Leng

On Golarion

The runelords of Thassilon were aware of Leng and often conjured its inhabitants to do their bidding; Karzoug the Claimer even built his capital, Xin-Shalast, in a location where the border between Leng and the Universe was unusually thin to take advantage of the dimension's eldritch energies.
Knowledge of Leng on Golarion is sketchy at best, limited to glimpses in nightmares and drug-fueled visions and its origin is unknown. Some believe it to exist physically on the Universe or another dimension. Others claim it is a surviving piece of a dead plane from before the current multiverse. Yet others believe that Leng is the nightmares of mortal dreamers shaped into a physical place.

What Fay Knows

Leng is a remote location on another plane, one mostly unknown to Golarion’s brightest scholars but referred to obliquely in certain ancient and blasphemous tomes. The runelords of Thassilon knew of it and often drew creatures from its frozen, inhospitable reaches to do their bidding. Leng is a desolate realm inhabited by semi-human cannibals, immense intelligent spiders, flying draconic creatures, and other horrific monsters. Immense mountains, some of which seem impossibly tall, hedge in Leng from every side.