The Final World

The Final World is an original world in Kingdom Hearts III.

Settings and Areas
The Final World consists of an expansive sea and sky that seems to reach out into infinity. Deeper into the realm lies a labyrinth that consists of a multitude of crystalline cubes and pillars, suspended in the sky of what appears to be a, such that falling off the structure results in falling down to the structure. It is possible to rotate the structure by striking the glowing cubes.

According to Chirithy, The Final World is the edge of reality just before a heart ceases to exist, and so is nested closely to the Station of Awakening and the realm of sleep. Those who dream and/or visit the Station have a chance to catch a glimpse of The Final World, as Sora does when he begins his journey. More generally, the Final World is inhabited by faded remnants of hearts, usually of people with attachments so strong that they cannot pass on.

Kingdom Hearts Dark Road
Heimdall, Helgi, Sigrun, and Hoder end up in the Final World as hearts while fighting Maleficent's dragon form.

Between Kingdom Hearts and Kingdom Hearts 3D: Dream Drop Distance
Whenever Sora performs a Dive to the Heart, he unknowingly passes into the edges of the Final World. Chirithy observes him during these events, but does not interfere due to the fact that Sora does not do this voluntarily.

Kingdom Hearts III
When Sora begins his Dive to the Heart, he unknowingly passes into the edges of The Final World, where he fights a semi-transparent Darkside.

After Sora and his friends are defeated by Terra-Xehanort and consumed by a Demon Tide, he ends up in a featureless plane. Soon after, he meets Ventus's Chirithy, who is shocked that he retains a faint physical form. Chirithy explains that this is a sort of limbo for those who have not been fully claimed by death, but since Sora and his friends tethered to the Realm of Light, Sora can return to life by collecting the pieces of his body scattered throughout The Final World.

While reforming his body, Sora encounters a multitude of stars, the remnants of those whose bodies have faded away but whose bonds with the living have kept some part of them there, waiting for their loved ones to join them. Sora encounters Naminé's star, having been slain alongside Kairi. She explains that Kairi is the one keeping Sora and the rest of their friends tethered to the Realm of Light, and that she will make contact with Terra via the Lingering Will. Sora promises to save her and finally thank her in person, while she promises to send the Lingering Will to help when Sora is sent back given a second chance to survive his previous battle. A Nameless Star tells Sora that she is waiting for someone who has been changed beyond recognition and no longer remembers her, but would miss her if they were restored. Sora offers to help the star, and she whispers her loved one's name to him.

After restoring his body, Sora asks the Chirithy why it is there and it explains to him they are waiting for their Keyblade wielder, but does not inform Sora that this is Ventus. When Sora asks how to save his friends, the Chirithy alludes to the power of waking, and instructs him to look for the light in the darkness.

Sora then begins a Dive to the Heart in an effort to save his friends. Although he appears to find himself in other worlds - Olympus, Toy Box, Kingdom of Corona, Monstropolis, Arendelle, The Caribbean, and San Fransokyo - his friends take the form of ephemeral images in the same way that Sora did, indicating that the search takes place in The Final World. He repeatedly fights the Lich in order to save his friends' hearts, successfully bringing back Donald, Goofy, Riku, Lea, Aqua, Ventus, and Mickey. San Fransokyo is the last world he appears to visit, where Young Xehanort attempts to warn him of the dangers of using the power of waking recklessly. After Xehanort departs, Chip and Dale call Sora on the Gummiphone, revealing that a new path to the Keyblade Graveyard has been found. Sora then departs the Final World, finding Kairi along the way.

After defeating Master Xehanort, Sora returns to The Final World in order to save Kairi after Xehanort crystallized her. Chirithy meets him once again, and Sora explains the situation. Chirithy attempts to warn Sora of the dire consequences of going through with this, as he had misused the Power of Waking too many times already. However, Sora is unfazed and willing to go through with it anyway.

After rescuing Kairi, Sora brings her to The Final World to meet Chirithy. Sora offers to bring Chirithy to Ventus, and they all depart together.

One year later, while Kairi is asleep in Ansem the Wise's lab, she dreams of The Final World.

Sora awakens in a nighttime version where he meets Yozora, who explains that he was tasked to save Sora. However, Yozora transports the pair to Quadratum where they fight. After Sora wins the fight, he will return to the daytime version of The Final World.

Kingdom Hearts Melody of Memory
Following Sora's disappearance, Kairi ends up in a replica of The Final World after searching through her memories. Recalling how Terra-Xehanort had sent her away from Radiant Garden, she encounters her own heart shattering in to pieces and chases its fragments. Though she is able to obtain almost all of them, she is accosted by a vision of Master Xehanort. After he confesses his role in drastically altering the course of her fate and dooming Sora, he promptly battles Kairi and nearly incapacitates her before Sora controls her body, defeating Xehanort and freeing the final fragment of Kairi's heart. Weakened, Xehanort tells Kairi that Sora is in a world beyond reality before disappearing.

In the real world, after Kairi reveals Xehanort's message to Ansem the Wise, the Fairy Godmother takes her and Riku to The Final World, revealing that both of them are two keys needed to find Sora with the third being the Nameless Star. When Riku recounts his dream, the Nameless Star recognizes the city he described as Quadratum, a world beyond reality. As the Nameless Star is a resident of Quadratum, she acts as a gate between reality and fiction. Though Riku is able to travel, Kairi chooses to stay behind to train further.

Trivia

 * Enemies normally do not appear in this world, but should an enemy be spawned inside of The Final World, the battle theme from Scala ad Caelum, Edge of Existence, plays.