Sleeping Worlds

The Dreaming Realm is the main setting of Kingdom Hearts 3D: Dream Drop Distance. It was created following the defeat of Ansem, Seeker of Darkness, as a portion of the worlds attacked by the Heartless remain submerged in sleep. As this created barriers not even the Heartless could penetrate, a new form of enemy emerged in the Dreaming Realm, known as the Dream Eaters.

Kingdom Hearts 3D: Dream Drop Distance
As part of their Mark of Mastery exam, Yen Sid sends Sora and Riku to the Dreaming Realm to unlock the seven Keyholes of Sleep, and return the worlds to the Realm of Light.