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The Realm of Darkness is a realm made up of Darkness in people in every world. As with the Realm of Light, this realm has a Keyblade, Kingdom Key D, 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 Ansem, Seeker of Darkness here in Kingdom Hearts.
Sora, Donald, and Goofy visit the Realm of Darkness after being saved from Saïx by Maleficent in Hollow Bastion, where they encounter a man in a black coat, 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.
Corridor of Darkness
Corridors of Darkness are unpredictable pathways that interlink the worlds of the Kingdom Hearts universe. The Heartless use these extensively to reach other worlds, as do members of Organization XIII, their subordinate Nobodies, and many other characters. These pathways are formed out of Darkness, so it is unsafe for a normal person to travel them, because the constant use of these corridors would corrupt one's heart. However, the Heartless, the Nobodies, and the Princesses of Heart don't have to worry about this, since the Heartless already have corrupted hearts, the Nobodies do not have hearts to corrupt, and the Princesses of Heart have hearts made out of pure light. An example of the Princesses of Heart's immunity would be, when Axel was kidnapping Kairi, he took her through a Corridor of Darkness. Kairi was wearing normal clothes, yet she wasn't affected by the Darkness, as her heart was made out of pure light. One upside to the Darkness in the Corridors is that it provides a faster means of travel than the Gummi Ships.
Other characters who have used these corridors include Sora, Donald Duck, Goofy, King Mickey, Kairi, Riku, Naminé, DiZ, and even Pluto. Of these, only Riku, Naminé and DiZ have been responsible for summoning the portals, and only Heartless, Nobodies, and Princesses of Heart aren't affected, with two noted exceptions; according to Nomura in the Director's Secret Report XIII, Riku and Mickey aren't affected by darkness at all, due to some "secret similarities". Ironically, in the first Kingdom Hearts, after the face-off in Neverland, Riku's heart was significantly weakened as he carelessly used a Corridor of Darkness to return to Hollow Bastion. Nomura goes further to state that DiZ and the Beast can successfully use the Corridors of Darkness, and perhaps open them, due to their particularly strong feelings. Furthermore, despite Sora having used the Corridors of Darkness several times before, he has not used them frequently enough to allow the Darkness to stain his heart. The influential power of Darkness depends on the strength of a person's heart.
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 and Braig, it is not so severe as they only have 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 known that Pete travels the Corridors as well. He is known to be protected from the Darkness by Maleficent's magic.
The black coats worn by Riku, King Mickey, and Organization XIII are said to protect wearers from the effects of darkness, and from detection by those who use darkness. Thus, those who wear these coats can traverse Corridors of Darkness without suffering the negative effects.
The Dark Margin (referred to by Tetsuya Nomura as the Dark Coastline) is a mysterious place in Kingdom Hearts. It is first seen in "Another Side, Another Story" (Deep Dive) video.
In Kingdom Hearts II and In Kingdom Hearts 358/2 Days, Roxas converses with Xemnas at the Dark Margin. He talks to Roxas, telling him how much he looks like Sora.
It reappears in the end of Kingdom Hearts II. After defeating Xemnas, Riku and Sora end up on the Dark Margin. Suddenly, after Sora reads a letter from Kairi, a door appears leading them to Destiny Islands.
According to Another Report, Nomura states that the Dark Margin is "the tip of a world", i.e. the boundary between Darkness and In Between. Sora and Riku were on the coast of the side of In Between, and were looking over the sea in the Realm of Darkness. Having said this, the Door to Light which appeared in the ending scene is reminiscent of a quote which recurs in the Kingdom Hearts series, albeit in different words: "No matter how deep the Darkness, a light shines within."
In Kingdom Hearts Birth by Sleep's secret event, Aqua finds Ansem the Wise sitting on the coast of the side of darkness. The scene takes place some time after the machine Ansem used to encode Kingdom Hearts as data exploded in Kingdom Hearts II. Instead of killing him, the machine's explosion transported Ansem to the dark realm and caused Ansem to slowly lose his memory (he likely made his way there after Sora and Riku left). Aqua and Ansem the Wise have a conversation about the worlds and how Ansem came to know Sora. This leads Aqua to shed tears of hope and say Sora's name, knowing that he would save her and her friends.