I think every child is convinced of the existence of ghosts and bogey-men.
Being asleep and dreaming used to seem as "real" and vivid as being awake. Dreams were the place where we found monsters to scare us. They came in forms that had no loyalty to the conventions of the day-world of mothers and fathers and siblings and people in general.
(Often these nightmares pursued us. Sometimes through dark woods, in swamps, in poorly lit and derelict mansions, inside our own homes. They might eventually speak, but often what they have to say is incomprehensible.)
Once in a while, monsters from dream crossed the border and would seem to occupy the real world. How many of our beds once had monsters living under them who might creep out of hiding at any time (but especially at night)?
It's a twisted sense of nostalgia that makes me recall the imaginary creep at the bottom of my stairwell. It's true that fear -- in the correct dosage or from the proper distance -- can be pleasurable and even educational.
Zones are realities accessible through magic and advanced psionics. Each zone bears some resemblance to "standard reality" but with a few of its own peculiar natural laws and conditions.
Travel from zone to zone is rare and typically accomplished by sorcerers and other psychic transients. Mastery of esoteric drugs (including exotic jale spindle lotus), meditation and auto-hypnosis are required for a human to successively materialize in another reality.
Travelers tend to congregate in old, odd places. These hubs are typically back-alley operations where cults might be formed, where information and disinformation could be exchanged in equal measure.
There is no color in the weird, monochrome labyrinth of Chune Ch'abbu. Its partially collapsed maze-walls are overgrown with rank black vines and poisonous fungi. At the labyrinth's heart sits the giant white toad, Yattu, the self-appointed ruler of Chune Ch'abbu. The Toad is said to gift magical trinkets to its beloved servants and sycophants. Contact with Yattu has caused many of its followers to mutate into blind, zombie-like phantoms.
The Cult of the White Toad -- a modern revival of an ancient coterie of idolaters -- seeks to coax Yattu back into our reality. No less than twenty-seven virgins must be ritually sacrificed to supply Yattu with the psychic energy necessary to materialize. At least one of the chosen nubiles is a relative of one or more of the PCs. Familial obligation would be the nature of the McGuffin here. The means to subdue and possibly destroy the Toad may be found somewhere in Chune Ch'abbu.