The Realm of Darkness (in Japanese: 闇の世界, Yami no Sekai), also known as the Dark Realm, is a supernatural dark world that is so named because it is entirely comprised of pure darkness within the people of every world and a location in the Kingdom Hearts universe. As with the Realm of Light which is it's opposite, this realm has a Keyblade, Kingdom Key D, which was discoverd and obtained by King Mickey. Kingdom Hearts is to be found here within it's deepest depths, though it can only be accessed when the Final Keyhole is unsealed. This is also the home of all the Heartless and where the most powerful ones can be found.
Kingdom Hearts: Birth by Sleep
After Terra-Xehanort turns the Keyblade on himself, he fell into a dark portal leading to the Realm of Darkness. Aqua then dives in and sacrifices her Keyblade and armor to rescue her friend Terra-Xehanort. However, Aqua herself is now trapped in the Dark Realm.
Between Kingdom Hearts: Birth by Sleep and Kingdom Hearts
Ansem the Wise, the fair king of the Radiant Garden who has been betrayed by his apprentices and his former experiments put into dangerous use, was banished into the Realm of Darkness, a world he calls the realm of nothingness in his reports, due to nothing existing there save for the darkness itself. While he is in this realm, he learns to use the power of darkness to exact his revenge, but does not allow the darkness to control him as it has controlled his beloved apprentices.
After the battle on End of the World's recreation of Destiny Islands, the false Ansem, the Seeker of Darkness, transports Sora, Donald and Goofy to the Endless Abyss, where the Door To Darkness resides. This is the closest Sora had gotten to the Realm of Darkness until then.
Kingdom Hearts II
Sora, Donald, and Goofy visited the Realm of Darkness after being saved from Saïx by the reluctant Maleficent in Radiant Garden, where they encounter a man in a black coat, later revealed to be Riku. Riku leaves behind 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 Gate, that leads them back in their Gummi Ship to revisit all the worlds they had previously been to deal with new problems plaguing them.
Also, after defeating Xemnas and ending the Organization XIII's reign of terror during the battle at their castle in the World That Never Was, Sora and Riku end up on the edge of the Realm of Darkness, at a beach. There, a letter from Kairi allows Sora to open the Door To Light, returning him and Riku home to their world and their dear friends.
In the secret ending of Kingdom Hearts: Birth by Sleep, Aqua finds Ansem the Wise sitting on the Dark Margin on the side of darkness. The scene takes place some time after the Kingdom Hearts Encoder exploded in Kingdom Hearts II. Instead of killing him, the machine's explosion transported Ansem to the Dark Realm and caused him to slowly lose his memory.
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, knowing that he would save her and her friends.
Kingdom Hearts 3D: Dream Drop Distance
After having defeated the Anti Black Coat and having reappeared in the World that Never Was, Riku is contacted by Ansem, who opens a portal of Darkness in order to bring Riku into the Realm of Darkness.
The Realm of Darkness is first seen in Blank Points as the pathway that Aqua follows while lost in the darkness. Here, she encounters a numerous multitude of Darksides. While she is about to give up and let herself be swallowed by the darkness, Terra's and Ventus' Keyblade, the Earthshaker and Wayward Wind rescue her before vanishing, renewing her hopes.
The Middle Zone is an open area coming right at the end of the pathway. The area is filled with stairs-like terrain, high cliffs, and strange column-like structure with glowing blue marks. There are also strange orbs of darkness that, when approached, will push Aqua away, and are useful for reaching higher areas by jumping on them. It is in this area where the player first encounter the Heartless.
The Lower Zone is a maze-like area, with such countless cliffs and twisted pathways populating this zone. In order to advance through here, it is necessary to make good use of the dark orbs that push you away. There are zones where the pathway is vanished, and just noticeable for a subtle purple glow. For the path to appear, Aqua must defeat the Heartless that spawn near them.
The Upper Zone is an area consisting of various hovering platforms surrounded by twisted, glowing structures. Should Aqua fall in the pits below, she will return to the Lower Zone. The Heartless in this place only spawn once in the places where the ground is purple.
Valley of Darkness
The Valley of Darkness is an open space within the Realm of Darkness. This "valley" seems to be covered by a thick mist, and surrounded by twisted, dark trees in the background. It is here that Aqua is attacked by the Hunter of the Dark and engages in a fight with the Heartless. A mysterious vortex is seen in the background, surrounded by dark thorns.
Darkness's Call is a place within the Realm of Darkness. It has two areas, a sky of darkness, and a valley with dark rocks.
In Kingdom Hearts 3D: Dream Drop Distance, Ansem brings Riku here in an attempt to get Riku to embrace his darkness.
The Dark Margin (referred to by Tetsuya Nomura as the "Dark Coastline") is a mysterious place in Kingdom Hearts. It is first seen in the "Another side, Another story [deep dive]" FMV.
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 at the Dark Margin. Suddenly, after Sora reads a letter from Kairi, a door appears, leading them to Destiny Islands.
The Dark Margin is also seen in the secret ending of Kingdom Hearts: Birth By Sleep. Taking place after Kingdom Hearts II, Aqua finds Ansem the Wise here and is curious where he came from. Upon further discussion, he tells her about a Keyblade-wielding boy who has saved the entire universe from darkness on multiple occasions. When she asks his name, Namine, Axel, Roxas, Xion, Terra, Ventus, and Ansem the Wise are all shown saying Sora's name with a renewed sense of hope.
Corridors of DarknessThe Corridors of Darkness are swirling blackholes that were unpredictable pathways which interlink the worlds in 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 slowly corrupt one's heart. One upside to the Darkness in the Corridors is that it provides a faster means of travel than the Gummi Ships.
However, the Heartless, the Nobodies, and the Princesses of Heart do not 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 full 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 was never affected by the darkness, as her heart was made out of pure light.
Other characters who have used these corridors include Sora, Donald Duck, Goofy, King Mickey, Kairi, Riku, Naminé, DiZ, Beast, and even Pluto. Of these, only Riku, Naminé and DiZ have been responsible for summoning the portals.
Ironically, in 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, the vessel that make his own Organization XIII both have tan skin and amber eyes. In the case of Ansem the Wise as 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 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.
- According to Another Report, Tetsuya 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 theme which recurs in the Kingdom Hearts series: "No matter how deep the Darkness, a light shines within".
- According to Nomura in the Director's Secret Report XIII, Riku and Mickey also are not affected by the darkness at all, due to some "secret similarities".