Skulks originally descended from a tribe of nomadic humans who had wandered Calimshan after the defeat of the Djen. They were abducted by drow and forced into servitude in the city of Guallidurth for centuries, their descendants kept passive by their fear of the darkness surrounding them. They finally escaped when a giant lizard appeared from the darkness, drove off the drow overseers, and led the slaves away. The lizard was an avatar of Ibrandul known as the Stalker and through it, the humans learned his worship.
While some of these men and women returned to the surface to spread Ibrandul's faith, the rest remained in the Underdark, surviving by raiding the farms and caravans of Guallidurth as well as those of human settlements on the surface. Over time (and continued use of a particular spell provided by their god) these people became the known as skulks and could thereafter be found living near many entrances to the Underdark.