K'n-yan (or Xinaián) is a fictional, subterranean land in the Cthulhu Mythos. The underground realm was first described in detail in H. P. Lovecraft's revision of Zealia Bishop's The Mound (1940), in which it is discovered by the 16th century Spanish Conquistador Zamacona. Lovecraft also mentions K'n-yan in The Whisperer in Darkness (1930) and in his revision of Hazel Heald's "Out of the Aeons" (1935). The people of K'n-yan are sometimes referred to as the "Old Ones", a term of variable meaning in Lovecraft's fiction.
K'n-yan is a blue-lit 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 (a title sometimes used to identify 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.
The people of K'n-yan also utilize undead slaves, reanimated from the bodies of ancient enemies or inferior members of the ruling class. They attained immortality and subjugated other races before them thanks to their technology and ability to dematerialize and rematerialize at will. The underground people also customarily engage in several forms of sadism, depraved practices, ritualistic orgies and unspeakable horrors such as extended public torture, random body modifications and mutilations of other slave species as entertainment, in order to gratify their time-dulled senses.
Despite their advanced technology and mental powers, the people of K'n-yan usually dress in ancient fashion and are only equipped with melee weaponry for their protection.
Yoth is a red-lit 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. 
- Harms, Daniel (1998) . "K'n-yan". The Encyclopedia Cthulhiana (2nd ed.). Oakland, CA: Chaosium. pp. 169–70. ISBN 1-56882-119-0.
- Joshi, S. T. (1989). "Lovecraft's Alien Civilizations: A Political Interpretation". Selected Papers on Lovecraft (1st printing ed.). West Warwick, RI: Necronomicon Press. ISBN 0-940884-23-2.
- Lovecraft, Howard P.; Zealia Bishop (1989) . "The Mound". In S. T. Joshi (ed.). The Horror in the Museum and Other Revisions (4th corrected printing ed.). Sauk City, WI: Arkham House. ISBN 0-87054-037-8.CS1 maint: Extra text: editors list (link)
- Lovecraft, Howard P.; Hazel Heald (1989) . "Out of the Aeons". In S. T. Joshi (ed.). The Horror in the Museum and Other Revisions (4th corrected printing ed.). Sauk City, WI: Arkham House. ISBN 0-87054-037-8.CS1 maint: Extra text: editors list (link)
- Price, Robert M. (1997). "The Mythology of Hastur". In Robert M. Price (ed.). The Hastur Cycle (2nd ed.). Oakland, CA: Chaosium. ISBN 1-56882-094-1.CS1 maint: Extra text: editors list (link)
- Lovecraft may have derived the name K'n-yan from Robert W. Chambers' Kuen-Yuin, a mystical international brotherhood from Chambers' The Maker of Moons. (Price, "The Mythology of Hastur", The Hastur Cycle, pp. iv–v.)
- According to S. T. Joshi, this view reflects Lovecraft's own disgust for the mechanization that was changing the character of his own society. (Joshi, "Lovecraft's Alien Civilizations", Selected Papers on Lovecraft, p. 7.)
- Lovecraft & Bishop, "Out of the Aeons", The Horror in the Museum and Other Revisions, p. 276.
- Lovecraft & Bishop, "The Mound", The Horror in the Museum and Other Revisions. p. 152.
- Lovecraft & Bishop, "The Mound", The Horror in the Museum and Other Revisions, p. 134.
- Harms, "Yoth", The Encyclopedia Cthulhiana, p. 348.
- Harms, "N'kai", The Encyclopedia Cthulhiana, p. 213.
- Cthulhu in Mesoamerica, an essay by Richard L. Tierney