Terra-Xehanort no longer exists. After the tragedy by the heartless attack Xehanort (Terra-Xehanort) became a heartless (Ansem the Seeker of Darkness), and created a nobody as well. This could be explained if Xehanort offered himself to become part of the experimental guinea pig, at the time they were attempting to become darkness in order to gain power. They used their artificial heartless technology as part as their experiment, which is why you see the "artificial symbol" on "Ansem the seeker of darkness", in hopes of controlling them, and could explain why only Xehanort's heartless stayed in a humanoid form possessing immense power and self consciousness.
Xehanort, at the time of the experiment, still did not recover all his memories, but did happen to summon the keyblade. This can be proved since both Xehanort's nobody and "Seeker of darkness" both refer themselves as "Ansem" and have no prior memory of their real past.
Remember that Terra-Xehanort have become one heart, battling each other, thus the heart succumbed to a coma like state, forgetting all memories from the past and becoming an entirely new person. But since Xehanort was able to summon his keyblade he does in fact have the ability to gain his memories back, when this happens the battle of the two-hearts-in-one will begin again, eventually. Only a keyblade wielder, like Sora, is able to unlock their hearts and put them in their right places. 3D drop distance will answer these questions and most likely Sora will have to defeat "Ansem the seeker of Darkness" and "Master Xehanort" (the original) as well. This story plot is highly likely since the trailer states Sora to "restore them in their proper places."
The question is, how does "Ansem The seeker of Darkness" revert from being a heartless to a human again? Since heartless' are just manifested hearts. If the heartless were to be destroyed, it would end up in Kingdom Hearts. Sora most likely has to do something about Kingdom Hearts and the hearts that are trapped in them.
Due to the final boss battle between Vanitas and Ventus, Ventus is placed in a camotose state because he lost his heart due to the destruction of the "X-Blade". However, his heart finds its way into Sora's giving Sora the ability to use the Keyblade while Ventus's body remains in the Chamber of Awakening created by Aqua. Terra, on the other hand does not get his body back. Xehanort and Terra, or Terra-Xehanort, are still having an internal conflict for control, thus making Xehanort turn the Keyblade on himself which backfired. Xemnas(the Nobody of Xehanort) was created. Have you every stopped to think about the common facial characteristics Terra, Terra-Xehanort and Xemnas have in common? I think that Terra's body is still there, just taking different forms of people.
His body is taken over by Master Xehanort.
**SPOILER WARNING** Terra is sent out to find Master Xehanort and the source of the Unverse under orders from Master Eraqus. In the process, he is manipulated into doing terrible things by many people. This manipulated into killing Master Eraqus, and he leaves the Land of Departure unprotected for Master Xehanort to destroy. He then confronts Master Xehanort and fights him in The Keyblade Graveyard. After Vanitas creates the x-blade, Master Xehanort stabs himself with the keyblade and forces his heart into Terra's body. Master Xehanort manages to take over Terra's body, but Terra wont accept it. His armor rises from his will and soul, and attempts to reclaim his body. He manages to defeat Master Xehanort, but after the x-blade shattered, the body was lost in the confusion, and the armor-known as Lingering Sentiment- remained in the Keyblade Graveyard. Afterwords, Aqua confronts 'Terranort' (Master Xehanort possessing Terra's body) in Radiant Garden, and she tries to reclaim his body. In the end of the fight, Terra tries to eject Master Xehanort from his body by stabbing himself with the keyblade. This opens a dark portal which Terranort begins to fall into. Aqua rescues him, but is lost in the darkness, and Terranort is left knocked out in Radiant Garden. Braig then brings Ansem the Wise to see the knocked out man, and Ansem the Wise adopts the man as an apprentice. Ever since then, he's been called Xehanort, and has been the main villain of the series up until now. (Mind blowing, right?)
terra-xehanort or simply xexanort :)
Smash him with Terra and Lightning Will.
Yes, Terra dies at the end of "Kingdom Hearts: Birth by Sleep." After battling Master Xehanort, his body is taken over by Xehanort, leading to his transformation into Ansem, the Seeker of Darkness. Although Terra's heart is lost, his spirit and will continue to play a significant role throughout the Kingdom Hearts series.
*NO SPOILERS*Ven and Roxas have some sort of connection through Sora. We know that they are all connected by some relation, but nothing is known yet. More will be explained in the upcoming 'Kingdom Hearts: Birth by Sleep'.Note:Please don't spoil anything, guys! Wait until the game comes out!-RoxasDive
When Master Xehanort took control of Terra's body, Aqua didn't realize this until he attacked her. In order to free Terra's heart, she sacrificed her armor and her keyblade. She was then sent to the Realm of Darkness, which she remains for the rest of the story. Xehanort gets sent to Radiant Garden where he is apprenticed to Ansem the Wise. However, in Kingdom Hearts 2 Final Mix, Xehanort is seen talking to her empty armor.
Xemnas AKA Ansem AKA Xehanort AKA Terra
He merges with xehanort to become 'xemnas' and is accepted by ansem the wise to work for him.
Successfully complete the Prison Guard Challenge in the Mirage Arena
No one really went "missing". Xehanort faked his going missing just so that him and Vanitas can lure Ven out of the Land of Departure and so that Xehanort can use Terra.