End of the World

The End of the World is a world from . It is the last new location to open in the game, becoming available upon locking Hollow Bastion.

End of the World is a by-product of all Heartless's destructive nature. Upon taking a world's heart, a tiny fragment of what is left of that world is sent drifting through the void of space until it eventually meets and merges with End of the World. It is, in essence, a Heartless of the destroyed worlds, as well as the location of the Kingdom Hearts of worlds' hearts. In nature, it can be seen as the opposite of The World That Never Was, which is a similar artificial "Nobody" world without its own heart.

Development
Originally planned for Kingdom Hearts was a boss battle with Riku at the End of the World, in an area called Crumbling Island. The area was cavernous with purple walls; it had a large amount of running water on the walls, and a large hole in the ceiling. It is reminiscent of Destiny Islands while it was being consumed by darkness. The map for Crumbling Island was placed out of the way within the remains of the World Terminus, where it can be found through hacking.

Kingdom Hearts
End of the World opens on the Yami e no Iriguchi (闇への入口), a cave leading into the Hate no Makū (果ての魔空), which appears to the eye as an empty field of darkness dotted by snowy fragments of land, and showered by infrequent dark orbs being drawn deeper into the world. However, being sucked into one of those orbs reveals the true shape of the area, a snowy wasteland with plentiful outcroppings of crystals, and swarming with powerful Heartless. A vortex at the end of the area leads to a colorful, quiet canyon with Gummi Blocks scattered all over the place, called the Kyodai Kurebasu (巨大クレバス), which spans several levels, connected by green web-like formations.

At the bottom of the canyon is a passage to the fragmented World Terminus (世界の牢獄), which is a series of pillar-like structures spread over what appears to be a vast empty space. Each pillar contains a green swirl of light which connects to the previous pillar, a blue swirl of light which connects to the next pillar, and a terminal to one the worlds whose Keyholes Sora can potentially lock, in the form of a beam of roiling darkness. If Sora has locked the respective world's Keyhole, its terminal is violet and the blue swirl of light is present, but if he hasn't, the terminal is blue and the blue swirl of light will not appear until the terminal is completed.

Each terminal takes Sora and his party to a location from its respective world, and each of these contain Heartless native to that world and have all but one exit blocked off. In order, Sora is taken to the Third District at Traverse Town, the Rabbit Hole at Wonderland, the Coliseum Gates at Olympus Coliseum Gates, the Bamboo Thicket at Deep Jungle, the Plaza in Agrabah, the Calm Depths at Atlantica, the Torture Chamber at Halloween Town, and the Galley at Neverland. Upon completing each terminal, Sora receives a treasure that symbolizes that world; in the original Kingdom Hearts, these are copies of the accessories he received from each world's respective boss, while in Kingdom Hearts Final Mix they are synthesis materials and other items. These terminals remain accessible after completion. As in the original Atlantica and Halloween Town, Sora is transformed into his merman and vampire forms within the appropriate terminals. However, the Olympus Coliseum terminal has the dusky lighting of the Hades Cup, and the Neverland terminal does not allow Sora free flight.

However, the final two terminals are outliers. The green terminal for 100 Acre Wood, which is composed of sparkling light, leads to the Meadow and contains a save point, instead of enemies. When Sora uses this terminal, he dissolves into starlight rather than the dark mist of the other terminals.

Meanwhile, the red terminal for Hollow Bastion, which is composed of roaring flames, leads to the Kenkyūshitsu (研究室), an area which Sora had not previously visited. This lab consists of a short hallway blocked by a large Heartless emblem at one end, with a small room to the side that opens after Sora defeats several Heartless. The side-room contains a large machine with a computer console and five capsules on it hooked up to a tank of unknown liquid. Red, orange, and black warps in space dot the hall and lab. The computer has a data log for Sora to read:
 * Ones born of the heart and darkness, devoid of hearts, ravage all worlds and bring desolation. Seize all hearts and consummate the great heart. All hearts to be one, one heart to encompass all. Realize the destiny: the realm of Kingdom Hearts. The great darkness sealed within the great heart. Progeny of darkness, come back to the eternal darkness. For the heart of light shall unseal the path. Seven hearts, one Keyhole, one key to the door. The door of darkness, tied by two keys. The door to darkness to seal the light. None shall pass but shadows, returning to the darkness. Ones born of the heart and darkness, hunger for every heart until the dark door opens.

After reading the message, Sora is once again attacked by Heartless. Once Sora completes the terminal, it vanishes and is replaced by a hole, rendering the Laboratory inaccessible.

Jumping into the hole will lead Sora down to the Majin no Ji (魔人の地) and a boss battle with Chernabog. Within the Evil Grounds, Sora has free flight, and must fly up to reach the World Terminus. After defeating Chernabog, the top of Bald Mountain opens, allowing Sora and his party to fly through the Kakō (火口). The Crater runs into the Funkakō (噴火口), with entrances to it at the top and bottom of the crater. The Volcanic Crater is a tunnel that extends through four rooms, and contains remnants from worlds that have fallen into darkness - for example, trees and small towers from Enchanted Dominion, the demonic trees and the front door of the Dwarfs' house from Dwarf Woodlands, stone tapestries from Castle of Dreams, and a small glass sphere from Beast's Castle.

At the bottom of the mountain is a glowing chamber called the Majiriau Sekai (交じり合う世界), in which Sora and his friends are forced to fight a swarm of Heartless, beginning with a battle against a Behemoth, in order to break open the path to the Saigo no Kyūsoku (最後の休息). This room is the location of the final save point of the game, as well as the door which first appeared in the Awakening and led to a strange version of Destiny Islands. Just as before, this door leads to the remnants of Destiny Islands, in a room named only Omoide no Shima (想い出の島). After two fights against Ansem, Sora and party are transported to The Void where they face the final boss, World of Chaos, which even includes tiny rooms within itself called Portals of Darkness. Once Ansem is defeated once and for all he opens the Door to Darkness which is located in a room called Saigo no Tobira (最後の扉).

Kingdom Hearts
After sealing the Final Keyhole at Hollow Bastion, Sora, Donald, and Goofy arrive at the End of the World. Goofy takes note of its composite-world nature, and Sora states that all of the worlds would be restored if they managed to defeat Ansem, Seeker of Darkness, but wonders what would happen to the world itself and them by extension. Goofy says that, as a "Heartless world", it would just disappear, but assures them that, even if that did happen, they would not meet the same fate. Sora takes out Kairi's lucky charm, and remembers his promise to return it.

After fighting through several legions of Heartless and the World Terminus, Sora, Donald, and Goofy run into the giant demon Chernabog and manage to defeat him after a lengthy battle. Afterwards, they descend into the crater of Bald Mountain and press onwards. The group reaches the Final Rest area where a door appears like the one that was in the Awakening earlier in the game. Through the door, they come to the remnants of Sora's homeworld, the Destiny Islands, which soon takes on a much darker appearance.

Ansem appears in the form of Riku, and states that Riku was desperate to escape the islands by any cost, and then takes on his own form. Sora calls out to Riku, but Ansem informs him that any effort to reach him is futile. The villain goes on to explain that all worlds and hearts begin and end in darkness; thus, darkness is the true essence of a heart. Sora denies this, stating that, while hearts may be weak and sometimes even give in, deep down, there is a light that never goes out. Unimpressed, Ansem says, "So you've come this far, and still you understand nothing" (mirroring his words to Sora at the beginning of the game), and engages in battle with the trio.

Sora comes out victorious, and then the mountains of the islands split open. Ansem escapes into the opening with Sora, Donald, and Goofy in hot pursuit, but only Sora is able to get past a preset barrier. After defeating another Darkside and Ansem, the latter uses his power to send himself and his foes to a dark void.

Ansem fuses himself to the World of Chaos, a giant, organic Heartless battleship, and sucks Donald and Goofy into a Corridor of Darkness. Sora begins to fall into the void, but the encouragement of Riku gives him the strength to fight back. Sora rescues Donald and Goofy, and the trio destroy the World of Chaos.

Badly injured, Ansem opens the Door to Darkness, informs Sora that not even the Keyblade can seal the door, and pines for Kingdom Hearts to fill him with the power of darkness. Sora shouts that Kingdom Hearts is, in fact, light, which proves to be correct; the light of Kingdom Hearts shines out of the door and hits Ansem full force, seemingly destroying him.

However, the threat is not over; several thousand Heartless lie in wait behind the door. Sora, Donald, and Goofy attempt to close it, but even with help from the restored Riku, their efforts are futile. Just when things appear to be over, King Mickey arrives with a golden Keyblade, and Sora, Riku, Donald, and Goofy manage to close the door. Before it does, Mickey assures them that there will always be a door to light; thus, they will be able to find their way back. Riku asks Sora to take care of Kairi, and Sora and Mickey use their respective Keyblades to seal the Door to Darkness forever.

Soon afterwards, Kairi appears, and Sora runs off to talk to her. However, the worlds are beginning to restore themselves. Sora promises to return to Kairi when his obligations are done just as the End of the World breaks apart; Kairi is sent back to the Destiny Islands alone, while Sora, Donald, and Goofy end up on the pathway to Castle Oblivion, where they resolve to find Riku and King Mickey, though they are unsure of where to start looking.

Kingdom Hearts Chain of Memories
A facsimile of End of the World is created by Ansem within Castle Oblivion for his and Riku's final fight.

Kingdom Hearts II
Sora and his friends' time at the End of the World appears in Roxas's dreams of Sora's memories. Later, when King Mickey explains the truth of Ansem's identity, Sora and his friends briefly recall their battle at the Final Door.

Trivia

 * The Halloween Town fragment of the World Terminus is the only time in which Oogie's torture chamber can be explored freely. The real one only appears during the boss battle with Oogie Boogie.