Terra-Xehanort

Terra-Xehanort is the result of Master Xehanort forcing his heart into the body of Terra, thus making him a fusion of the two Keybearers. He is the final antagonist of Terra's story and the Final Episode in Kingdom Hearts: Birth by Sleep.

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 tries to shield himself with his armor, but fails as his darkness allow Xehanort to take over, 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 the χ-blade (and Vanitas) by Aqua and Ventus. 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 symbiote.

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 Terra out. However, this weakens Terra-Xehanort enough for his symbiote to open a portal of darkness and drag him into the Realm of Darkness. Aqua dived in after him and sacrificed her armor and Keyblade to save what is left of Terra's consciousness.

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 the following day, where the amnesiac Keyblade wielder lies unconscious with no memory at all. Upon Ansem's request, the amnesiac Terra responds that his name is "Xehanort" On Ansem's order, the amnesiac young man is taken back to the castle by Braig while Dilan takes Aqua's armor and Keyblade.

Personality
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's ruthless and cunning.

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".

Appearance
Terra-Xehanort's appearance is almost identical to Terra's, only with Xehanort's silver hair, darker skin and yellow eyes.

Abilities
Terra-Xehanort, being a possessed version of Terra, 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 like Ars Solum, a fast combo comprised of multiple hits. He can use Meteor and Quake as well, and is 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, Seeker of Darkness, 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.

Weapon
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, Way to the Dawn, 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 lost his memories, he lost his ability to wield this weapon, but as Yen Sid later claims in Kingdom Hearts coded that one Keyblade Master will not be enough to defeat the revived Xehanort; it is possible that Xehanort has become Terra-Xehanort once again, along with his memories and ability to wield the Keyblade.

Trivia

 * The first few sentences that Terra-Xehanort says after coming into existence are a nearly word-for-word quote of what "Ansem" (Xehanort's Heartless) says when Sora finally confronts him in the original Kingdom Hearts.
 * In the Japanese version, Terra-Xehanort is voiced by Akio Ōtsuka, the son of Chikao Ōtsuka, the Japanese voice actor of Master Xehanort. Akio also voiced "Ansem" (Xehanort's Heartless) in Kingdom Hearts and Kingdom Hearts: Chain of Memories and Riku-Ansem and Xehanort in Kingdom Hearts II.

Quotes

 * "This heart belongs again to darkness. All worlds begin in darkness, and all so end. The heart is no different. Darkness sprouts within it. It grows, consumes it. Such is its nature. In the end, every heart returns to the darkness whence it came."
 * "You have lost! Yield!"
 * "Who...am I?"
 * "Terra's heart has been extinguished! Smothered by the darkness within him!"
 * "I will guide you to the depths of darkness."