Tempting as it is to simple do a version of the Corbitt House from the "The Haunting" (aka "The Haunted House"), the original Call of Cthulhu RPG scenario, I need to do a map for my current game that'll fit the theme, so I'm doing that.
In "The Mound", one of the stories Lovecraft "co-authored" while doing the vast majority of the work, the reader learns of K'n-Yan, a vast underground realm, lit with a blue radiance and inhabited by an ancient, immortal, and decadent race of humanoids. Also, down here, thousands -- millions -- of years of biological experiments, psychic power, and sorcery have created beings that serve various roles as domesticated beasts or slaves. Additionally, they can reanimate the dead to serve as telepathically-controlled zombie-like creatures. While the main region is under Oklahoma, with the population largely moved to their capital city, my focus is down in Texas, not far from the Mexican border, where my players' characters have gained access down to the blue-litten realm of K'n-yan. Here they will find R'desh, once an important town engaged in scientific research. But it is all but vacated, the inhabitants long-departed, save one.
R'desh is haunted by Glot-tesh, the Y'm-bhi, and the Gyaa-yothn. Additionally, the people of K'n-yan were possessed of great psychic power. Some potent psychic impressions may linger, still active after millennia. Glot-tesh is a man of K'n-yan, a biological and cybernetic researcher who reach decadence more slowly than most, focused on his work. He grew alienated from his fellow citizens as they drifted away and now lingers. Like all of his people, he lives in a half-spirit state, capable of dematerializing himself and spending much of his time wandering the Dreamlands, though intruders in the city will get his attention soon enough. The Y'm-bhi are reanimated dead, serving various functions, or standing forgotten where they last worked centuries ago. The Gyaa-vothn are horrific creatures built like beasts but with human or anthropoid features and a single horn growing from the forehead.
The area around R'desh contains a number of old experiments of various types (the player-characters have already encountered old products of K'n-yan's weird science, somewhat hybridized and cybernetically augmented bears, coyote-wolves, and a mountain lion) as well as a tribe of degenerate humanoids of the troglodyte caveman variety. These last will have a witch-queen, though I haven't decided yet whether she'll be a terrifying old crone centuries-old or full pulp-fiction anomaly, not only of superior intelligence but also beautiful to the surface-human's eye.
As I've been going through Lovecraft and other Mythos readings lately, I have come to realize that Gygax's Lovecraftian influence is shown as much in his penchant for underground expeditions as in the various horrible elder gods imprisoned around the world of Greyhawk awaiting their release to wreck havoc on the world. It's surprising that there seems to be little game material devoted to K'n-yan, the original Underdark.
For the challenge, I'll be producing a map of R'desh itself, though I'll have to work out additional maps of the surrounding caverns and some of the more important buildings for game use.