A demiplane of existence either coexistent or coterminous with the Dimension of Dreams. Incredibly difficult to get to: no known mortal magic will allow for plane shifting there.
In Chapter 93, it was learned that the Bloodwind is able to travel there.
In Chapter 110, the party reviewed the way dreaming 'works'–information they learned from The Chain of Nights–and processed that the Plateau of Leng is actually a dreamscape created one layer below the Dreamlands by an unknown dreamer sleeping inside the dream.
First Reference: Chapter 62
Other Notable References: Chapter 78, Chapter 90, Chapter 93, Chapter 97, Chapter 110, Chapter 123