Leng is a fictional cold arid plateau in the Cthulhu Mythos, whose location seems to vary entirely from story to story. The Plateau of Tsang, referenced by H. P. Lovecraft and other authors, is probably a region of Leng.
Abdul Alhazred describes it as a place where different realities converge, which might explain why its precise location cannot be pinned down.
Appearances in Lovecraft's work
Lovecraft first described Leng in "The Hound" (1922) in which the dreaded Necronomicon places it in Central Asia and says it is inhabited by a human corpse-eating cult.
In At the Mountains of Madness, an expedition from Miskatonic University explores a plateau in Antarctica and discovers an ancient and apparently abandoned city built by the Elder Things. One member of the expedition, who has encountered references to the Plateau of Leng in ancient texts, forms the hypothesis that the plateau they are exploring is Leng. In common with the High Priest's abode in The Dream-Quest of Unknown Kadath, the walls of the buildings atop the plateau are covered with detailed frescos that are disturbing to read. However, it is never explicitly stated in the text — or in any of Lovecraft's later works — that this Antarctic city actually is Leng. In fact, it seems more likely that the city is simply an outpost of the Elder Things which came to Earth not to conquer but to live in isolation.
In Stephen King's novel Needful Things, Mr. Gaunt gives Ace Merrill some cocaine said to be fabricated in "the plains of Leng", though no other explanations are given. The novel also contains other references to Lovecraft's work. It is mentioned again in his novel The Eyes of the Dragon, where it is described as the place where Flagg's spellbook was written, by a man named Alhazred. This implies that Flagg's spellbook is the Necronomicon itself.
In Brian Lumley's Cthulhu Cycle Deities Novels the plains of Leng are supposed to be located in Earth's dreamland.
In Alan Moore's Neonomicon the plateau of Leng is described as a projection into a higher mathematical space, which makes up the universe as observable by humans.
In Darrell Schweitzer's short story "The Adventure of the Death-Fetch", featured in the anthology The Improbable Adventures of Sherlock Holmes, a mystery revolves around an expedition to the Plateau of Leng.
In the comic series Locke & Key, demonic Lovecraftian spirits from beyond a buried stone doorway are known as the Children of Leng.
^Abnett, Dan (2009). "Blood Games". In Kyme, Nick & Priestley, Lindsey. Tales of heresy (print). Horus Heresy [book series] 10. Cover art & illustration by Neil Roberts (1st UK ed.). Nottingham, UK: Black Library. pp. 9–60. ISBN978-1-84416-683-1. [Context at pp. 20, 27].