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, but they come into conflict with the Dream Eaters and a mysterious young man with ties to Master Xehanort.

Eventually, Sora and Riku manage to seal the seven keyholes in the Dreaming Realm, but thanks to a set-up by Organization XIII, Sora ends up trapped in the World That Never Was and his heart is damaged during a fight with Xemnas. With help from Mickey, Donald, Goofy and Lea, Riku manages to save Sora from becoming a new vessel for a fragment of Master Xehanort's heart and they return safely back to the Realm of Light.