Leng is originally a Lovecraft creation (my standard disclaimer about my lack of familiarity with the Mythos goes here. The little we know about Pathfinder’s Leng is tucked away in Pathfinder #6: Spires of Xin-Shalast (courtesy of Greg A. Vaughan), the Bestiary 2, and The Great Beyond (though it appears we might learn more as the Shattered Star Adventure Path progresses). But we have glimpses of an arid, arctic plateau that exists half in the realm of dreams and madness, where the humanoid denizens of Leng and Leng spiders eternally war.
All well and good. But what if you want to go beyond the Leng spiders of legend? There’s always a reason for scary CR 14 spider-monsters, especially ones that spin their own illusions and weapons…
The mages of the Collegium have long dismissed as sophistry any theories linking phase spiders and the more mysterious spiders of Leng. But when blink dog sages the world over begin to have nightmares about asymmetrical spiders from another world, these same mages grow concerned—for if the Leng spiders learn the phase spiders’ ability to ethereal jaunt, the world could be doomed. Unraveling the mystery will find the adventurers facing off against blink dog apostates, night hags of the Ethereal, and perhaps even the norns…not to mention the denizens and spiders of Leng itself.
Arachnids used to be part of the portfolio of an ancient goddess of crafts know known only as the Weaver. When she was slain by a drow goddess and her urdefhan champions, the trauma of her death throes turned the Weavers’ high priestesses into the first maker spiders. Instead of spinning sacred patterns, wise tales, and the knowledge that women share as they work the shuttles, the subterranean maker spiders craft weapons, illusions, and grisly deaths in their eternal war against the drow and the urdefhans.
The City of Sleepers is an impossibly tall city of terraces and spires that floats at the border of Air and Dreams. Djinn, leprechauns, sylphs, sirens, brass dragons, and even relatively benevolent lamias and caulborn relax and dream in lush gardens and orchards tended by animate dreams. No one (but mortal visitors) tends to notice when dreams go missing…or asks about the spider-webbed cocoons hidden in the City’s many forbidden spires.
—Pathfinder Bestiary 2 176