Disclaimer: I do not own Kingdom Hearts or anything else I use except for the plot and OCs.
Silence. It ran throughout the chamber. Many were unable to comprehend what had just happened.
'That didn't just happen. Tell me that didn't just happen,' said a devastated Sora.
'It would serve no purpose,' Sam stated, 'we haven't much time. We must think of our plan soon, before the castle is overrun with Heartless.'
'What do you suggest we do?' asked Riku.
'Yeah, we don't even know anything about this here Ultameca,' Goofy said.
'Ultimecia!' Donald corrected.
'That is not entirely true,' said Sam, 'of what I recall, she is a Sorceress, like my mother. If Ultimecia was to kill her, I am sure she will become stronger.'
'So, she'll be a target then?' Riku asked.
'Most likely,' said Sam.
'Where is she?' Sora asked.
'I believe she is with the queen in the library.'
'That's not that far, let's go,' said Sora as he led the others back up the stairs and out of the throne room.
As soon as they were out. A Shadow lunged at Sora, but he quickly summoned his Keyblade and slashed it. Immediately after, he noticed that they were surrounded by Shadows like that he had just vanquished.
'Damn, they moved fast,' Sora stated.
Donald fired an icicle and impaled one of the Heartless. Several Shadows lunged at Donald, but he surrounded himself in flames. Flames, which engulfed a large number of the Heartless that had jumped forwards at him. Donald unleashed a thunderbolt, which struck and destroyed one of the Heartless. A Shadow sunk into the ground and scurried behind Donald, before rising. As it rose, it clawed at Donald's behind. That made Donald jump. In the air, he spun around in a one-hundred and ninety degree angle, and brought his staff down on it, which made the Heartless retreat a few steps. Donald shot a fireball forward and took it out.
Goofy charged forward and smashed many Shadows away, destroying a lot of them. A Shadow jumped forward from behind, but Goofy spun around and smacked the Heartless away. A Shadow slid past him and slashed his leg. Goofy spun around and slammed his shield down upon the unsuspecting Heartless, destroying it. Goofy then began spinning around, as well as charging at a cluster of Heartless. Many Shadows tried to jump at him, but they all met nothing but their end. A few of the Heartless tried to escape but they were not fast enough, and only met their end like the others.
Sam jumped to the air and sent several projectiles towards several different Heartless. After that, he slammed down to the ground, turning it black. He raised his free hand and slowly clenched. Chains then sprouted from the ground and captured many of the Shadows before crushing their small forms. When the captives were each vanquished, the free chains lunged forward and impaled many other Shadows. A Heartless jumped at Sam from behind, but as its claw encountered his body, he dematerialized, and his body became darkness. Then, unknown to the Heartless, the real Sam Heartily appeared from behind and brought Outcast down upon it, vanquishing the Shadow.
Kairi brought down Destiny's place upon a Shadow and vanquished it in the single strike. The next move Kairi made was charging forward towards another Shadow and crushing it. Suddenly, Kairi noticed the floor go black, most likely a Heartless trick of some kind. Then she saw chains rise from the ground and decimate many of the Heartless. There was only one who could do this if not the Heartless. Sam. She continued to trace his attack. The only thing more horrific then those horrible chains was the look of pure disgust upon his face.
'Take it easy,' she whispered. Sam turned to her, obviously hearing her, to Kairi's surprise.
'These things are but evidence of the weakness of all things,' Sam stated, 'what's more, they are not even amusing to dispose of. They do not bleed. Do not scream. They do not even remain after death. At least a cadaver becomes nutrients for the soil. Why should I take it easy? They deserve destruction, not mercy.'
Riku lunged forward and impaled a Shadow, vanquishing it. After that, Riku performed a back flip and slashed a Shadow that had scurried behind him. Several Shadows surrounded him; however, he spun around and slashed them all with ease. A Shadow jumped forward, but Riku slashed it while it was still in the air. He then ran forward and cut down a few Shadows that he came across, all of which went down in one strike each.
Shadow was instantly surrounded by Shadows. Interceptor raised his upper lip and bared his teeth at them. Shadow quickly drew his blades and charged in-between two of the Shadows. Quickly, they dematerialized in a flash of darkness, their captive hearts floating away. A Shadow jumped at him; however, Interceptor jumped and crushed it in his teeth. Shadow threw striker at one of the Heartless. As it dematerialized, Shadow sped in front of the weapon and caught it. A Shadow jumped at him, but he met it with man-eater, and destroyed it before cutting another Shadow that had jumped at him.
Sora ran forward and slashed a Heartless as it jumped at him. After that, he launched a fireball towards another Shadow. A cluster of Shadows jumped at him, but Sora took to the air before throwing his Keyblade at them. It slashed and destroyed them before returning to him. Another Heartless scurried behind him and slashed his leg. After which, Sora spun around and impaled the Heartless. As Sora looked about the room, he noticed only one more cluster remained. He lunged forward and slashed a few before launching a thunderbolt that consumed those which remained. With that done, Sora swiftly dismissed his Keyblade.
'The library is this way,' Riku pointed out.
'Then let us hurry. I feel a great darkness has already descended upon this castle, and that a force with an even greater magnitude approaches from the shadows,' Sam acknowledged.
Without hesitation, the group made their way down the hall, towards the library. Sora had rushed in front of the others; he quickly opened the door and lunged into the room. He was surprised at what he saw though. Many of his comrades were on the ground, severely injured or unconscious. Rinoa was standing, if only barley. She was covered in her own blood, head to toe. Behind her stood the queen, a horrified look upon her face.
'Sora, run!' Rinoa shouted.
'What's wrong?' Kairi asked as she and Riku rushed forwards to the room, but was suddenly sent flying back.
'Huh, what happened?' Sora asked. Then, he was suddenly shocked as his fallen friends, as well as the queen and Rinoa, lifted into the air, and one by one, flew from the room. Immediately afterwards, a barrier arose, blocking the doorway.
'You must be Sora, am I correct,' Sora turned and out of thin air, appeared a silver haired woman. 
'Who are you?' Sora asked.
'It is Ultimecia, our enemy!' Sam called through the barrier.
This surprised Sora, and he called Heroes crest forward.
'Feisty, willing to fight are we. Such a trait is to be expected form a young Paramekian such as yourself,' Ultimecia stated, 'yet it is strange that you do not fight alongside your own people. Especially considering your royal blood.'
'How do you know who I am!?' Sora demanded.
'To the point I see,' Ultimecia stated, 'one must know their opponent, and I feel you may be a worthy adversary, indeed.'
'Well I know you too, you're a witch!'
'Such a bitter tongue,' said Ultimecia. She quickly disappeared and materialized close to Sora, who was suddenly thrown back, 'yet you do not know me. You only see what all humans see.'
'Sora,' Sam called, 'she is the Sorceress of time, watch that. Do not let her touch your flesh either.'
'Don't worry, I can beat her!' Sora replied. 
He charged forward, but suddenly, in less than a blink of an eye, the distance had grown significantly between the two, and black energy spears surrounded Sora. The blasts all simultaneously lunged at him, engulfing Sora in an enormous explosion. As the smoke cleared, Sora could be seen on his knees.
'Those clothes, they absorbed the majority of the blast,' Ultimecia observed.
Sora used his Keyblade to push himself up. Anger was evident on his face. He charged forward, and slashed Heroes crest about. Ultimecia evaded the strikes as if they were nothing. Sora's frustration grew rapidly, and Ultimecia noticed this. 
'Why can't I hit her!?' Sora asked himself furiously.
'I've already told you!' Sam called in Sora's head, 'She's the Sorceress of time, and she is slowing time when you try to strike her so that she may escape.'
'Then how do I win?'
'You must out last her, she will tire out eventually, that is when you must strike.'
'This is pitiful. You are not even worth killing,' Ultimecia mocked, 'wait, what am I saying,' and with a mocking smirk upon her face, she sent three blasts from her fingers into Sora's chest, one penetrated his heart. His eyes widened in pain as he fell backwards, unable to even scream as the shock and pain overtook his body. A girl screamed out his name, and that was the last thing he heard before his world went black.
'Sora!' Kairi screamed out, she rushed forward, but the barrier just pushed her back.
'This cant' be happening, I ...I just...cannot believe it' were Riku's thoughts.
Donald and Goofy were too devastated to form coherent thoughts.
'He is dead. Then that means...Relm,' Shadow thought.
'And so his life goes to the darkness,' Sam thought, 'yet, there is something strange about this. His life should be fading, but instead something is rising within him. Something of darkness,'
Ultimecia was about to turn her back, but then felt it too. She quickly turned around, and saw Sora's body covered in darkness. Slowly he rose to his feet. His eyes were completely yellow. Darkness soon completely covered him, now giving him the appearance of a Heartless. In desperation, his heart and body temporarily had submitted to Anti-Sora.
'What is this?' thought Sam.
'What is this?' Ultimecia asked herself curiously. 
Anti-Sora, driven by its own primitive savagery, lunged at the only other being in the room, Ultimecia. Anti-Sora slashed with his claws at Ultimecia, who managed to evade. It continued to lunge through the air. As it hit the wall, Anti-Sora pushed himself off the library wall and back towards Ultimecia. She was not expecting the strike. Anti-Sora's claw slashed through her flesh, spilling blood onto the ground between them as Ultimecia jumped backwards. Ultimecia launched several icicles at it, all, which Anti-Sora dodged before jumping at her, but was swiftly forced back as she blasted him with a bolt of lightning. Anti-Sora got back up once again, and rushed forwards towards Ultimecia, however she created a wall of fire, which she then sent hurdling towards Anti-Sora. The flames consumed it, but stalled him only for a few short seconds. This was all which Ultimecia needed however, as she immediately appeared behind Anti-Sora, and blasted him with dark beams from her fingertips, sending him back.
'Maelstrom!' Ultimecia called as darkness surrounded Anti-Sora. It looked around to find a way to escape, but found none. The darkness began to push down, harder and harder, all but crushing it. Then, before he was completely compressed, the pressure was released, and replaced with the agony of his very life force feeling as if it was being torn from his being. Then, with a click of her fingers, the pain ceased, and the darkness vanished.
Anti-Sora turned its gaze upon the witch, who merely said, 'I have seen all that I need to. I see I am unable to dispose of you...yet,' with this said, a Corridor of darkness opened behind her, and Ultimecia turned her back on Anti-Sora, and left. The half Heartless, half Keybearer chassed after her, but the Corridor closed before it could reach it in time.
As soon as Ultimecia had left, the barrier disappeared.
'Sora!' Kairi called as she rushed forwards, but Riku grabbed her arm.
'He's dangerous right now,' Riku informed her.
'But I-I have to help him. I have to try at least,' Kairi replied.
'And what would you do?' asked Sam, 'would you attack him?'
'Then what?' Sam asked, 'Oh, I see. You would embrace him, and then he would somehow gain control of himself again. Such a foolish action would do nothing more then bring an end to your life,' Sam entered the library.
'What are you doing?!' Riku asked.
Sam did not answer. Anti-Sora was about to jump at him, but chains came from the ground and pulled it down, and held it there.
'Subete no seimei wa, kurayami no naka Ni kawaru kurayami no naka de, shohi denryoku ga umare, sono chikara de, wareware wa eien no void e no subete no seimei o kaeshimasu,' mumbled Sam, covering himself in a purple aura. Anti-Sora began to struggle, but shortly after, it seemed as if it were paralyzed. The darkness that coated Sora began to vanish, retracting to his chest until it was all but gone. The chains vanished, leaving a very tired, very dazed Sora on his knees. Sam passed Sora's friends as they rushed to his side. 'I feel that it may be of use in the near future.'
'Sam,' said boy turned to Riku.
'What?' Sam Nonchalantly asked as he lit up a cigarette as he commonly did.
'You said something was coming. Does Sora have time to recover?'
'No,' Sam answered, blowing smoke into Riku's face. He had to turn away from the unappealing smell. Riku was not a fan of cigarettes, 'it is already here.'
1. I wrote this while listening to 'succession of witches.'
2. I believe this fight scene goes well with the song 'Premonition' from the final fantasy 8 soundtrack.
3. Premonition pauses here.
4. Premonition starts again.