Leng (or Y'Pawfrm e'din 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.
Read more about Leng: Appearances in Lovecraft's Work, Other Mentions