From publisher blurb:
The duergar, often called grey dwarves, live well-hidden in the darkness of the cold ground, revelling in mischief and evil deeds of all sorts and despising their cousins still living on a surface world now forbidden to them. Once a mysterious people, more has been discovered about their way of life, and sages hope that with the information provided by their texts one will be able to understand the intrinsic nature of these creatures and be aware of the danger they represent.
Duergar are masters of treachery and deceit and are engaged in an everlasting struggle with the other races of the underdark in their search for dominance over those realms. Drow are their fierce competitors in this domain, but duergar have surrounded themselves with such ingenuity that even dark elves have learned to respect them as capable adversaries. Duergar retain much of the dwarven race’s traits even if their metabolism has changed somewhat to adapt to their environment, and their communities have evolved into a well-oiled fighting power to be reckoned with. Duergar are not to be taken lightly. They are cunning and intelligent and, most of all, can be very unpredictable.