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The Realm of Darkness (闇の世界 Yami no Sekai?) is a realm made up of the Darkness in people of every world. As with the Realm of Light, this realm has a Keyblade, the Kingdom Key D, which King Mickey managed to obtain. Kingdom Hearts is found here within its 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.
The Realm of Darkness appears as a playable world known as Dark World in Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage- with Aqua as the main protagonist.
Birth by Sleep
In the Final Episode, after Terra-Xehanort turns the Keyblade on himself, he falls into a dark portal leading to the Realm of Darkness. Aqua then dives in and sacrifices her Keyblade and armor to save Terra-Xehanort. However, Aqua herself is now trapped in the Dark Realm.
Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage-
At some point in time following her fall into the dark realm, Aqua encounters the sunken remainder of the Castle of Dreams. The world appears to her with the castle towering above in the distance. She travels via the main road into the area, musing about the events of the past whilst she walks. Among other things, Aqua comments that time does not move within the Realm of Darkness. As such, she states that she has no clue for how long she has been alone within the realm.
Aqua's lonely walk soon brings her to the bridge separating the main castle from the remainder of the world. However, as she steps forward to continue, the castle clock travels forwards five hours. This is quickly followed by the immediate collapse of the majority of the bridge on which Aqua stands. Managing to save herself from falling into the darkness below, Aqua notices the appearance of 'gears' in the area surrounding the castle, much of which is suspended in mid-air. She strikes a nearby gear with the Master's Defender, and the hour hand on the grand clock travels back one hour, reinstating a portion of the crossing. Understanding what she needs to do, Aqua goes about the task of locating and striking the four other gears. This completed, the Keyblade Master makes her way back towards the bridge. However, upon reaching the courtyard preceding it, she is spontaneously assaulted by a hoard of Shadow Heartless taking the form of a tornado. Once defeated, the hoard disappears into the ground, and Aqua makes her way undeterred to the castle. Inside, she catches herself wishing once again for the past, before being greeted by a memory of Terra, as he walks down the carpeted steps. The memory speaks Aqua's name, but thereafter does not respond to anything she says. When Aqua tries to take the figure's hand, it fades away. She then muses that perhaps his appearance is a means to inspire her to keep going.
Aqua continues, and finds herself in a fallen version of Dwarf Woodlands. She spies a glass coffin and quickly makes her way to it. Inside, she sees an unconscious Ventus, who disappears in a similar way to Terra when she places her hand on the coffin. Aqua speaks to the empty coffin, speaking aloud the promise she once made to Ventus to be there to wake him up someday. Aqua walks up to a mirror, from which a copy of her hand appears to forcefully pull her inside.
Between Kingdom Hearts Birth by Sleep and Kingdom Hearts
Ansem the Wise, betrayed by his apprentices, was banished into the Realm of Darkness, a realm he calls the realm of nothingness in his reports, due to nothing existing there save for the darkness. 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, Ansem, 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 Kingdom Hearts II.
Kingdom Hearts II
In Kingdom Hearts II, Sora, Donald, and Goofy visit the Realm of Darkness after being saved from Saïx by 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, 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.
In Kingdom Hearts Birth by Sleep's secret ending, 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.
Kingdom Hearts Birth by Sleep Final Mix
Realm of Darkness
The titular Realm of Darkness (referred as that in the game data) is first seen in Blank Points as the pathway that Aqua follows while lost in the darkness. Here, she encounters a numbers 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.
Realm of Darkness: Middle Zone
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.
Realm of Darkness: Lower Zone
The Lower Zone is a maze-like area, with 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.
Realm of Darkness: Upper Zone
The Upper Zone consist 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 in the Realm of Darkness. It 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 location inside 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.
Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage-
The Castle Town is a sprawling, segmented town comprised of the remnants of the Castle of Dreams. Within, time has ceased, as evidenced by certain setpieces appearing completely frozen. The area is mostly populated by small streets lined with houses and shops, many of which lead to an open fountain square. In addition, there are parts where large crystal barriers have formed as a result of the destruction. House fragments can also be seen floating in the air; some of them double as functional platforms. One segment hides a second plaza that is sunk underground due to fragmentation. The castle itself can be seen in the distance.
The World Within
The World Within is a location within a magic mirror located in the remnants of the Dwarf Woodlands. It is a small area comprised of a central chamber leading into numerous small rooms, each with a mirror allowing a view into what lies within. There are four types of rooms: a small, house-like room that houses Heartless or treasure, a never-ending staircase interspersed with Heartless and wall-like mirrors, a fragmented mine shaft that is manipulated by mirrors, and a large room fit with arranged pillars and a mirror floor. In addition, the latter three rooms possess mirrors that lead to a ghost-like arena not unlike the Magic Mirror's, where the Phantom Aqua is encountered.
Forest of Thorns
The Forest of Thorns is a world fragment comprised of the remains of the Enchanted Dominion. It consists largely of a sprawling, vine-ridden forest surrounded by rock walls. Many large "rooms" in the forest hide minor nooks and crannies. Between areas are vines that Aqua can Rail Slide on to traverse. The actual center of the forest has been disintegrated and taken over by numerous Darkside, who are gathering orbs of darkness.
Depths of Darkness
The Depths of Darkness are a bleak landscape within the Realm of Darkness. Much of the map is comprised of fragments of earth haphazardly spliced together into a single, continuous area. Between pathways are medium-sized rooms which have a tendency to host Heartless, as well as a large arena where the Demon Tower is fought.
Destiny Islands appears in the Realm of Darkness as it does in the End of the World, having been transported to the Realm of Darkness. It is largely identical to its appearance in World Terminus; however, a large piece of island land is isolated as the arena where the Demon Tide is fought.
Kingdom Hearts II
Corridor of Darkness
Corridors of Darkness are unpredictable pathways that 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 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. According to Nomura in the Director's Secret Report XIII, Riku and Mickey also aren't affected by the darkness at all, due to some "secret similarities".
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 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, the vessel that make his Thirteen Darknesses 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.
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 Birth By Sleep, Blank Points. 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 worlds 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.
0.2 features a brief appearance of the Dark Margin during a cutscene in which Aqua walks up to it, leading into Blank Points as seen in Birth by Sleep.
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 theme which recurs in the Kingdom Hearts series: "No matter how deep the Darkness, a light shines within".