Realm of Darkness

The Realm of Darkness is a realm made up of Darkness in people in every world. Time has no meaning there. As with the Realm of Light, this realm has a Keyblade, Kingdom Key D, which chose King Mickey as its wielder. It is where Ansem, Seeker of Darkness formed the Kingdom Hearts of Worlds' Hearts, and is the home of all Heartless.

Settings and Areas
After defeating Terra-Xehanort, Aqua awakes within an immense, eerie cavern known simply as the Yami no Sekai (闇の世界). Starting at the cavern's Chūsō (中層), she ventures through the Kasō (下層) and Jōsō (上層) before finally finding a way out. These caves are infested with Heartless and strange rock formations, as well as geysers and barriers formed from pure darkness.

Just outside of this cavern is the Yami no Hazama (闇の狭間), a twisted and decaying forest drenched in darkness. Past the valley is the Yami Shinderera-jō (闇シンデレラ城), which is all that remains of Castle of Dreams after that world was swallowed by the darkness.

At the furthest reaches of the Realm of Darkness, on the border between it and the Realm Between, is a moonlit shoreline called the Dark Margin (闇の海岸). Like the caverns, this beach is littered with strange, almost organic rock formations.

Kingdom Hearts Birth by Sleep
While wandering lost through the Realm of Darkness, Aqua is confronted by a Darkside. She prepares to fight it, but three more Darksides join the fight and she loses hope. However, she is rescued by the Earthshaker and Wayward Wind Keyblades, which make quick work of the Darksides and renew her hope. She looks at her Wayfinder and remembers all the friends she has made throughout her journey across the worlds. Smiling, Aqua vows to escape the Realm of Darkness.

However, as she moves onward, a crack of light forms in the cavern's roof and a beam of darkness fires through it. Aqua's Wayfinder stops glowing and she senses something is wrong. Aqua ventures deeper in the Realm of Darkness and comes across the Heartless, enemies she does not recognize. As residents of the Realm of Darkness, these enemies prove to be stronger than the Unversed, but Aqua manages to fight her way through them until she reaches the Dark Hide. Although the beast is vicious and powerful, Aqua finally emerges victorious after a lengthy battle. Venturing forward, Aqua is horrified when she spots the destroyed Castle of Dreams jutting out of a sea of darkness in the distance.

Kingdom Hearts 358/2 Days
Six days after Roxas's induction into Organization XIII, Roxas visits Xemnas at the Dark Margin, where they briefly talk before Xemnas takes his leave.

Kingdom Hearts
After Ansem takes Riku's body, Riku's heart escapes to the Realm of Darkness, where he hears Mickey calling for him. Mickey lets him know that he has retrieved a Keyblade of darkness, with which they can close the Door to Darkness. Riku agrees to help, and continues onward.

Later, after Sora reaches the deepest depths of the End of the World, he finds Ansem preparing to open the Door to Darkness, so that he can enter the Realm of Darkness and the Kingdom Hearts of Worlds' Hearts laying within it. Though Sora defeats Ansem, the door has already opened and darkness begins spilling out. As Sora and his friends struggle to shut the door, they see Mickey and a restored Riku on the other side, and together, they manage to close the door.

Kingdom Hearts Chain of Memories
Riku awakens from his slumber in the Realm of Darkness when an unseen presence, later revealed to be DiZ, gifts him a magic world card and asks him to choose between slipping back into sleep, or taking the card and making his way toward the truth. Riku decides that going back to sleep would be boring, and confidently takes the card. He is taken by the card through a corridor of darkness to the lowest basement of Castle Oblivion. As Riku makes his way through the castle, he is contacted by Mickey, who is still trapped within the Realm of Darkness but encourages Riku to persevere. As Riku reaches the top of the castle, Mickey finally finds a way out of the Realm of Darkness in the form of another card, which showed him Riku's heart and led him through a corridor of darkness constructed by Organization XIII.

Kingdom Hearts II
After running into Mickey at Twilight Town, Sora and his friends decide that they must enter the Realm of Darkness to find Riku.

When they encounter Saïx at the Battle of the 1000 Heartless, Sora even begs Saïx to let him into the Realm of Darkness, but the Nobody coldly refuses and prepares to attack. Maleficent begrudgingly rescues Sora and his friends by transporting them to the Realm of Darkness, where they run into a disguised Riku-Ansem. He leaves them a sea-salt ice cream bar and a photo of Roxas in Twilight Town as clues to get to The World That Never Was, rescue Kairi and destroy Organization XIII. They follow the clues to the Old Mansion, hoping that the Simulated Twilight Town will have another path to the Realm of Darkness.

After defeating Xemnas within his "The World of Nothing", Sora and Riku end up at the Realm Between's side of the Dark Margin. They patiently wait there until Kairi's Letter washes up on the shore. The Door to Light appears as they read it, allowing them to return home to Destiny Islands.

Blank Points
Eventually, Aqua finds Ansem the Wise sitting on the coast of the side of darkness, having been sent there by the explosion of the Kingdom Hearts Encoder, though he is slowly losing his memories again. Aqua and Ansem 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 will save her and her friends.

Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage-
<!--rewrite in-universe, place flashback where it belongs in timeline using storylinks, use natural English, stop saying albeit multiple times

After the events of Kingdom Hearts 3D: Dream Drop Distance, Riku, having officially named a Keyblade Master, is told by Mickey of events that had happened just before the beginning of the original Kingdom Hearts.

Being told as a flashback, the story starts at where the Secret Episode in Kingdom Hearts Birth By Sleep Final Mix ended; Aqua finds the remains of the Castle of Dreams while traversing the Realm of Darkness. She explores the fallen remnants of this world, along with the Dwarf Woodlands, Enchanted Dominion, and even the Destiny Islands, all while encountering walking memories of Terra and Ventus, and even confronting her own phantom, remarking that she never thought that she would become her own worst enemy. While exploring the fallen Enchanted Dominion, Aqua meets the actual Terra and Ventus, albeit just visions of them. Terra is surprised that Aqua can see him, while Ventus remains silent, as he is still comatose in the Realm of Light. Terra, unable to see Ventus, asks Aqua what happened after they were separated following the battle with Master Xehanort at the Keyblade Graveyard. As she explains, Terra is slowly overtaken by Xehanort, who used Terra in order to lure Aqua into revealing where the Chamber of Waking. Before she is able to reveal where Ventus is, the real Terra shows up behind Terra-Xehanort and attempts to restrain him, and a struggle ensues. Aqua and Ventus are grabbed by two hands that appear out of the darkness; Terra, in a latch ditch effort to safe his friends, releases his full power, similar to the Lingering Will conjuring the Will's Cage, and Aqua passes out.

Aqua, ready to let herself fade into the darkness, is saved by none other than King Mickey. It is then revealed that 10 years have passed in the Realm of Light, and that the worlds are in danger. Sensing the danger, Mickey set out to enter the Realm of Darkness and find a "Keyblade of Darkness" in order to close the door letting darkness escape into the Realm of Light. Eventually, the two come upon the ruins of the Destiny Islands. Aqua reminisces that this is the same island where she met the two boys she considered passing the Keyblade to, and Mickey tells her that those same two boys are the ones helping him close the door to darkness. Speculating that since this is where Sora and Riku obtained the Keyblade, he concludes that the dark counterpart to must also be here. They set out towards the Secret Place, and open the door inside that was originally the Destiny Islands' Keyhole; inside is the Kingdom Key D, the Keyblade of Darkness Mickey was looking for.

Armed with the Kingdom Key D, Mickey explains that the door is actually Kingdom Hearts, albeit a different one than the one they encountered at the Keyblade Graveyard. The camera pans back to reveal that the room they are in is the same one that is seen inside the door during the ending of the first game. Riku is seen running towards the door, but a swarm of Shadows are chasing after him. Aqua and Mickey rush to his aid, but Aqua is swept up by the swarm into the door and is forcefully ejected outside to the beach. As she battles it, Mickey and Sora successfully close Kingdom Hearts. Having defeated the Heartless, Aqua is lying on the beach as the world is restored around her, in a scene similar to the ending of the original Kingdom Hearts,where the stars become visible again and lights start showering. The beach around her starts disappearing as the Destiny Islands are restored. However, she is not able to be taken back to the Realm of Light, as creatures of light are unable to pass through the door to darkness. She resolves to be a wayfinder for people who end up in the Realm of Darkness, and once again she falls into darkness.

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Trivia

 * According to the Director's Secret Report XIII, the Dark Margin is the tip of a world in the Realm Between reaching into the Realm of Darkness. The Kingdom Hearts II game files and the official Kingdom Hearts website refer to this world as End of Sea (エンド・オブ・シー).

Notes and References
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