Duergar, otherwise known as dark dwarves or gray dwarves, are a malevolent breed that typically exist at extreme depths underground. They appear to be emaciated, nasty-looking dwarves, with complexions ranging from medium to dark gray. They prefer drab clothing designed to blend into their background. Duergar sometimes wear jewelry, although such items are usually dull. Duergar society is very harsh, because of the hostile environment in which they are usually found. They are more intelligent and dexterous than their dwarven cousins, but usually physically weaker and less hardy.
Duergar have infravision, from many years living in the dark, and are very resistant to poison, disease, and mental attacks. Like their dwarven counterparts however, they are very vulnerable to drowning, and dislike water very much. In addition, because of their many years living in near complete darkness, Duergar are very vulnerable to light and attacks based on light. In fact, like Drow, Duergar will even tend to recover from wounds and fatigue faster in darkness than in light. Duergar are also well practiced in sneaking and hiding, and get these skills for free.
Duergar are good at a wide variety of professions, but tend to be best as thieves, mages, spellfilchers, and even warriors in some cases. They speak the same tongue as their dwarven relatives, but do not get along well with them at all, often attacking each other on sight. Other races are not often friendly to them either, but that is alright with the Duergar, who'd rather not deal with them whenever possible anyway.
Duergar have a common past with dwarves, but none of the races like to talk much about that.