K'n-yan is a blue-litten cavern beneath Oklahoma. It is inhabited by a human-like race that resemble the Native Americans of the area, though they are actually extraterrestrials who arrived in prehistoric times. They are immortal and have powerful psionic abilities, including telepathy and the ability to dematerialize at will. They are also technologically advanced, using machines that employ principles of atomic energy, though they have largely abandoned their mechanized culture finding it unfulfilling.
The most populous city is Tsath, the capital of K'n-yan. It is named for Tsathoggua, a deity once worshiped there, but later deprecated after the inhabitants found out the true nature of the god. Other deities include Shub-Niggurath, Nug and Yeb, Ghatanothoa, and the Not-to-Be-Named One (Hastur?). The two most important ones, however, are Tulu (Cthulhu) and Yig. The denizens of K'n-yan often place idols of these deities in near proximity, as in the following passage from "The Mound": "[In] a pair of vast niches, one on each side, [the] monstrous, nitre-encrusted images of Yig and Tulu squatted, glaring at each other across the passage as they had glared since the earliest youth of the human world.
In ancient times, the people of K'n-yan traded with the humans of the surface world. But when geological calamities caused the continents of Atlantis and Lemuria to sink into the ocean, the people of K'n-yan sequestered themselves below ground, thereafter having no further dealings with the outer world.
When the denizens of K'n-yan rejected their mechanistic lifestyle, they turned to a sort of socialistic aristocracy, controlled by a ruling class made "highly superior through selective breeding and social evolution. Individual behavior is dictated more by established social norms than the rule of law. Although K'n-yan attained great advances in art and science, its people have become increasingly hedonistic, decadent, and cruel.
Yoth is a red-litten cavern beneath K'n-yan. It was once inhabited by the Serpent Men who fled here to escape the destruction of Valusia. In Yoth they built great cities, of which only ruins remain. Explorers from K'n-yan visited Yoth frequently to learn more of the serpent people's scientific lore.
The downfall of the serpent people came when they brought up idols of Tsathoggua from N'kai, abandoning their patron deity Yig to worship their new god. As retribution Yig placed his curse upon them, forcing his few remaining worshipers to flee to caverns beneath Mount Voormithadreth.
Explorers from K'n-yan met with disaster when they first visited N'kai, encountering the deadly, amorphous spawn of Tsathoggua. Thereafter, entrances to N'kai were sealed off.