Wait, sorry, wrong link. My bad.
In D&D 3.5, skum were the piscine servitors of the aboleths. They perform the same role in Pathfinder’s Golarion, but there they have more agency—as the aberrant fish lords have turned inward, they have left the skum more and more to their own devices.
Of course, you don’t even need the aboleths to use skum in your campaign; they’re good amphibious adversaries no matter what. And their need for human sexual partners to reproduce automatically brings them into conflict with surface civilizations. Which means what starts out as an ordinary kidnapping scenario can take a very dark turn (and an especially Lovecraftian one) once your PCs discover that skum are involved.
The skum of Glassmere inherited the worship of the Dark Destroyers from their now-deposed aboleth masters. The irony of subterranean creatures that have never seen the stars worshipping Powers from beyond them is not lost on the fish-men. To remedy this, Glassmere skum make an annual pilgrimage to the surface to gaze upon the blackness of space. Along the way they raid human and svirfneblin settlements for masterwork weapons and breeding stock.
A ceratioidi grande dame of crime has found a new way to recruit heavies into her organization: She implants the parasitic males under the skin of human “recruits”—typically fascinated captives—before caging and lowering them into nutrient-rich baths. The resultant skum provide compliant muscle for the fishy crime family’s more valued ceratioidi members, who have begun to spread their operations through the Bog Cities of Least Coast.
Despite the South having won both the American Civil War in 1863 and the Couatl Rebellion of 1877, the Underground Railroad has endured, with gnome rebels smuggling human and dwarven slaves North year after year. Now though, the Railroad has broken down. Slaves and their conductors have started to go missing, and the disappearances seem linked to the glum, watery-eyed inhabitants of a sleepy Massachusetts town known as Innsmouth.
—Pathfinder Bestiary 253
Alan Moore and Jacen Burrows’s Neonomicon is a decent cheat sheet for a Lovecraftian skum adventure. Be warned, though: He explicitly set out to put Lovecraft’s racism and sexual discomfort on the surface, and he delivered.
Also: parasitic male anglerfish!