Dark Seeker Saga

The Dark Seeker Saga, informally known as the "Xehanort Saga" (ゼアノート編), is the phase of the Kingdom Hearts series that details the battles between Sora and Xehanort, as well as various other heroes of the light swept up in the conflict. According to series director Tetsuya Nomura, the Dark Seeker Saga is only the first phase of the Kingdom Hearts series, which will continue after the saga is complete.

Becoming a Master
Xehanort, a young man from Destiny Islands, yearns for a way to leave his home for a more adventurous life. Eventually, he manages to do so, by means unknown, and comes to live in the Land of Departure, where he becomes a Keyblade wielder under a Keyblade Master and befriends a fellow apprentice, Eraqus. Xehanort and Eraqus eventually come to see each other as brothers, but their friendship is shattered by their conflicting beliefs in the balance of Light and Darkness. While Eraqus believes Darkness must be destroyed, Xehanort believes Darkness can be controlled with enough training. Xehanort and Eraqus eventually pass the Mark of Mastery exam and are deemed Keyblade Masters, but Xehanort, having learned much of Keyblade lore, particularly the events surrounding the famed χ-blade and Kingdom Hearts, sets out on a mission to merge with Kingdom Hearts and remake the universe in his own image, despite the lives it will cost. His first step is to find a vessel to transfer his heart into from his now-aged body, but his apprentice, Ventus, proves to be too frail for this, so he instead decides to use Ventus for the χ-blade by extracting the Darkness from his heart into a separate being, Vanitas. Though Ventus survives the ordeal, Xehanort takes him to Eraqus to be trained in Light. In the process, Xehanort meets Eraqus's two students, Terra and Aqua, and sets his eyes on Terra as his new vessel.

Quest for the χ-blade
Four years later, Xehanort begins to manipulate Terra into turning against his friends and loved ones and luring him toward the Darkness. First, he sabotages Terra's Mark of Mastery exam and then stages his own disappearance. Along the way, Xehanort comes into contact with an evil fairy, Maleficent, and tricks her into beginning a quest to gather the Seven Princesses of Heart, maidens with no darkness in their hearts. Once "found" by Terra, Xehanort "guides" Terra in the ways of the darkness, and stages his own kidnapping at the hands of a lackey, Braig, in order to provoke Terra's anger and vulnerability to darkness. Along his way, Terra comes across Destiny Islands, where he meets a young boy, Riku, and chooses him as his successor to wield the Keyblade one day. Meanwhile, on Xehanort's orders, Vanitas goads Ventus into chasing after Terra, who Ventus has come to love like a brother, in order to make him stronger for the χ-blade. Through Xehanort's machinations, Eraqus almost ends Ventus but is stopped by Terra, who battles him and injures him, while Xehanort delivers the final blow. With his plans revealed, Xehanort goads Terra to meet him at the site of the Keyblade War, the Keyblade Graveyard, while Ventus and Aqua are instructed to do the same.

At the Keyblade Graveyard, after an epic battle, Xehanort finally puts his plans into action. He transfers his heart into Terra's body and becomes Terra-Xehanort, but Terra's Keyblade Armor comes to life as the Lingering Will and defeats Xehanort. Vanitas succeeds in possessing Ventus and creating the χ-blade, but Aqua, aided by fellow Keyblade apprentice, Mickey Mouse, manages to defeat Vanitas and shatter the χ-blade, creating a corridor which scatters everyone present to different locations. Xehanort ends up in the Radiant Garden, but with his memories slowly leaving due to Terra's heart resisting him. Aqua finds him and manages to defeat Xehanort after an intense battle, allowing Terra to gain the upper hand, but Xehanort retaliates by stabbing himself with his own Keyblade and falling into the Realm of Darkness. Aqua sacrifices herself to save Terra-Xehanort, ending up trapped in the Darkness. Xehanort, now with apparent amnesia, is found by Ansem the Wise and taken in as one of his apprentices.

A Heartless and a Nobody
A year later, however, Xehanort slowly begins to regain his memories as Master Xehanort. He and Braig successfully manipulate Ansem into experimenting with the heart in order to produce Heartless, and when Ansem refuses, they and the other apprentices, Dilan, Even, Aeleus and Ienzo banish him to the Realm of Nothingness and Xehanort steals Ansem's name. Eventually, Xehanort regains his memories completely and apparently turns himself and the other apprentices into Nobodies, becoming the founders of Organization XIII, a group of thirteen Nobodies with the intention of harboring a piece of Master Xehanort's heart as an alternate way of creating the χ-blade. But knowing full well that over time, the Nobodies will begin to develop hearts and personalities of their own, Xehanort's Heartless steals his teacher's name, travels back in time and contacts Xehanort as a young man, granting him the ability to travel through time and begin gathering various incarnations of himself together as a backup plan.

Now split into a Heartless and a Nobody, Xehanort continues his plans in two different directions. As Ansem, he leads an army of Heartless across several worlds in order to swallow as many as possible, while as the Nobody Xemnas, he begins searching for more Nobodies to join Organization XIII.

Nine years later, however, Xehanort's plans begin to fall apart due to the intervention of another Keyblade wielder, Sora, who defeats Maleficent and her council of villains, and successfully defeats Ansem at the End of the World. Meanwhile, Xemnas completes his Organization, the thirteenth member being Sora's Nobody, Roxas, who can also wield the Keyblade and use it to gather hearts for an artificial Kingdom Hearts. As a backup to ensure a Keyblade wielder loyal to their cause, Xemnas creates a Replica of Sora, Xion, for the purpose of absorbing Roxas and becoming an obedient clone of Sora, but this plan falls apart when Xion develops her own personality and is defeated in battle by Roxas.

Sora, meanwhile, eliminates the members of the Organization one by one until only Xemnas is left. Xemnas merges with Kingdom Hearts and engages Sora in a final battle, but is defeated and fades into darkness.

Return
With the destruction of both Ansem and Xemnas, it appears that the madness of Xehanort has been destroyed forever, but it is soon revealed that this is merely the beginning, as this will inevitably lead to the revival of Master Xehanort himself. To counter him, retired Keyblade Master Yen Sid puts Sora and Riku through the Mark of Mastery exam in order to make them strong enough to defeat Xehanort. However, a mysterious youth hinders their journey, eventually guiding Sora back to The World That Never Was and capturing him.

Riku tracks Sora to the Castle That Never Was, where he is confronted by the youth. The young man reveals himself to be Xehanort from the past, given the ability to travel through time by Ansem in order to gather the many versions of Xehanort to attend the revival of Master Xehanort. Upon his revival, the Master admits that he miscalculated his previous attempt to forge the χ-blade with Ventus and Vanitas, but resolves to forge it correctly by pitting his thirteen Seekers of Darkness, composed of himself and his twelve other vessels from across time and space, against the seven Guardians of Light, either the seven princesses of heart or seven Keyblade wielders protecting them. However, as Master Xehanort prepares to place a fragment of his heart inside Sora's body to complete his group, his plans are foiled by reinforcements and he is forced to retreat with his incomplete Organization, promising that they will all meet again at the predestined final battle.

Notes and references
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Chroniques du Chercheur des Ténèbres