Seven Kingdoms: Seowyn's Crossing
During the Dawn War, the race of the Dwarves languished in slavery to the Giants in the frozen northlands of Jotunheld. Some, like Balgon Fivebraids councilled patience, knowing that The Thanes of the Five Halls would give them a chance to strike back when all was in readiness.
Some dwarves, however, were not content to wait. Turning from the Gods that had created them, they sought aid from darker powers, wanting a more immediate freedom and vengeance. Some turned to Asmodeus, himself now a captive god, and pleaded with him for the power to overthrow their captors. The Lord of Devils agreed, and the dwarves now sworn to him were transformed into Duergar, earning their freedom a century or more before Fivebraids led his followers south.
Or so they thought.
After slaying their former masters, the gray-skinned dwarves found the cold, blindingly bright lands of Jotunheld too hostile to them, so they retreated underground. Guided by Asmodeus, they found their way down into deeper places, eventually into the Underdark, where they began to toil and dig.
With time, the duergar have come to realize that they have only exchanged service from the giants to Asmodeus, and that they’re as firmly enslaved by him as they ever were. In a gesture of terrible irony, they have taken strongly to the slave trade themselves, setting themselves up as some of the premier flesh-merchants of the Underdark, hating their dark new god, even as they adore and serve him.