Kingdom Hearts Birth by Sleep
Terra-Xehanort first appears at the Keyblade Graveyard when Master Xehanort uses his Keyblade to unlock his heart and possess Terra after the creation of the χ-blade, seeking to use Terra's body as a younger and more powerful weapon as he ventures deeper into the mystery of the Keyblade War. Terra fails to shield himself with his armor, becoming Terra-Xehanort, a younger incarnation of the elderly Keyblade master. As he walks off, Terra-Xehanort is forced to battle the Lingering Will, Terra's armor come alive through Terra's mind. Terra-Xehanort is knocked unconscious and is then sent to the Radiant Garden in a blast of light caused by the destruction of Vanitas's χ-blade by Ventus and Aqua. The Lingering Will continues to reside in the Keyblade Graveyard from then on, Kingdom Hearts vanishing above it.
Aqua, learning from Master Yen Sid that Terra's heart has been lost, is led to Radiant Garden through Terra's voice after hearing it when she leaves a comatose Ventus to rest peacefully in a chamber in the newly reshaped Land of Departure. It is in the Radiant Garden that Aqua meets Terra-Xehanort. Confused about his identity (due to the conflicting hearts Terra and Master Xehanort within him), Terra-Xehanort attacks Aqua with Master Xehanort's Keyblade and a Heartless-like guardian.
After the climatic battle, Terra begins to get the upper hand in his and Master Xehanort's internal struggle, which forces Terra-Xehanort to use the Keyblade to force him out. However, Terra-Xehanort falls into the Realm of Darkness, Aqua diving in after him and sacrificing her armor and Keyblade to save what is left of Terra. After these events transpire, Braig takes fellow apprentice Dilan as well as their master Ansem the Wise to the location of the battle between Aqua and Terra-Xehanort, where the amnesiac Keyblade wielder lies unconscious with no memory at all, except the name "Xehanort". On Ansem's order, the amnesiac young man is taken back to the castle, along with Aqua's armor and Keyblade.
Some time later, Braig attempts to start conversation with Xehanort, who he believes is still Terra-Xehanort, only to realise that Xehanort's amnesia seems to be genuine. Nevertheless, Braig promises to look out for Xehanort.
Between Kingdom Hearts Birth by Sleep and Kingdom Hearts
Over the following year, Xehanort becomes Ansem's foremost apprentice. Eventually, Ansem begins to worry about the darkness that lingers within the hearts of others and seeks a way to stop it before it consumes his people. Ansem constructs a laboratory under his castle where he, Xehanort, and Ansem's five other assistants (Braig, Dilan, Even, Aeleus, and Ienzo) conduct experiments on darkness.
Xehanort, who Ansem has by now come to fully trust as an apprentice and friend, offers to use himself in the experiments. Ansem agrees, also hoping to unlock Xehanort's memories in addition to uncovering a way to repress the darkness within hearts. Xehanort, however, has other plans in mind though and decides to conduct his own experiments to go deeper into the research.
Xehanort, Braig, Dilan, Even, Aeleus, and Ienzo carry out their own experiments where they abduct countless people and lock them within the depths of the laboratory where Xehanort conducts horrible experiments on the subjects. After using many test subjects, one set of experiments leads to the collapse of their hearts. The subjects then go into unrecoverable conditions and are locked up in the dungeon of the castle. Eventually, Xehanort checks on the subjects and sees that they have transformed into creatures of darkness, which he names "The Heartless" as they apparently lack hearts. At this point, Xehanort has taken Ansem the Wise's name as his own and writes his own report under this stolen name.
Xehanort then presents the Heartless with living and non-living samples; the Heartless only respond to the living. Xehanort witnesses the Heartless consume the living creatures and then multiply into more Heartless. Xehanort deduces that the Heartless seek the hearts of other things, which explains why they only react to living samples, in order to create more of their kind. Xehanort and his associates soon begin to slowly accept the darkness, allowing them to have limited control over the Heartless.
They in turn begin to look less and less human, a factor that Ansem himself realizes. When Ansem learns that his apprentices have continued with the research without his permission, he calls them together and orders them to cease their efforts immediately, all the while questioning what is happening to his loyal apprentices. Xehanort, however, sees his master as a fool with no understanding of research and secretly keeps the research operating.
After observing the behavior of the Heartless and their responses to living things, Xehanort takes one aside and observes its behavior. The Heartless begins to move throughout the castle until a door appears which Xehanort somehow manages to open. Behind the door is a humongous mass of energy to which the Heartless react immensely. The opening of this door leads to the collapse of the world's boundaries, seen in the form of a meteor shower. Xehanort eventually deduces that the door actually leads to the heart of that world and that the opening of this door has led to the collapse of the walls between worlds.
This allows inter-dimensional travel, and eventually a visitor named King Mickey Mouse arrives, having constructed a Gummi Ship, a spacecraft capable of breaking through world boundaries, out of materials from the meteors and used it to visit many different places in search of answers of recent events. Xehanort and Mickey meet and have huge discussions in which Xehanort, once again, learns of the Keyblade, a weapon that Mickey also possesses.
Xehanort also hears legends about the seven princesses with pure hearts and how they have a connection to the Keyblade. The Keyblade can also be wielded by many, as long as their hearts qualify, and it seems there is a legend of one Keyblade bearer who went on a journey of destiny to seal off the doors to other worlds. Since this is not in Xehanort's interest, he seeks a way to find this weapon, which is ironic, given that his former self once wielded one, but was now lost. Mickey eventually meets and befriends Ansem the Wise and the two of them exchange theories and ideas in his office about what the Heartless could be after. Ansem decides to resist the urge to test their theories while expressing deep remorse for his actions, unaware that Xehanort has gone further than he himself even dared. Xehanort enters the office during their discussion, asking Ansem's permission to test an experiment. Ansem forbids Xehanort to conduct it and tells him to forget everything he learned from the research.
This, however, does not deter Xehanort, who continues with the experiments and creates a machine that creates artificial Heartless, all of which are marked with an emblem created by Xehanort in order to differentiate them from the pure-blood Heartless. King Mickey suspects something odd about Xehanort and tells Ansem to check his lab, collect all his data, and see if everything is under control.
Ansem does so and finally learns the full extent of Xehanort's treachery. Ansem orders his apprentices to seal off the laboratory and cease their scheme for knowledge. The order is followed, but Xehanort eventually banishes Ansem to the Realm of Darkness and ravages the world with Heartless. The Heartless, under the command of Xehanort, begin spreading to other worlds and destroy numerous ones in their mad search for hearts. One side effect of the destruction of so many worlds is the creation of Traverse Town, made up of the remains of the ruined worlds and populated by the few survivors of those said worlds.
Xehanort then abducts and sends Kairi, a resident of Radiant Garden, to the oceans of other worlds. The reason for this is because Xehanort believes Kairi possesses a connection to the Keybearer, as she seems to posses power comparable to that of a Princess of Heart, which is rumoured to posses a connection to the Keyblade. He hopes to use her to find the Keyblade master and prevent him from sealing off the doors to other worlds.
Xehanort makes the final pages of his report, which outline his intentions as well as those of the Heartless. The Heartless seek to consume the hearts of all living things. However, they not only seek human hearts, but the hearts of the worlds themselves. Their ultimate goal is to collect enough hearts to form the biggest heart of all, the heart in which all that exist are evolved from: Kingdom Hearts. Xehanort decides to use the Heartless' instincts to make his own plans: like Master Xehanort before him, to find Kingdom Hearts in order to gain ultimate power, control, and knowledge. Xehanort also learns that the creation of a Heartless can also lead to the creation of another creature called a Nobody, beings that are left over bodies of those whose hearts have been stolen away. They transform into an inhuman state and do not truly exist as they lack a heart to make them truly live.
Xehanort and the others eventually give themselves to the darkness to gain the ability to control all Heartless and wield the powers of darkness. They lose their hearts in the process and become powerful Nobodies. However, Xehanort is the only apprentice to have both a human Heartless and a human Nobody, both possessing the memories and personality traits prior to their creations. Since they possessed strong hearts and wills, Xehanort and the others are able to retain their human forms as Nobodies. Radiant Garden is almost completely destroyed, save for the castle and the mutated waterways, which had several modifications done to it by Xehanort and the other apprentices, and most of its people are wiped out while some (like Leon, Aerith, Yuffie, Cid, and Merlin) manage to escape to places like Traverse Town.
Sora finds a painting of Xehanort while investigating Ansem the Wise's study. At first, he believes that Xehanort is Ansem the Wise, given that he is the original being of "Ansem", but following discussions with Tron and King Mickey, Sora finally realises the truth about Ansem and Xehanort, and seethes with fury at Xehanort's painting, labelling him as the cause of all the chaos and mostly Riku and Kairi's situation.
Xehanort himself appears in King Mickey's flashback, where Xehanort requests permission to proceed with an experiment but is denied by Ansem, a scenario which Mickey witnessed ten years ago. This memory is what leads Mickey to remember Xehanort's name and identify Xemnas, the leader of Organization XIII, as Xehanort's Nobody.
In a flashback, Xehanort is confronted by Braig in the computer room of Hollow Bastion about the unconscious bodies of Even and Ienzo. Xehanort responds by summoning his old Keyblade, and as Braig begins to question as to whether Xehanort's memories have returned or if he never lost them in the first place, Xehanort stabs him in the chest, unlocking his heart. As Braig loses consciousness, Xehanort tells him, "My name is Ansem".
Following the restoration of Master Xehanort due to the destruction of Ansem and Xemnas, Terra-Xehanort's status is currently unknown.
Upon his "birth" in Kingdom Hearts Birth by Sleep, Terra-Xehanort appears to retain Master Xehanort's personality, including his knowledge of Darkness, thus being a mere younger incarnation of the elderly Keyblade Master. The only difference is the occasional interference by Terra, shown when Terra-Xehanort briefly asks Aqua who he is, and when he is briefly forced to stand still during the subsequent fight with her.
Shortly after Terra-Xehanort is defeated by the Lingering Will, and loses some of his memories, he ends up in Radiant Garden, and begins to question his existence upon meeting Aqua - by strangling her. By the time he is found by Braig, Dilan, and Ansem, he has lost all of his memories entirely—except for the name "Xehanort", although at one point, Braig insinuates that the amnesia is a mere ruse.
As one of Ansem's apprentices, Xehanort appears to have had a very professional and no-nonsense attitude, shown by his determination in his studies. He appeared to be easily annoyed by Braig and was shown to be equally as interested in the darkness of hearts as his original incarnation. Xehanort appeared frustrated when Ansem forbade him from continuing his studies and experiments, and ultimately disobeyed his master. When speaking to Xemnas, Ansem the Wise refers to Xehanort as "foolish" and that any world of his "would be an empire of ignorance".
Terra-Xehanort's appearance is identical to Terra's, only with silver hair, darker skin and yellow eyes. He also bears a striking resemblance to Master Xehanort in his youth, and some of his physical traits are later carried on to Ansem and Xemnas.
As an apprentice under Ansem the Wise, Xehanort wears a white labcoat with rather loose sleeves that is partway zipped down. The coat also has pockets roughly where Xehanort's hips are. He wears black boots that are identical to those worn by Ansem, Xemnas, and the members of Organization XIII, with a white/silver lining along the top and an indentation on the sides. Underneath his coat, Xehanort wears a grey vest overtop a white collar shirt, along with a purple ascot around his neck. During this time, his hair is worn long, with two bangs framing his face. His eyes have gone from yellow to brown. His hair also sports three backward-facing spikes, one at the top of his head, and one on either side, just above his ears.
As a fusion of Terra and Xehanort, Terra-Xehanort is able to use many of Terra and Master Xehanort's old abilities. He wields Master Xehanort's Keyblade, and uses it to perform techniques Ars Solum, a fast combo comprised of multiple hits. He can use Meteor and Quake as well, one of the few bosses that is able to heal with Curaga. When in desperate need of power, Terra-Xehanort gains access to the Dark Impulse Command Style, as well as the Dark Volley and Ultima Cannon Shotlocks. Terra-Xehanort also appears to possess enhanced strength, able to effortlessly lift Aqua off the ground by the throat with one hand.
Xehanort also can call upon his "Guardian", a Pureblood Heartless-like symbiote that floats behind him and serves as both his main source of offense and defense. Its great power also makes it capable of slamming opponents with physical blows, possessing targets, and firing dark energy projectiles. While Ansem can also use the Guardian in other installments, its first chronological appearance is during Terra-Xehanort's battle with Aqua in Kingdom Hearts Birth by Sleep.
After Master Xehanort possesses Terra towards the end of Kingdom Hearts Birth by Sleep, Terra-Xehanort is able to wield Master Xehanort's Keyblade. The weapon has a spiky shaft, a gleaming eye found near the teeth, which look similar to a battle ax. It is black and silver in coloration, able to lock and unlock hearts, appearing similar to the Oblivion Keyblade. It also contains two demonic blue eyes; one above the handle and the other one on the head, similar to the Soul Eater and Void Gear. Terra-Xehanort uses the weapon to perform long, powerful combo attacks, as well as use several powerful desperation attacks. After Terra-Xehanort loses his memories, he loses his ability to wield this weapon, but a year later, he regains it and uses it to unlock Braig's heart.