A giant blue-litten cavern beneath Oklahoma, inhabited by a human-like race that resemble the Native Americans of the area. The most populous city is Tsath, the capital of K'n-yan. It is named after Tsathoggua, a deity once worshiped there, but later deprecated after the inhabitants found out the true nature of the god
Beneath K'n-yan can be found the red-litten cavern of Yoth, once inhabited by Serpent People, and deeper still is a vast unlit cavern, known as N'Kai - where the Tsathoggua dwells. Yoth's civilisation came to an end when its inhabitants began to worship the god from N'Kai instead of their patron god Yig. The entrances to N'Kai have long been sealed by explorers from K'n-yan.
The world of K'n-yan might have much in common with other tunnels that travel deep into the ground (such as those dug by Ghouls) which end up in the fantastical Dreamlands. Perhaps K'n-yan and its deeper neighbouring caverns are sunk deep into the realm of dreams, rather than physically existing beneath American soil.