Land of Dreams (Endhaven Supplement)
From D&D Wiki
Where do you go when you dream? Why are dreams prophetic? Are dreams a place? Who are you when you dream? Are dreams from Rem, the Goddess of Dreams? Or are they her world? Or does she merely visit them, too?
There is no known world without dreams.
The Land of Dreams may only be visited while sleeping. In this land, the world is both less and more real than it ever was. The visitor may gain knowledge and prophecy.