Lovecraft first set Leng down in “The Hound” (1922) as a plateau in Central Asia where a corpse-eating cult lived, then relocated it in “The Dream-Quest of Unknown Kadath” to the far north of the Dreamlands. Which is correct was never settled. The Men of Leng live there, hooved and hungry, alongside the enormous spiders they have fought for ages, and in one prehistoric monastery the High Priest Not To Be Described sits alone.
What has kept the plateau in use is not its monsters but its position. Leng refuses to be fixed on a map. In one story you can walk there; in another you must fall asleep first. A place with no settled location exists only in the records of those who reached it — and those records disagree.