Realm of Darkness

The Realm of Darkness is a realm made up of Darkness in people in every world. According to Aqua, time doesn't exist in this realm. 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.

A Fragmentary Passage
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-
Once she gets to the Castle of Dreams, Aqua sees a ghost of Terra from their time in that world. Aqua is left wondering where the people of the world went when it was swallowed by darkness. She next finds herself in the remains of Dwarf Woodlands, where she sees a phantom of Ven in Snow White's glass coffin. Wondering what her friends are trying to tell her, Aqua sees the Magic Mirror and is pulled into the world inside of it by her own reflection. Fighting off illusions of herself in the mirror, Aqua manages to escape and arrives in Enchanted Dominion, where phantoms of Terra and Ventus walk through a forest of thorns. Eventually catching up to them, Aqua is shocked to find Terra, who reveals that he is no longer himself. As Aqua tells Terra that she put Ven somewhere safe, Terra becomes Terra-Xehanort; however, Terra's spirit holds him at bay, explaining that Xehanort is looking for Ven for a dark purpose.

The struggle between the two summons a Darkside, which pulls Aqua into a port of darkness. Ready to let the darkness swallow her, Aqua is surprised to find Mickey rescuing her. Mickey explains that it's been ten years since she fell into the darkness, and that Sora and Riku are helping him to try stopping the darkness from destroying the worlds. To to this, they need the Dark Realm's Keyblade to help Sora seal the Door to Darkness. They eventually find it in the remains of Destiny Islands, and head off to find the door. However, when they get there, Heartless attempt to attack Riku, so Aqua fends it off at the cost of getting separated from Mickey again.

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.

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. Although this world is not officially named, the Kingdom Hearts II game files refer to it as End of Sea (エンド・オブ・シー).
 * The playable version of the Realm of Darkness that appears in Kingdom Hearts 0.2 is referred to as the Dark World. However, the Japanese game uses the typical term yami no sekai, with the English phrase "Dark World" only appearing in the world's logo.

Notes and References
🇰🇲