The Realm of Nothingness
The Realm of Nothingness, also known as the Realm of Twilight, is along with the Realm of Light and the Realm of Darkness one of the three main parts that make up the universe. It is not a dimension similar to Slipspace or the Warp, nor is it another universe; it is an integral part of the very universe itself. The Realm of Nothingness does not truly exist, in a way, for it is nothing. As the Sky King once aptly explained: "Consider it in a mathematical way with the Realm of Light being positive numbers, the Realm of Darkness being negative numbers, and the Realm of Nothingness being zero."
The Door to the Realm was found by the Sky King two thousand years ago when renovating the Castle That Never Was to serve Edolas' purposes, and it has been used for training, to test dangerous technology, and to self-isolate the Sky King in order for him to think about how to solve problems ever since. Time in this Realm passes much slower than in the Realm of Light, for just as the Realm of Darkness warps space, this Realm warps time. What people see when entering the Realm of Nothingness, grey and white moving patterns which all move towards bright swirling voids, is what life invisions the Realm to be like; it is not truly representative of the realm itself, as -again- it does not exist.
The Realm is home to strange creatures called Nobodies, who are mainly the souls of ancient Ventus placed there during the Great Daimao War for unknown reasons, but can also be from other races and empires. They come in many different classes, Dusk, Dragoon, Sniper, Samurai, and Sorcerer among them. The most powerful of the Nobodies are the enigmatic Twilight Thorns, who possess great power; but at the cost of sanity. The intelligence of a Nobody is determined by their class, but Nobodies generally are not capable of feeling emotion due to their lack of hearts. Keyblades can sometimes circumvent this, for an unknown reason. The Sky King is capable of using these Nobodies for training purposes, as they follow his commands for an unknown reason.
