Realm of Darkness

The Realm of Darkness (闇の世界), also translated as Dark World in its playable incarnation in Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage- and Kingdom Hearts III, is a realm made up of darkness. It lies opposite to the Realm of Light, and the normal passage of time does not exist here.

The Realm of Darkness is the home of all Pureblood Heartless. It is an exceptionally dangerous place, and very few have survived a trip to the Realm of Darkness. The Realm of Darkness is also home to a special type of Keyblade, Keyblades of darkness; the only known Keyblade of darkness being Mickey's Kingdom Key D.

When a world or its heart falls to darkness, it is swallowed into this realm. The Realm of Darkness also has its own natural landmasses, often stone-like with blue glowing cracks in them. Apart from falling into the darkness oneself, the only ways to reach the Realm of Darkness are by using a Corridor of Darkness or with the power of waking.

Kingdom Hearts
The Realm of Darkness isn't playable in Kingdom Hearts, but can be briefly seen through the doors of Door to Darkness located at the Final Door area deep within End of the World. The doors lead to a cave-like area covered with organic-looking rocks, shaped like cylinders, shooting out from all over the walls. The rocks are often covered with cracks, from which a bright blue light pierces through. At the far back is a white cold light slightly illuminating the area. The cave is also covered with hordes of Heartless, with two Darksides arising as well.

Riku can also be briefly seen trapped in another part of the realm, which consists of a narrow, free-hanging grey path, continuing on into the unknown, and surrounded by seemingly endless darkness.

Kingdom Hearts II
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). In Kingdom Hearts II Sora and Riku arrive at this area during the game's ending. Like the caverns, this beach is littered with strange, blue, and almost organic formations of rocks, with several huge arches stretching above and across the sea. The Dark Margin is also a part of the world known as End of Sea.

Kingdom Hearts Birth by Sleep
In Kingdom Hearts Birth by Sleep Final Mix the Realm of Darkness appears playable for the first time in the series during the game's. 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. The various paths twist and wind and reconnect, and navigating the areas can be a struggle. The Upper Zone consists of many rocks hanging in the air, with risk of falling back down to Lower Zone. Towards the end of the Upper Zone is a path towards a spiraling purple and violet light, further leading out of the cavern and to the Yami no Hazama (闇の狭間), a twisted and decaying forest drenched in darkness.

After defeating the Dark Hide Aqua finds herself arriving from a gloomy forest where she discovers that Castle of Dreams has fallen to darkness at the area fittingly named Yami Shinderera Shiro (闇シンデレラ城).

Kingdom Hearts χ
In Kingdom Hearts χ the Realm of Darkness appears during a few missions in the form of Yami no Kairō Toraiaru? (闇の回廊トライアル), with each area being simply called Toraiaru Eria? (トライアルエリア). Within these corridors the ground is made of streams of darkness in the color of dark purple and magenta, and there's purple smoke and clouds passing by. Far off in the back is small portal of light, in which the darkness swirls towards. Within the game's files the world is simply known as as Danjon? (ダンジョン).

Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage-
Past the valley is the Dark Cinderella Castle in the distance, which can be reached through a crumbling forested path sparsely lit with lanterns called the Kaidō (街道). Surrounding the castle is the Jōkamachi (城下町), which is filled with empty houses, business stalls, and a fountain frozen in time in the center. Pieces of buildings float around the area. At the edges of the Castle Town, the remains of the world are crumbling into blue crystals. Beyond that is the remains of the seven Dwarfs' cottage from Dwarf Woodlands, and a mirror leading to Kagami no Naka no Sekai (鏡の中世界) the Magic Mirror, which resembles both the realm of the Spirit of the Magic Mirror and the Queen's dungeon. Past the mirror is the Ibara no Mori (茨の森), a corruption of the dark forest of Enchanted Dominion which ends at the Forbidden Mountain. The Forest of Thorns is divided in three parts: the Saki no Mienai Michi (先の見えない道), the Kewashī Michi (険しい道), and the Michi no Hate (道の果て). From there, a portal leads to similar cavernous area known as the Depths of Darkness (闇の深部). At the end of the path is a sandy terrain leading into a bright light; it leads to Homecoming, where Kingdom Key D is hidden.

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 0.2 Birth by Sleep -A fragmentary passage-
Aqua continues her journey through the Realm of Darkness, passing through the remains of Castle of Dreams, Dwarf Woodlands and Enchanted Dominion. Aqua questions what happens to the people who have lost their worlds to darkness, while finding herself thinking back to when she was with her friends. She is eventually confronted by Phantom Aqua, an illusion created from the doubt in her heart by the darkness. Once it is defeated, Aqua finds manifestations of Terra's and Ventus's hearts; Terra is able to converse with her, while Ven remains comatose. However, Xehanort attempts to use this his link to Terra-Xehanort to find out where she hid Ven. Terra fights off Xehanort, who summons the Dark Figure to drag Aqua further into darkness.

Aqua is rescued by Mickey, who informs that an entire decade has passed since the battle with Xehanort at the Keyblade Graveyard. Mickey informs her they need to find Kingdom Key D, to help Sora and Riku seal the Door to Darkness to keep the Heartless from escaping and destroying the worlds. Upon finding the Keyblade they need, the duo finds a Demon Tide attempting to attack Riku; Aqua tethers it with her magic, getting pulled into a secluded part of the Dark Realm. Mickey is left to focus on sealing the Door and hoping to find Aqua again. Once she defeats the Heartless, Aqua rests; she is content to wait for her rescue, knowing now that the Realm of Light is safe with Sora and Riku there.

Kingdom Hearts
When Riku continues to resist against his possession, Ansem banishes the boy's heart to the Realm of Darkness. Riku's heart manifests as his former self, lamenting over his foolishness in abandoning his home and friends all for the sake of dark power. He hears Mickey call out for his help. 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 to find the door.

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
When they encounter Saïx at the Battle of the 1000 Heartless, Sora 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 III
Later on, Ansem, Seeker of Darkness confronts Aqua and Ansem the Wise at the Dark Margin, seeking Ansem the Wise's hidden work. Aqua tries to defend against Ansem, Seeker of Darkness, but without her Keyblade, she is hit by a glob of darkness and thrown into the seas of the Dark Margin, where she is corrupted into Anti-Aqua.

King Mickey and Riku arrive shortly afterwards, but are repelled by a Demon Tower that breaks Riku's Way to the Dawn and damages Mickey's Kingdom Key D; they retreat to better arm themselves. They later return after failing to find other leads, only for Anti-Aqua to imprison Mickey in a Demon Tower and steal his Star Cluster, planning to trap them in her place. However, Sora is able to use the Master's Defender to reach them. Destroying the Demon Tower and freeing the King, Sora battles Aqua and liberates her from the darkness. The Keyblade wielders return to Destiny Islands.

After Xehanort's defeat, Terra, Ventus, and Aqua entered the Realm of Darkness to find Sora, who had disappeared after the battle.

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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