The Realm of Darkness is a realm made up of Darkness in people in every world. As with the Realm of Light, this realm has a Keyblade, Kingdom Key D, which chose King Mickey as its wielder. Kingdom Hearts is found here. This is also the home of all the Heartless, as this is where the most powerful ones can be found. Sora fought Ansem, Seeker of Darkness here in Kingdom Hearts.
Sora, Donald, and Goofy visit the Realm of Darkness after being saved from Saïx by Maleficent in Hollow Bastion, where they encounter a man in a black coat, later revealed to be Riku. Riku leaves 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 pathway. This is the first time that Sora, Donald, and Goofy are able to make their way to the Realm of Darkness in Kingdom Hearts II.
Settings and Areas
In A fragmentary passage, the Realm of Darkness is explored further, though nothing notable is shown. The different areas are given names like Middle Zone and Lower Zone, and each feature domes of darkness which sprout different kinds of Heartless which will vanish once all the Heartless have been defeated. Some of these domes form pillars which allow the player to advance through the world. A final, arena-like setting is where the battle between Aqua and the Unnamed Heartless takes place.
The Dark Margin (闇の海岸 Yami no Kaigan ) is a mysterious place in Kingdom Hearts. It is first seen in Another Side, Another Story... (Deep Dive) video.
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 on the Dark Margin. Suddenly, after Sora reads a letter from Kairi, a door appears leading them to Destiny Islands.
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 quote which recurs in the Kingdom Hearts series, albeit in different words: "No matter how deep the Darkness, a light shines within."
A fragmentary passage
While wandering lost through the Realm of Darkness, Aqua is confronted by a Darkside. She prepares to fight it, but two 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.
But as she moves onwards, a crack of light forms in the Realm's ceiling 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 some enemies she has never encountered before. 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 leader. After a lengthy battle, with Aqua barely keeping up with the creature's speed and stealth tactics, Aqua finally emerges victorious and defeats her enemy.
Venturing forward, Aqua stumbles across the Castle of Dreams jutting out of a sea of darkness in the distance.
Kingdom Hearts: 358/2 Days
Kingdom Hearts II
In order to save them from Saïx, Maleficent transports Sora, Donald and Goofy to the Realm of Darkness, where Riku, his face hidden, 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.
Following the defeat of Xemnas, Sora and Riku end up on the coastline and sit in front of the water for a while before a portal to the Realm of Light leads them back to Destiny Islands.
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 instead of being killed as previously believed, 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.
It is theorized that time does not move forward in the Realm of Darkness as it does in the Realm of Light, explaining Aqua's lack of change in appearance over the years.
Corridors of Darkness
Corridors of Darkness are unpredictable pathways that interlink the worlds of 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 corrupt one's heart. 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. One upside to the Darkness in the Corridors is that it provides a faster means of travel than the Gummi Ships.
Other characters who have used these corridors include Sora, Donald Duck, Goofy, King Mickey, Kairi, Riku, Naminé, DiZ, and even Pluto. Of these, only Riku, Naminé and DiZ have been responsible for summoning the portals, and only Heartless, Nobodies, and Princesses of Heart aren't affected, with two noted exceptions; according to Nomura in the Director's Secret Report XIII, Riku and Mickey aren't affected by darkness at all, due to some "secret similarities". 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.
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 Keyblade Armor worn by Terra, Ventus, Aqua, Master Eraqus, and Master Xehanort functions in the same way as the black coats.