Earth-8544

Earth-8544 is a reality centered on the heroes and villains of Kingdom Hearts. There won't be just Final Fantasy or Disney, but there will be more.

The World
Instead of being a plethora of worlds like in the original series, it is instead one world.
 * Destiny Island
 * Traverse Town
 * Hollow Bastion/Radient Garden
 * End of the World
 * Castle Oblivion/The Land of Departure
 * Twilight Town
 * The Land That Never Was
 * Keyblade Graveyard

Birth by Sleep
The story begins in the ancient Land of Departure, were three youths, Terra, Ventus, and Aqua live to complete their training under Keyblade Master Eraqus. During the Mark of Mastery exam to learn who will be a Keyblade Master, the aged Master Xehanort and his apprentice Vanitas rigs the exam with shadow monsters. Due to this, only Aqua becomes a Master, while Terra is considered to be unable to contain his darkness, and Ventus too young.

Eraqus studies the shadow monsters and deems them Unversed, monsters created by unstable emotions. Along with finding Xehanort and Vanitas, the three students are sent to eliminate the Unversed in order to prove that they can become Keyblade Masters. Immediately, the three's journey diverges. Terra's adventure involves his manipulation by villains, Ventus' quest to find his friends, and Aqua searching for a way to purge Terra's dakness.

While staying in the kingdom of Radient Garden, Aqua meets a girl named Kairi and unknowingly uses the Keyblade Inheritence Ceremony. Eventually, all three finds Xehanort's original home: The Destiny Islands, now a playgroung for boys Sora and Riku. While Terra sees Riku has the potential of being a Keyblade wielder, Aqua asks Sora to protect Riku if he ever becomes lost, deeming more than one friend wielding a keyblade too dangerous. And Ventus learns that Xehanort created Vanitas when he expelled Ventus' darkness before he contracted amnesia while Xehanort's original student. Now, he wishes for the two to merge back together to create the X-Blade, the original keyblade created alongside Kingdom Hearts, the light of worlds.

The students return to the Land of Departure to confront Eraqus on this information, who admits that he knew but didn't want to hurt the children. But he admits that he doesn't "Want to aid Xehanort start the Apocalypse out of Curiosity." So he fights them. After the fight, Eraqus realizes that he's actually attacking his students, who he considers his students, and is disgusted with himself, but he is assassinated by Xehanort, who plunges the Land of Departure in darkness.

Xehanort tells the students to find him and Vanitas in the Keyblade Graveyard, the original site of the Keyblade War over Kingdom Hearts and the X-Blade. Here, Xehanort admits to his plan of using the X-Blade and Kingdom Hearts to restart the Keyblade War, which he believes will bring balance to the world. He also reveals that he wishes to use the darkness in Terra to be used as a more powerful and youthful body. In a vicious battle, Ventus and Vanitas merge, creating the X-Blade, and Xehanort successfully merges his heart with Terra's, creating a new, youthful Xehanort.

However, Terra's mind refuses to give up, and possesses discarded armor to continue fighting. In his own mind, Ventus is able to destroy Vanitas and the Unversed that was created by him, but is left in a coma. Ventus, Aqua, Terra, and Xehanort are scattered across the world by the X-Blade's destruction. Aqua takes Ventus to what's left of the Land of Departure, and uses her status as a Keyblade Master to recreate the land as Castle Oblivion, a maze like castle where at the center is a sleeping Ventus.

Aqua confronts Xehanort, who is having an internal struggle with the hearts of Terra and Eraqus. After an arduous battle, Xehanort rashly unlocked his heart with his Keyblade, giving himself amnesia and sending Aqua to a realm of darkness. Afterwards, Xehanort was found by the Sage-King of Radient Garden, Ansem the Wise and his apprentices', who takes him in as a student and adopted son. Additionally, Ventus' wandering heart is found by Sora and merges with him.

Kingdom Hearts
The story jumps forward 9 years, with a grown up Sora and Riku continuing to spend free time at the Destiny Islands, away from home, but an amnesiac Kairi is now one of their new friends. While exploring the island, darkness invades the world with monsters of shadow attacking. Riku is absorbed into the darkness, and Kairi vanishes before Sora's eyes, leaving him alone with a Keyblade. As he flees the island as it is engulfed in darkness, he finds himself in Traverse Town.

Here, he finds the monsters, called Heartless, are attacking here as well. He also meets a band of warriors that have joined him in finding his, and their friends. The people of Traverse Town mentions a Sage-King named Ansem, who disappearred while trying to study the Heartless.

The warriors travel to different dungeons sealing massive keyholes that only the Keyblade can seal, stunting the growth of Darkness in the world. Along the way, the heroes meets an enigmatic witch who wishes to take over the world. And what's more distressing is that Riku has joined her to search for Kairi, refusing to work with Sora due to the witch's words. Riku manages to find Kairi, but her heart is missing, leaving her catatonic.

The heroes arrive in the final dungeon, a ruined castle called Hollow Bastion, the original home of Ansem and the witch's base of operation. Here, Riku takes the Keyblade, something that only a Keyblade Wielder can do, and reveals that Sora was "Just a delivery boy." Sora's allies, adamant in their quest to save the world, joins Riku's side. Sora, refusing to give up, rushes into the bastion, armed with a wooden sword, and challenges Riku. This act of courage, allows Sora's allies to rejoin him, and with their restored friendship, Sora admits his "Friends are his power," and the Keyblade returns to him. Afterwards, a cloaked figure that has spoken in cryptic messages, appears before Riku and disappears, leaving behind his cloak.

The heroes encounters the Witch and Riku, who is now acting strange and with a Keyblade that unlocks hearts. Riku shows to them Kairi, and when Sora asks who he is, he answers: "It is I. Ansem, Seeker of Darkness." Riku's possessed form shapeshifts into that of Ansem and reveals that Sora's a conduit of lost hearts, having merged his heart with Kairi's, who, along with the lost friends of the allies, is a Princess of Heart, a woman with no darkness in their hearts, who are needed to activate the Keyhole in Hollow Bastion, Sora uses Ansem's Keyblade to release his heart, restoring Kairi, but turning himself into a Heartless.