Please note that while this story will cover several canon events, it will not be canon compliant. There's just too many details in canon now for me to fit them all in.
Prologue
The first thing he's aware of is pain. He lurches off his stomach and onto his feet, as if he can run away from it. But it follows, it follows from a place deep within and he keels over until he's on his knees. There is a fire in his chest, hot and searing, cold and jagged. His heart quivers with each breath, like there's too much inside and it's about to explode.
A weapon comes to his hand. It's a long, sword-like thing with two crossed keyblades as its – it vanishes again as he cries out in agony. Now he's on his hands and knees. He wants it to stop. He just wants to go home and curl up until it stops. . .
Responding to his unspoken desires, a dark portal opens in front of him. He looks up and forces himself to trudge through the darkness. It spits him out in a room both strange and intimately familiar, and he crawls into the bed. He throws the covers over himself until he can't see. (Can't see the window and the land outside which was once so bright but is now shrouded in darkness.) He clenches his jaw and tries to sleep it off.
Back in the desert he just left, a blue-haired woman stirs.
Xehanort is down, but the heart is still corrupted.
The battered armour drags itself forward. Its rage is smouldering, but there is nothing to fight. Xehanort is down, and should it destroy the body, He cannot return. It wonders though, if the gain is worth the price. There will be no Xehanort. Aqua and Ven will be safe.
She is calling His name. The armour turns. It has no eyes with which to see, but it feels the pulse of her heart, and the soft hum of her keyblade. It extends a hand in acceptance, in loyalty. She is not yet here, but it knows the direction in which she lays.
The heart behind it stirs. It turns; the keyblade sharpens into a whip. But with the last of its strength, Xehanort's heart calls upon the shadows to swallow him. The darkness fades, and Xehanort is gone.
She is here. She calls His name. Her hand rests on his chest plate and she is disturbed at the lack of heart.
"What happened to you?" she asks. "Who are you?"
"I am what remains. Radiant Garden," it intones, because that is where He is. Because it was Him once, so it knows. "That is where you will find Him."
She knocks on his chest and listens to the echo. "Are. . . Are you going to be okay?"
"A will like mine does not fade," it tells her. "Find Him."
She nods, and she is gone. It cannot follow; it lacks the body to endure through the perils of space. It must stay. It is here Xehanort's plan will reach its climax, and here Xehanort will return.
"One day. . . I will set this right."
"I'm with you," she whispers. She smiles as she casts Terra off toward the light.
He wakes up, and he's engulfed by urges he doesn't fully understand. (Find him. Find her. Claim Kingdom Hearts. Rip out the Master's heart.)
His feet take him to the Keyblade Graveyard, the place where he expects to satisfy his strongest cravings. Terra and Aqua do not respond to his shouts, so he shifts to Plan B. The X-Blade comes to him (his soul sings because he knows this keyblade, because it means he is finally complete), ready to serve.
He should use it. He should summon Kingdom Hearts. It is his purpose.
His arms shake as he stands in the Keyblade Graveyard and points the X-Blade at the sky. He needs to do this. This is the reason he was created.
(Yet something at him screams otherwise. He wants to summon Kingdom Hearts, and he wants to die to stop the summoning. He wants them to put an end to him so this never, ever comes to pass –)
He pulls at his hair and screams. His knees hit the sand.
"What are you doing here?"
The voice echoes. He looks up at the gold and red armour. Terra?
"Ven . . . Why are you here?" Terra asks.
"I'm here to unlock Kingdom Hearts," he says truthfully.
A strange noise comes from Terra, like his bones are rattling in a cage. He swipes Ends of the Earth through the air. "No! That is Xehanort's plan. You cannot."
"No?" he repeats.
He looks up at the armour towering over him. The image flickers, and then it is Terra he is looking up at, Terra telling him to leave.
"O-okay," he says. He wipes tears from his eyes. He laughs bitterly because he was the one complaining about being smothered before and now he's just so glad to have Terra to guide him.
But something is wrong. He inhales and smells only metal. He knows the scent of Terra's darkness (from that time where he crawled in shadows, when he was a wretched soul chasing after the light) He knows the scent of Terra's heart.
"Terra, what happened to you? Where's your heart?"
"Radiant Garden," the armour intones.
"Do you remember your name?" the blond man asks him.
It takes a while to come, like it's wading its way to him through a thick sludge. "Xehanort."
Behind Ansem, Braig smirks.
"My name is Xehanort," he tells the boy that has suddenly appeared at his bedroom door.
There is a long silence. The boy is small, young, and yet Xehanort finds he cannot tear his eyes away.
"Is this some kind of joke?" the boy suddenly spits. And Xehanort steps back because he wasn't prepared for the frantic fury in those gold eyes. "Your name isn't Xehanort. You're Terra!"