From Ardrana

Duergar, or Gray Dwarves, are a malevolent breed that exist at extreme depths underground. Duergar appear to be emaciated, nasty-looking dwarves. Their complexions and hair range from medium to dark gray. They prefer drab clothing designed to blend into their environment. In their lairs, they may wear jewelry, although such pieces are kept dull.

Duergar have infravision, and speak the duergar dialect of the dwarven tongue, "undercommon" (the trading language of subterranean cultures), and the silent speech employed by some subterranean creatures.

On Ardrana, duergar are something of a scourge. They are found in nearly any underground area not inhabited by Dwarves, and some that are. Often, they will drive other creatures out, as they consider themselves the true owners of all the UnderWorld.

Note: Duergar originally appeared in this form in the 2nd Edition Monstrous Manual for the AD&D game from TSR. Their use here is for the purposes of providing context for the campaign only.