The mysterious deep grey dwarves are as notorious as the drow, but perhaps not as well known on the surface.  Any mountain dwarf can tell you, however, that the duergar pose just as much threat to the security of goodly races as the drow. The duergar are responsible for much of the mining, tunnel-making, and fortress-creation in the deepest regions of the Underdark.  Duergar expeditions are frequently sent out to find new ore resources, treasure, tactical locations for outposts, and slaving raids.  They are strong, fierce, stalwart, and extremely crafty.  Rather than challenge each other for superiority, the drow have reluctantly left duergar-controlled regions alone for fear of a war they cannot win.
DUERGAR RESOURCES

Duergar Humor
Duergar Info

Available related .pdfs

Slayer's Guide to Duergar
Races of Faerun
Thankfully the duergar rarely make their way to the surface, prefering to mine, craft, smith, and create twisted metal and stone-works in the darkest regions of the Underdark.  Most of the foulest and most powerful artifacts in the Underdark originate from duergar craftsmen. 
     Duergar presence in the Aglarond region includes:

Gracklstugh - duergar capitol in the region
The Iron Ring - a slaver consortium which the duergar "run"
Delvesdeep - the great Underdark market, which the duergar use
      to sell most of their wares and slaves
Duergar are a psionic race, possessing innate powers and abilities of the mind.  This has led to frequent conflicts with the illithids since they cannot be easily mentally dominated by the masters of psionic manipulation.  Wary travelers in the Underdark know of the duergar tactic of using their innate invisibility powers to ambush.  They may also grow to great size making them formidable foes.  One does not survive long, or keep their freedom, if they cannot deal with duergar war parties.

As an interesting side-note, if one can make an ally of the duergar, they find the deepest and most secret regions of the Underdark open to them.  They also know of, and control, many of the secret tunnels leading from one region to another.  This may be one reason the Deephammers' primary targets in the deep are the duergar fortresses.
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