World Shards
A World Shard is what is left of a world after it has fallen to darkness, or the World Heart has left said world without finding a Princess of Heart. It's either a piece of a world, or a world in itself - out there. Sometimes, these World Shards take the form of a giant world in space, filled with darkness. But yet sometimes, they may be found in other worlds; fallen and attracted to the power of other World Hearts.
Think of the Winnie The Pooh book for instance - a shard embedded into a book found in Merlin's House. Other examples for us are Figaro, which has fallen under the desert near Agrabah - and the Cloisters of Spira which have been littered along the worlds.
Some of these World Shards are revived on their own, if enough light remained within that world - but those Shards are rare. Most World Shards are dormant, locked... and they require a special kind of person to Unlock them. A special kind of Key to open that Lock. They need the power of a Keyblade Wielder.