Terranort (kinda) explained

Near the end of Terra's story, Xehanort manages to put his heart on Terra's body, creating Terranort. A part of Terra (his mind, his will, your call) remains on his armor and promptly fights Terranort before the X-Blade's destruction sends everyone god knows where. Aqua goes to find Terra and finds Terranort, she fights him and during it realizes Terra's still fighting inside, and by the end Terranort seals his heart and memories (plus he almost falls into the Realm of Darkness but Aqua saves him).
Terranorts remember nothing but his name (Xehanort), but Xehanort clued in Braig on everything before things happened so he's there to help and all the stuff that's the background plot of all Kingdom Hearts game happens. Also he renames himself Ansem after banishing the real one. Also if you thought Sora having Ventus' heart inside him was confusing then prepare yourself: Xehanort is apparently now the ruling heart of Terra's body, Terra's heart is inside Xehanort's but Eraqus' heart of all people is inside Terra's now... apparently, so the sealed memories and control over the body turns into a battle of attrition... which apparently Xehanort wins but it sure took longer than he hoped.
Xehanort gets at least some memories (plus his Keyblade) back, releases all the apprentices' hearts (plus Isa and Lea too I think?) including Braig and himself. Xemnas and Ansem, the Seeker of Darkness are born. Ansem gets bitchslapped by light in Kingdom Hearts, Xemnas gets a laser through the chest a la Dragon Ball's Special Beam Cannon in Kingdom Hearts II. They're both gone, which retcon magic from Coded and 3D now tells us reforms the original being... but who comes back, Xehanort or Terranort? We don't know. Not helping is the fact that, apparently, most members of the real Organization XIII traveled through time... so is Master Xehanort a time traveler, or the reformed being? Did Terranort split apart again like Braig did and how they did to Isa? Why did they even ran out of time at the end of 3D? I guess we'll have to wait for Kingdom Hearts III to (hopefully) find out.