Core of the Arcology

The Arcology's Core lay badly damaged. The staircase that once spiralled around the outside walls had large chunks missing, and several more in precarious places. Upon one lay Sora, not at his full strength. Something had been and still was sapping his power, and an unexpected encounter within the walls of the Arcology had left him feeling worse.
The floor of the Core was littered with the pieces of stair and shards of shattered glass. The thin staircase that lead back into the basement had been blocked off by a particularly large piece.
The Kingdom Heart of the Arcology was darkened, it's power also drained by the child Heart even though the two were no longer attached. That child Heart, with the Hearts of every member of the Arcology had taken on form. Sora's own Heart resided inside the form of his own old enemy revived.
Up at the top of the Core, upon the lone remaining transparent platform, were his friends and enemies. Kairi, Riku, Donald, Goofy, King Mickey, Jamie, Alex and of course Roxas. Zexion and Vexen, previously of the same Organization that Roxas had been a part of, stood there too.
Then in front of them was the revived enemy, formed from the child Heart.
In front of them, looking exactly as he had when Sora had confronted him alongside Riku, was Xemnas.
"I stopped you once before," Riku told him. "With or without Sora – I can do it again."
"No," Xemnas answered quietly. "You can't. None of you can. Some of you are subject to the authority of the Arcology. As I now possess the Heart of Sora, I can stop any of you even as he could." As if to emphasise his point, he pushed up one sleeve. On it were markings identical to Sora's – complete with 'Sora' in the ornate lettering.
"I'm not a part of the Arcology. Neither is Kairi."
"Don't forget about us and the King," Donald added.
Xemnas merely extended one hand and snapped his fingers. Sora's keyblade appeared in it.
"What is Sora's, is now mine."
The keyblade disappeared again.
As he looked bemused toward where it had been, he then said, "Looking for something?" this time overlaid with strong overtones of Sora's voice. Xemnas looked down sharply toward Sora. He was still stuck on the small chunk of stair, sat leaning against the wall with a cocky smirk, tossing his keyblade from one hand to the other.
"You might have my Heart," he told Xemnas, Xemnas seemingly forced to repeat his words in unison, "But you'll never control it – or anything else you try to take from me."
"You are not Xemnas," Jamie accused then. "Xemnas existed long after the Arcology's Kingdom Heart was created. He never had the chance to corrupt it."
"You stupid boy," Xemnas shook his head. "Why do you think Timeless River exists? The corruption of the Cornerstone was merely a diversion that allowed me to go back and create that link. Then you conveniently removed the only remaining link that could possibly have stopped me – Alex. As long as he remained on the Ancient Forest isle, the isle would never fall to the ravages of fire, and it would protect the past from me."
"Hold on a moment. I visited there after you claim to have done, and you did at the same time Sora did. You're talking about a paradox."
"No. Timeless River is true to it's name – Timeless. When I made my visit, yours had already taken place. When you made yours, unaware of what you were doing, you actually made it before Sora made his. I really must thank you Jamie... it was ultimately you who allowed me to be again. You've served me better than Vexen has this time, and you didn't even know it."
While this had been going on, Zexion had been in a whispered conversation with the King.
"Try it," Mickey told him.
Zexion nodded, flicking open his Lexicon. Xemnas' reflection drew up out of the platform, then vanished again.
"What's wrong, Zexion?" he taunted. "Can't make me face myself? I embody nothingness. You cannot create nothing from nothing."
"Then we'll just have to take you on the old-fashioned way," Mickey asserted. "Everyone take arms – we'll do this together!"
Weapons were sprouted from all, even Kairi managed to find a keyblade.
"I forbid all those of the Arcology to stand against me," Xemnas told them.
Roxas' keyblades and Axel's Chakrams vanished. Jamie's spear did not vanish, instead he was forced to take a more passive stance.
"I don't," Sora said through Xemnas. "Stand against him all you want. I exempt you from having to follow commands issued by any using my Heart or position until we're finished here."
The weapons reappeared again, and Jamie was able to take up battle stance once more.
"You're beginning to irritate me, boy."
"Only beginning to? Damn, I can do better than that."
Xemnas drew out two of his ethereal blades, but they quickly vanished as Sora's Keyblade reappeared in Xemnas' hand.
"What are you doing?" Roxas shouted down to him.
"Trust me!" Sora called back.
"I hate it when he says that."
In moments the allies, Vexen now included, had surrounded Xemnas and were on the attack, jabbing and stabbing, hacking and slashing, with magic and blade, staff and shield, spear and wits.
Sora's reasoning wasn't apparent. With the Keyblade at his disposal, any defence any of them mustered was easily cut through. Donald's magic was more often reflected back at them than not. Zexion conjured whole armies of illusions and mirror images, but few of them got to land anything before they were dispelled.
Goofy was repeatedly thrown back, his shield no help at all. Through it all, Roxas, Riku and Mickey seemed to achieve the most, their own keyblades doing more than anything else. Xemnas had to continually block not one, but four keyblades.
Every time Xemnas broke a large enough gap through them, Jamie appeared there and hurled a wedge at him from the tip of the spear, emphasised with the name of the element and backed up by a ball of whatever the chosen element was direct from the Arcology's power itself. This close to it's source, their power was magnified immensely, making up for the drain Xemnas was placing on them. Only Sora seemed unaffected by this boost.
Through it all, Axel threw fire alone around, skilfully guiding it away from the allies and toward Xemnas. Whenever Xemnas made an attempt for Kairi, his fire was there to stop him. At one point, all but he, Jamie and Kairi were scattered to the walls. They stood between him and her, and both launched every fire-based attack they had in their arsenal while Donald and Roxas went around casting Cure on everyone else
Xemnas retaliated with his barrage of lasers, but this time Kairi came to the rescue, using various reflection spells to send them straight back at him, allowing their allies to get back to the attack.
Just as Xemnas seemed to be getting to grips with the Keyblade, Sora caused it to vanish again at a critical moment, allowing several attacks to land unhindered.
Xemnas brought back the twin ethereal blades, resuming his assault. With these blades, he had twice the power, and was able to block twice as often and prevent most attacks from harming him, but also seemed unable to break through their concerted defences to harm them.
Once more, as Xemnas was readjusting back to his accustomed weapon and preparing to block attacks, Sora switched the weapons. The howls of frustration were edged with snarls directed at him.
This time round, having more experience with the Keyblade made the fight biased toward him, but the allies quickly picked up on Xemnas' routines and habits, and through their own experience battling him brought the balance back again.
Sora changed the weapons again and again, with increasing frequency as Xemnas began to anticipate the timings between them until it made no difference any longer. Sora left him with his ethereal blades for the safety of his friends, then waited.
Xemnas grew increasingly frustrated as his attacks were blocked time after time, and his attackers began to learn and exploit his defences against him until at last, he vanished and appeared in mid air beside Sora.
"So you finally got back to me," Sora greeted him. "Am I really that low of a priority to you that you've only just worked around to my attention?"
"Silence, boy," he growled. "You've troubled me long enough. You are a Nobody now, a mere shell, and I shall destroy you like any other."
Sora made no move to stop him, crossing his arms defiantly and continuing to grin even as Xemnas made attack after attack.
Kairi cried out, running to the part of the platform above him and kneeling as if it would bring him back to her. Similarly shocked expressions were on all faces except those of Alex and Jamie. Theirs were kept carefully blank. Unlike the others who rushed to various positions, they remained behind and apart from them. After a few moments whispered conversation, Jamie fetched Mickey and Donald, continuing the conversation with them present.
Sora closed his eyes, and seemed to fade away just as any Nobody of the Organization had done.
Kairi cried out again as he disappeared, only to be comforted by Riku. She clung to him even as he looked down on Xemnas with a mix of disbelief and hatred.
"I've done it," Xemnas whispered. "I've beaten him. I've... destroyed Sora..."
There followed a moment of complete silence. With a single glance toward one arm, the Keyblade appeared once again, and he laughed.
He laughed as he held the Kingdom Key of the Realm of Light, and knew he had triumphed at last over it's previous wielder.
He laughed as the friends of Sora grieved.
He laughed until some unseen force struck him with such strength that he was smashed into the floor below, sending a shockwave of glass shards outwards to stick into the walls of the Core, and scattering chunks of debris behind them. More parts of the stairway fell, one piece landing on him.
It was quickly flung off as the last of the debris settled again, and Xemnas rose again like the wrath of kings. His eyes flashed with malevolence, but nowhere in the Core could he see the source of this attack.
He vanished, reappearing just underneath the platform, searching from above when once again he was struck by the unseen force and hurled back to the floor almost too quickly to see it happen.
He rose again.
"Who's there? Show yourself!"
"No," a voice breathed so quietly that it could not be identified, but at the same time loud enough for all to hear.
"I demand you show yourself!"
"You make no such demand of me."
"Who are you?"
"Who do you think?"
With an angry grunt, the entire Core below the platform was bathed in the barrage of lasers, but none hit anything beyond the opposing walls.
"Where are you?"
"Where you least expect it."
"Are you too afraid to reveal yourself to me? Do you fear my power so much?"
"I cannot feel fear in any way, Xemnas. Surely you of all people should know this? Do you not realise that even if I could, I would not be afraid?"
"Then what have you got to lose in facing me?"
"Nothing," the voice said, taking on form at last. The glass from the floors and walls drew up sharply, then cut through the air to embed themselves in other parts of the walls, cutting the air as they passed through it. With every cut, the outline of a new form appeared in the air in the centre of the Core.
At first it was merely an outline, then it became a hooded robe. Boots and gloves formed, followed by colour. Not merely black, it shared the designs of white that Xemnas himself wore.
The wearer stood seemingly motionless in the air, then pushed back the hood. There were gasps of astonishment from above, and from below a snarl of, "You!"

A/N: I might have the next chapter available tomorrow... but the chances are slim. So for now, merry christmas to those of you who celebrate it, to those of you who don't, pick the next closest thing, and we'll see who this mysterious newcomer is just as soon as it's all over with!

(You're welcome to make guesses of course... and no doubt some people have an idea - but I'm not saying anything until the next chapter)