Duergars are indeed Grey Dwarves (their very common name, based on the color of their skin), but can also understandably be called dark, due to the fact that they are almost universally of Evil alignment.
spydar, the "hole" that you're looking for is in the eastern end of the Svirfneblin settlement. Go up a corridor and talk to the Svirfneblin guarding a door, that he wouldn't let you through before. Enter and do your stuff. Hope you've got a +3 weapon.....
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Volothamp's Comeuppance
Everything you ever needed to know about the entire Baldur's Gate series......except spoilers.
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