullen and insular, greedy and ungrateful, deeply cynical of others' motives and dutifully tracked and nursed every grudge, whether or not any offense was meant. Though their vices were many, the moral failings of the gray dwarves could be traced to three primary principles: bottomless greed, unceasing conflict, and rejection of emotion.
Abilities
Duergar were in some ways even better adapted to underground living than other dwarves. While other dwarves lacked the capacity to see completely in the deepest darkness, this was not a problem for duergar. Duergar were also immune to many of the ancient techniques used by the Mind Flayers to control them, such as paralysis, phantasms, poisons, or some types of illusion.
Duergar were also a sneaky, crafty people, unlike their honor-bound cousins, and often excelled at setting up ambushes or moving out of sight. Conversely, many were also good at detecting hidden objects. A few duergar also possessed natural abilities akin to the enlarge and invisibility spells. This came at a cost, however, and duergar, like drow, had a special vulnerability to sunlight.