Fallen World

A fallen world is rarely retrievable. The heartless have found their way to its World Heart and devoured it. The hearts of the people and other such entities of this world are sent into Limbo for all sakes and purposes. Only few with strong hearts find their way safely out. Upon its fall, on rare occasions, such a world will also shatter into shards and send them out across the realm.

To return such a world to light once more, one needs to gather all the shards to recreate the world, and then lift it from the dark realm by bringing light to its keyhole once more by use of a keyblade wielder. This venture is exceptionally dangerous, as heartless infest these worlds and the strongest kinds of heartless defend it from any who dare come there.

The only worlds that once fell and have since come back in recent memory are the Castle of Dreams and Manhattan.