Kingdom Hearts χ

"Far away there is a World of fairy tales.
It is a World filled with light.
The light gave birth to many lesser worlds, and then embraced them.
The source of light is Kingdom Hearts, and the key that protects it is the χ-blade.
So long as it endured, it was believed that the World's light would shine eternal.

However, bright light casts shadows that beget darkness.
Darkness brings anxiety and fear, awakening desire.
Formerly hidden in the depths of the heart, darkness turns to chaos and begins to spread across the World.
To halt the advancing darkness, those who desire light forge Keyblades in the image of the χ-blade and set out on an adventure across the World."
—Message from the KINGDOM
Kingdom Hearts χ[chi] opens in pop-up book format, showing Kairi and her grandmother in Radiant Garden. Kairi's grandmother tells Kairi her favorite story of light and darkness.

In Dive to the Heart, Player is given a choice to choose which of the five Unions to join: Anguis, Leopardos, Unicornis, Ursus, and Vulpes. Once Player has chosen, darkness surrounds them and Player summons forth a Keyblade. After being enveloped in light, Player appears in Daybreak Town and is immediately attacked by a Darkside Heartless. Player is easily overcome by the large Heartless, but is saved by their Foreteller, who forces the Darkside back into the Corridor of Darkness and follows it in order to destroy it.

Player is then introduced to Chirithy, who explains that Player had been chosen to become a Keyblade wielder to fend off the darkness that is quickly covering the worlds and collect Lux, and it has been tasked by Player's Foreteller to act as an aide to the novice Keyblade wielder throughout their training. Chirithy indicates that Player's Keyblade is incomplete and still requires the power of "Cards". When Player goes to pick up a nearby Card, a Moogle takes it instead. The Moogle tells Player that they have many Cards that they can offer, but for a price before it gives Player the Card as a sample of what they have to offer.

After exploring the area and defeating several Heartless, Player is approached by their Foreteller, who explains that they will need to call upon the help of others to defeat stronger Heartless. When another Darkside appears, numerous Keyblade wielders from Player's Union arrive to help Player take down the powerful Heartless. Following their victory, the Foreteller warns Player that there are others who appear to be collecting light in order to fend off the darkness, but other Unions do not necessarily strive for peace across the worlds and are gathering and hoarding Lux for their own selfish desires. The Foreteller instructs Player to discern whose darkness is hiding behind a façade of light for themselves. Once the Foreteller leaves, Chirithy sends Player to other worlds to fight the darkness via the Lanes Between.

Player visits future worlds Dwarf Woodlands, Wonderland, and Agrabah, where they defeat hordes of Heartless, but they are unable to locate where the Heartless are originating from. Upon returning to Daybreak Town, Chirithy decides to tell Player the story of the Master of Masters and how he bestowed a copy of the Book of Prophecies to each of the five Foretellers, which contains a prophecy of the Keyblade War. In order to save the future from being engulfed in darkness, the five Foretellers began using the hidden power within the prophecies via the Cards. Chirithy reminds Player that even though the five Foretellers have the same goal, their wills differ, which is why Player must not lose sight of who they are. Later, Chirithy discovers that the Darkballs that they found in Agrabah had come from another world and sends Player to Olympus Coliseum to defeat the Heartless.

Afterwards, Player returns to Daybreak Town to help several other Keyblade wielders fight off Heartless and meets another Keyblade wielder named Ephemer, who is from a different Union. Ephemer wishes to explore the Foretellers' Chamber, and despite Chirithy's protests, Player agrees to accompany him, though Ephemer eventually goes ahead without them. When Player arrives at the Foretellers' Chamber, Foreteller Ava challenges Player to a fight to test their strength and defeats them soundly. Afterwards, Ava notices Player that they have sadness in their heart, which is linked to darkness, and tells the Player to return home. During a mission at Beast's Castle, Player meets another Keyblade wielder named Skuld, who is from the same Union as Ephemer and invites Player to explore the Foretellers' lair. They are suddenly shocked to see two Foretellers fighting each other and encounter strange Heartless as they continue their mission. After the two defeat them, they also encounter a Nightmare Chirithy, who informs them that the Heartless they defeated are Keyblade wielders consumed by darkness.

To further understand the mysteries of the Foretellers and the future, Player, Chirithy, and Skuld re-enter the Foretellers' Chamber, where they are caught by a Foreteller. They are told that Ephemer is a traitor to his Union and that he has disappeared. Player then challenges the Foreteller to a duel, assuming Ephemer has been captured, but are no match. Afterwards, the Foreteller is revealed to be Foreteller Ava, and that Player's quest was a test of their character to see if they are worthy joining the Dandelions and going on to the Realm of Light to avoid the impending Keyblade War. Skuld accepts the offer, but Player remains uncertain if they should join Ephemer, who has gone on to the Realm of Light, or stay and fight to save the future. Later, they overhear two Keyblade warriors accusing each other of stealing Lux from their Unions, and Player is forced to fight Foreteller Aced, who defeats them with ease. When Ira arrives to stop the fighting, Aced claims that he is collecting more Keyblade warriors to prepare for the upcoming war before he departs. Ira tells Player, Skuld, and Chirithy that the war cannot be avoided and tasks the trio to search for Foreteller Gula, who may know of a solution to avert the war.

When Gula is found, he reveals that the only one who could stop the Keyblade War is the Master of Masters, who had disappeared along with the sixth apprentice, Luxu. Meanwhile, Ava tracks down Luxu, who had been ordered to watch the future unfold by the Master after discovering the contents of the "Lost Page" in the Book of Prophecies. Once Luxu reveals the truth about the events that have been occurring and the identity of the true traitor, a shocked and furious Ava engages Luxu in a duel, ringing the clock bell that signals the beginning of the war. Meanwhile, Skuld returns to her Union and Player encounters Nightmare Chirithy, who scorns them for avoiding the war. Nightmare Chirithy then summons the corrupted Keyblade wielder Heartless from earlier, forcing the Player to fight them. When Player emerges victorious, Nightmare Chirithy dissolves into darkness, but vows to meet them again.

A few days later, the Keyblade War begins and Player battles the Foretellers and despite their ability to persist and survive, earning the praise of the Foretellers, the wounds they suffered made them collapse in exhaustion. The war ends with numerous Keyblades planted in the ground, forming the Keyblade Graveyard. On the brink of death, Player loses consciousness as Chirithy, Skuld, and Ephemer approach them.

Some time later, Player wakes up to Chirithy at a simulation of Enchanted Dominion. Chirithy claims that the events of the Keyblade War have been a dream Player has had ever since turning down Ava's offer to join the Dandelions, and the two decide to rest after confronting a wall of thorns. On the other side of the thorns, Maleficent congratulates herself with seemingly traveling back in time to the Book of Prophecies' hologram of Enchanted Dominion. Confident that Sora and company cannot meddle in this world, she contemplates on conquering the new world that she has entered in.

(Summary from KHWiki.)