Realm of Darkness
The Realm of Darkness (闇の世界) is the second half of the Kingdom Hearts universe and is made up of Darkness. As with the Realm of Light, this realm has a Keyblade which chose King Mickey as its wielder. Kingdom Hearts is found here.
Corridor of Darkness
The Corridor of Darkness is a pathway in the Realm of Darkness that is used by Naminé, Riku, and Organization XIII to travel between worlds.
Xehanort and Ansem the Wise used the Corridors of Darkness for travel, too. Anyone who has travelled in darkness for too long would show a few physical traits: their skin darkens and their eyes turn amber. This is especially true for Xehanort, whose Heartless and Nobody both have tan skin and amber eyes. In the case of Ansem the Wise, it is not so severe: he only has amber eyes.
Finding the Box
After being saved from Saïx by Maleficent in Hollow Bastion, Sora, Donald, and Goofy are transported to the Realm of Darkness, where they encounter a man in a black cloak, later revealed to be Riku. Riku leaves a box with a picture of Hayner, Pence, Olette, and Roxas and a bar of sea-salt ice cream. In this same realm, the Keyblade reacts to the ice-cream and Sora opens a pathway. This is the first time that Sora, Donald, and Goofy are able to make their way to the Realm of Darkness in Kingdom Hearts II.
