Realm of Darkness

The Realm of Darkness (闇の世界) is a Plane of Existence made up of Darkness in people in every world. As with the Realm of Light, this realm has a Keyblade which chose King Mickey as its wielder. Kingdom Hearts is found here. This is also the home of all the Heartless, as this is where the most powerful ones can be found. Sora fought Xehanort's Heartless in Kingdom Hearts.

When Ansem the Wise was trapped here, he is quoted from his journals in saying that a person who is lost here can only stay sane and not turn into a Heartless by thinking about his memories. This could be one of the reasons that Organization XIII is able to walk the Corridors of Darkness freely. Another way he kept himself from turning into a heartless was through the authorship of the Ansem Reports.

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 traveled 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.

Maleficent is also believed to travel the Corridors. Though she does not show the physical changes. This could be due to her trying to command the Darkness, instead of becoming one with it. It is also know that Pete travels the Corridors as well. He is known to be protected by the Darkness by Maleficent's magic.

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.