Realm of Darkness

The Realm of Darkness (闇の世界) is a Plane of Existence made up of Darkness in people in every world. As with the Realm of Light, this realm has a Keyblade 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 Xehanort's Heartless in Kingdom Hearts.

Corridor 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 and the Nobodies don't have to worry about this, since the Heartless already have corrupted hearts, and the Nobodies do not have hearts to corrupt. However, one upside to this is that it provides a faster means of travel than the Gummi Ships.

Other characters who have used these corridors include Riku, Naminé, DiZ and even Pluto. Of these, only Riku, Naminé and DiZ have been responsible for summoning the portals, and only those lacking hearts to corrupt (Nobodies), and those who are darkness (Heartless) 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 faceoff 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 or hatred. 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, and the influential power of Darkness depends on the strength of a person's heart.

Effects
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, whose Heartless and Nobody both have tan skin and amber eyes. In the case of Ansem the Wise, it is not so severe: 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 know that Pete travels the Corridors as well. He is known to be protected by the Darkness by Maleficent's magic.

In Kingdom Hearts Re:Chain of Memories, whilst DiZ gives out black cloaks to King Mickey and Riku, he says the cloaks protect the wearer from the darkness.

Dark Meridian
The Dark Meridian (referred to by Tetsuya Nomura as the Dark Coastline) is a mysterious place in Kingdom Hearts. It is first seen in "Another Side, Another Story" (Deep Dive) video. In the first cutscene of Kingdom Hearts II, Roxas on a dark, moonlit beach, where Xemnas appears. 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 Meridian. Suddenly, after Sora reads a letter from Kairi, a door appears leading them to Destiny Islands.

In Kingdom Hearts 358/2 Days, Roxas converses with Xemnas at the Dark Meridian.

According to Another Report, Nomura states that the Dark Meridian 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 thus, 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: "Deep in the Darkness, there is Light".

Finding the Box
After being saved from Saïx by Maleficent in Hollow Bastion, Sora, Donald, and Goofy are transported to the Realm of Darkness, where they encounter a man in a black cloak, 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.

Trivia

 * The rocks of the Dark Meridian are very similar to the rocks of the End of the World, which befits this place as Riku calls it the "end of the road".
 * With the use of cheat codes in Kingdom Hearts II, you can access the Dark Meridian.