Duergar

Duergar are a sub-culture of Dwarves, considered to be "distant relatives" that split off centuries ago and underwent dramatic cultural changes. The Brightblaed Clan is currently heading a plot that involves the destruction, or dominion over, the Ten Towns. They have operated covertly enough to not draw the ire of most of the Ten Towns, and those who have noticed are doubted. Speaker Danneth, particularly, was not trusted when he called for aid.
The Duergar of Icewind Dale seem to come from The Spine of the World. Their clan leader resides in Father's House. The plans are still not entirely understood. However, some rumors gathered by The Naughty Boys indicate the ongoing construction of a mechanical Black Wyvern or Dragon out of the rare Shardulin substance.
Mountain Dwarves known them as an uncommon nemesis, and often have battle tactics passed down among generations in case of an underground Duergar breach. It's likely that a Mountain Dwarf or his father has encountered a Duergar once in their life. To Hill Dwarves, they are simply boogeymen- rarely seen enough to count as a threat, but known as a horrible legend of "The Clan That Dug Too Deep."
The first mention of the Duergar were mentioned during the séance to The White Lady, who refers to "Invisible Dwarves" towards Easthaven's Ferry. Gurney know them as boogeymen, and "The Clan that Left Moradin." Keth knew of their psionic descent from Illithids. Ambrose knew particularly that they enslave Half-elves.