Like drow and duergar, dark folk are—possibly?—humans (and halflings?) who sought shelter underground and were irrevocably changed by the experience.
While attacking morlocks in the deeps, a party of adventurers unwittingly saves a gang of dark creepers, who swear their loyalty to the surface-dwellers in thanks. For the next 48 hours, they serve as admirable guides, porters and foot soldiers—right up until the moment when a dark stalker appears and calls them to heel. They obey immediately, and then immediately attack the party to boot.
The beggars of Hornmoth and the dark creepers in the sewers below have come to quiet understanding, sharing the rags and discards of the wealthy city. When beggars start to go missing, are the dark folk to blame, or are they victims as well?
Perched on the edge of the swampy moors of Perrin, Night Town is two cities in one. By day, it is the lone large town in a backwater land, a place for farmers, trappers, tinkers, and swamp runners to eke out simple lives in a bleak landscape. By night, it is the one place where dark creepers live aboveground, carrying on much the same as their daytime compatriots. Being poor, the custom in Night Town is to share dwellings—humans and halflings work through the day, then return home just as their dark folk co-tenants are beginning their night. Despite rarely even speaking the same language, the people of Night Town make their system work—a system of hand signs, scrawled pictograms, and dogged persistence bridges the gaps where language fails. Unusually, there are no dark stalkers or slayers mingled with their smaller kin; in fact, Night Town’s dark creepers fear and avoid their cousins.
—Pathfinder Bestiary 53
You can tell that the Paizo folk love dark folk (or dark ones, as they’ve also been known). I’m pretty sure they appeared in 3.5’s Shackled City Adventure Path (along with a number of other Fiend Folio selects), I know they showed up in the Downer comic, and they certainly got catapulted into first-volume respectability with their inclusion in the Pathfinder Bestiary.
By the way, Night Town is inspired by a Dragon Magazine story, but I’ll have to do some digging to remind myself which one.