I changed the last part of the previous chapter so enjoy it and make it easier to write this. Also, hope you don't mind the original enemy base from other series to make it more original-ish.
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Earth
11:40 Am Japan, Mato, 9th Unit dormitory
Yuuki was slightly annoyed when they were having a nice conversation when the Shuuki wave suddenly appeared out of nowhere. He spread sense trying to find anyone controlling the Shuuki and found something peculiar, finding that his clairvoyance was being disrupted by something. His intuition at least told him that this was not an attack from Eight Thunder Gods knowing they already did something similar.
"Why did you suddenly pause?" Tenka worrieldy asked noticing the subtle changes in his mannerism and understood he was annoyed despite only a few months knowing each other was able to pick up some things about his behaviors and knows what's his thinking somehow.
"Did you bite your tongue or something?" Homare asked while her mouth was full and couldn't help but crave more of his food. She never had something delicious before and had been dying to eat his meal again like back at the Azuma Estate.
"The waves of Shuuki seem heading this way led by powerful Shuuki. It's unlikely this is caused by Eight Thunder God and learn more about another terrorist attack from another country." Yuuki explained and they didn't refute since they knew he had fought them and understood more about them but they were surprised that he was able to sense them.
"Don't tell me you have a clairvoyance ability too?" Maia exclaimed wondering to what end does his ability extend and certainly more versatile than their current commander.
"I do but the farther they are the less I can properly sense them. Alright, I'll let my clone handle it." Yuuki said summoning a clone of himself that he calls Kuro Oni. He didn't waste anytime and vanished from his spot heading to the battlefield. They wanted to joined but decided to continue eating since his clone can easily handle it while Yuuki made sure to give them show by placing hologram screen as the clone took out small camera robot connected to the device.
[ A/N: From now on if human clone Yuuki appear I'll name him, Kuro Oni while slave form would by their form names ]
Kuro Oni was walking towards the edge of the barrier where Shuuki was trying to enter, noticing that they weren't disintegrating and was a lot stronger than normal. He gauges their power giving them blow Kidoumaru but far outclass fodder Shuuki.
'Something or someone mutated them, most likely someone with the ability to influence Shuuki's biology.' Kuro Oni or rather Yuuki had studied them having peculiar biology unlike anything that made their body malleable yet can solidify that it been difficult to study them properly but gathered enough data that they cannot be easily be influence by experimentation and believed only ability can affect them like how normal guns won't work and needed a special one for Mato uses.
'Interesting…someone out there is going to entertain me!'
Kuro Oni wildly grinned at the thought that someone will have the power to make a stronger and harder fight for him that would be a lot more entertaining. Though, he doesn't plan on letting disaster affect the innocent and will try to find whoever the person is but for now he can enjoy the fight. In the end, his battle tendency still boils and would act differently when in a fight where he intended to kill. He took deep breaths that seemingly caused ripple around them and opened his eyes that had its color inverted while his hands turned black.
[ Hamon + Ultra Instinct + Asura Armament ]
He may be a clone but didn't try using [ Limit Removal ] as his body still had some damage and could affect his main body due to the mental after effect. Kuro Oni took a stance and leaped forward letting his heart pound as his fist collided with the Shuuki.
[ Explosion Heart Released Fist ]
Their body erupted from the inside and easily killed the Shuuki then within a few seconds the others around him also exploded. Kuro Oni felt that they were a lot more durable as the recoil of his fist was noticeable when it shouldn't as he released more force than his base with addition of [ Ultra Instinct ] and [ Asura Armament ] that negates normal durability.
'Still it isn't even an issue.' Kuro Oni was simply noting and didn't see it as any problem before clenching his fist taking a deep breath once more and eliminating hundreds of Shuuki that began to clamped together to become gigantic making it a lot easier to finish them off.
[ Renewal Taekwondo - Mach Punch. ]
Within a split second several massive holes were formed killing the Shuuki without any problem while Kuro Oni looked around to see the Shuuki still trying to fight him.
"Guess this would take awhile but…." He changed his style to something reminiscent of a wild dog and moved a lot more faster than before.
[ Watchdog man style + Formless ]
Kuro Oni became more precise and less restrained than before focusing on coordinated attacks rather than brute forces letting his body take action rather than his mind forcing him into a state of unrelenting killing machine that only knows to react removing his hesitation. He disliked using it considering he's intent was purely to kill as quickly as possible.
'If I use [ Raise Taiso ] then I could end this quicker but that drains me faster than other forms.' Kuro Oni has a massive reserve of stamina but he does have limits and depending on what he uses it will drain him faster or slower. They are some techniques and skills that have their uses in certain situations and he didn't want to show too much of his trump card. He didn't use his sword because it would be overkill and saw the opportunity to train while his main body hung out with the others who were watching.
'Though, this won't take me more than 10 minutes.' He became more like a streak of light forming after images at his speed vanishing and reappearing moments later before the Shuuki were torn to shreds. If Homare was present then she would be faster because she set code into her might that would make it easy to eliminate Shuuki.
[ Golden Hour ]
He stopped time over 5 seconds and took deep breaths then used [ Ame-no-Mitori ] to easily form a vortex that absorbed the majority of the Shuuki dwindling their numbers to near dozen. Time resumed and Kuro Oni simply beastly smiled, releasing an overpowering pressure into the Shuuki causing them to pause and kill them in real time.
'This was fun but this is just the beginning.' He stopped for a moment to look around while holding the head of Shuuki and stood on top of its body. Kuro Oni felt rather bored when he was slaughtering Cannon fodder knowing someone was watching.
"Hey! How long are you going to hide!? This is getting boring!" He shouted sensing the stronger enemy from a far and amazed it heard him as a dark mist appeared shocking him.
'Hell is this MHA Bs?' Kuro Oni shouted in his mind, taken aback by the mist even more when a monstrous beast appeared and glared at him instantly felt its power skyrocketed. The creature was humanoid yet had Shuuki features in them.
Roaaaarr!!!!
The deafening roar could be heard from miles away and Kuro Oni saw that his opponent was certainly not normal Shuuki nor blessed with Eight Thunder Gods. It growled at him as it vanished from his sight and reappeared inches away from Kuro Oni striking him straight to the face. The forces send shockwaves resulting in Sonic explosions and Kuro receiving a direct hit.
[ Defense Xiaoli + Water Stream Rock Smashing Fist ]
However, he anticipated the attack with foresight and absorbed the force discharge by its fist then redirected it back into his opponent.
[ Offensive Xiaoli ]
His fist collided and exploded upon impact but Kuro Oni noticed something was different and stepped back before noticing a pull as another fist slammed him into the ground resulting in a massive crater.
[ Water Stream Rock Smashing Fist ]
He deflected the attack and slipped from its grasp taking a stepped back noticing that his enemy wasn't the normal enemy that he faced before and understood this creature had been mutated by someone.
'Onmyou Agency shouldn't be the one responsible since I made sure…' He thought himself seeing the organization had experimented with human turn Shuuki but never went too far and mutated them. This is done by someone else and likely has the ability of transfiguration that affects humans and Shuuki.
"Psychokinesis...to think it can even affect my senses." Kuro Oni had a mental barrier that he made but his opponent was rather skilled despite how feral acts were. He understood that his future sight won't work since it still functions mentally and without proper barriers then it is inaccurate. He would have copied it's ability but didn't because it's redundant and rather focus on improving his skill instead.
He felt sudden force trying to pull in but use his Viltrumite physiology that allowed him to fly without problem and grinned using the momentum to travel unbelievable speed towards his opponent who anticipated his attack and clench their fist that collided producing a loud explosive forces that reverberate through the air and form crater around them.
'That actually hurts….' Kuro Oni was obviously holding back but he shouldn't be able to feel the force as his defense and durability was incredibly high and wondered if it's because it's trying to invade his mind that affects his body as a result. It was simply a slugfest where he bombarded his opponent with a rapid fist strike that created a sonic boom and his opponent was able to somehow keep up.
'Accelerated Growth as well? Interesting but I have no use for such ability.'
Kuro Oni already has insane talent that an ability would simply be a single drop in an endless ocean. He was confident whoever experimented in them was able to fuse multiple people into a single entity that has numerous powers.
'Whoever acquired an ability like this should have gotten them without anyone knowing.' He thought to himself concluding that a person ate a peach without supervision considering an ability like that would be sought after. Kuro Oni decided to not share what he knows and handle it himself to let the Anti-demon corp concentrate on Eight Thunder God since he doesn't trust the Omnyou Agency to keep themselves from trying to capture that person.
[ Formless ]
He ignored those thoughts and enjoyed his fight feeling his blood pumping increasing his power slowly improving his [ Relaxed state ] that lessened the energy that he spent hoping to help in using other sub abilities that he has.
[ Taekwondo - 360° Tornado Kick ]
Spinning his body at incredible speed straight to his opponent's head that sent his opponent flying several dozen meters only stopping using its Psychokinesis. The damage caused it to be shaken and if the person was truly once a human then its biology shouldn't have been that of Shuuki.
[ Shotokan Karate ]
Using another style of Karate focusing on more precise striking waiting for his opponent to arrive before deflecting and striking at the vital points. He hasn't studied every Karate, finding some to have been diluted by modern martial arts since they are too afraid of fatal injuries that made people laugh at them thinking it's fake.
Roaaaar!!
Giving a scream of anguish power through and managing to increase its speed then combined the pulling force of Kuro Oni and slammed its fist pushing him several dozen feet but able to stop himself and dusted himself looking at his opponent who was glaring at him then was taken aback when it suddenly took a stances.
"Hahaha bastard learned how to fight!" Kuro Oni finds it interesting that it is able to grow as they fight and thought it might have been reading his movement and memorizing it. He might have copied his basic martial art but won't ever learn the unorthodox martial arts like [ Water Stream Rock Smashing Fist ] without time and training.
"Perhaps I should take this more seriously…" He took a different stance and circles seem to appear below him that surround him until they reach several dozens of meters that was impossible to get him by surprise even without foresight relying purely on his experience and instinct to intercept incoming attack.
[ CQC + Jeet Kune Do ]