Yian is a fabulous city created by Robert W. Chambers for his short story "The Maker of Moons". It is mentioned in The Whisperer in Darkness, by H. P. Lovecraft, and "The Isle of Dark Magic", by Hugh B. Cave, although in the latter it might be the name of a being, as opposed to a location.
Description[]
A woman named Ysonde, raised in Yian for the first 16 years of her life, described it as a beautiful city, "sweet with perfume and the sound of silver bells all day long". She says that in Yian, "all the gardens are sweet and the river flows under the thousand bridges". Of the daily life of the inhabitants she knows little, as she was raised by her "stepfather", the legendary sorcerer Yue-Laou, who didn't allow her to leave her garden and interact with other people. From her garden, however, Ysonde could see the thousand bridges "and the white mountain beyond", and could see that millions of people lived in Yian.
The city's language is unknown, but it's not English. The main god worshiped in Yian is called Xangi, but there are also other deities, possibly including Yue-Laou and the Xin. The colour yellow is considered a symbol of faith in Yian.
According to Ysonde, Yian is located "across seven oceans and the great river which is longer than from the Earth to the moon". However, American Colonel Franklyn Barris, who has himself lived in Yian for years, dismisses this as "the allegories of another land". He claims that Yian "lies hidden in that gigantic shadow called China, vague and vast as the midnight Heavens,--a continent unknown, impenetrable". Nevertheless, Barris admits that Yian is the centre of activity of the Kuen-Yuin: a powerful society of sorcerers led by Yue-Laou, who appears to have the power to create and/or access places that are otherwise unreachable, including a house and a glade in Cardinal Woods that could never be found by anyone not allied with Yue-Laou (ADJ: "The Maker of Moons"). As such, the possibility that Yian might be only accessible by magical or supernatural means can't be ruled out.
Yian is one of the many things that Albert Wilmarth learned about while studying ancient myths that might be related to the Mi-Go or the entities worshiped by them. (HPL: The Whisperer in Darkness)
Behind the Mythos[]
It's not known whether Yian is related to the "shunned and dreaded" city of Yian-Ho, which is mentioned in "Through the Gates of the Silver Key" and "The Diary of Alonzo Typer". If so, it might also be associated with the Ancient One and the Plateau of Leng.