From the Dark
Duergars have an ancient culture living underground, deeper than any normal dwarves are willing to go. Their origins are mysterious and their motives more so. They have an inherent distrust of surface races and the feeling is mutual.
Few duergars ever go to the surface. There is no simply no need. And some on the surface believe them connected to the curse. Still, some do travel to the surface, for their own reasons. And those reasons usually bring them to the cursed lands.