/N: Boku no Hero Academia is written and illustrated by Kouhei Horikoshi. Kimetsu no Yaiba is written and illustrated by Koyoharu Goutoge. Rurouni Kenshin is written and illustrated by Nobuhiro Watsuki. I don't own the three of them.

How to read the story

In the… – Normal

"You're…" – Conversation

'Why…' – Thought and telepathic communication

Kagune – Foreign words

"Fired…" – Deep Voice

Tetsusaiga – Technique, Quirk and special weapons

Chapter 1

Return Back on Time

"Who's the one that said "I'm going to be the next leader" huh?"

"Did you come here to kill me?" Ask the once notorious villain known as Overhaul, or his real name is Kai Chisaki toward the current leader of League of Villains, Tomura Shigaraki.

Thanks to the Raid of his base, Shie Hassaikai, the last known Yakuza that still operated in Japan during the Age of Heroes by the combined forces of Police and Pro-heroes. It was all crumbled.

His dreams of reinstating Yakuza as the underworld super power once more through his Quirk-destroying Bullets.

His plans of eradicating Quirks from existence.

His dreams to becoming the next ruler of the night (villain) after the imprisonment of the most notorious villain of Japan, All for One.

But the most important of all, his dream to paid the debt to his boss who kindly takes him as a boy.

Now, all of that dreams is lost. He himself even lose to first-year U.A Academy student who unexpectedly outclass him in every way even after he fused with his underlings who had the ability to absorb the opponent's life force.

He's the same as a man who already give up on everything.

"Well yes." Tomura replied. "Actually I plan to make you suffer by cutting off your arm thus make you Quirk-less so you can watch helplessly from prison. However… I change my mind suddenly. Let me be blunt now… I hate your guts. Because you act so high and mighty."

Suddenly, Mr. Compress, one of Tomura's comrades is compressing a part of Kai's left arm using his Quirk, Compression. "I also feel the same." He said, agreeing with Tomura's statement.

Tomura then hold two small box on his hands which one of the boxes filled with four complete Quirk-destroying Bullets. One of them is already used to make Mirio Togata, one of the Big 3 from U.A become Quirk-less. "Two small box. Which one is the complete product?" He asked Kai before putting it into his pocket. "Well, not that I care."

Kai suddenly had the drive after seeing his hard-work for years is easily stolen like that. "Give it back!" He demanded.

"After all Overhaul-" Tomura later released his mask, which is someone's hand-palm. "-Someone who wants to extinguish Quirks from society… Doesn't need a Quirk right?" Tomura later grip Kai's remaining arm, starting to disintegrate it due to the power of his Quirk, Decay.

Kai face is showing shock, anger, and fear. He knows that's nothing he can do.

This is the end of the road for him.

"Sad isn't it?" Tomura looks toward Kai with a sick smile on his face. "All of your hard work till' now… IT'S ALL MINE NOW! YOU CAN WATCH US FROM HELL ALL YOU LIKE! I HOPE YOU HAVE A NICE TRIP TO THE AFTERLIFE!?" He yelled toward Overhaul body, who starting to disintegrate.

Overhaul only experience a state of shock and fear as the disintegration started to decaying his body.

'This is what I get!? No! I-' His thought never finished as his body finally decaying.

From the truck that used by Shigaraki and Mr. Compress, Spinner called. "Oi! Come quick! The police will arrive soon!" He warned.

Agreed with his comrades, Tomura along with Mr. Compress and Dabi is walking back toward their truck, leaving the decaying corpse of Shie Hassaikai's young master behind.

"Next… It's our time." Tomura declared with firm tone.

For Kai, he's now feel like he was in Limbo.

All of his hard-work, truly fails.

It is so wrong to want to repay your debt to someone? That's the thought that accompany him on this limbo.

'Grrh… Damn those heroes! League of Villains! They're just a bunch of sickos who view small true thing in front of them. Never look the bigger picture!' He thought the same thought when he was defeated by Izuku Midoriya. 'If only Quirk never existed! Pops would never have a sad look on his face! Damn you fake heroes! If only I can return to the time where Yakuza can go strong once more-'

"Khu khu khu."

'Someone's chuckling!' Kai thought. How's it possible? Isn't he dead by now?

"This human surely will be tasteful. It's been a while since I eat a muscular man like this." The voice of a man said in vile tone. It was revealed to be a malevolent being of a man with pale skin and a horn on his head. He then picks up Kai's left arm which developing hives on it. "Well then. Time to eat!" He said as he going to take a bite.

However, Kai's mysophobia reflectively makes Kai use his right arm to grip the malevolent being. "DON'T TOUCH ME!" Kai shouted in anger as he instantly activated his Overhaul on the being. The being can only look in shock as his face and every parts of his upper body starting to show veins before it become inflate and his upper body explodes completely, leaving shower of blood behind that some of it rain down to Kai's body.

Kai later got up and starting to clean the blood from his body due to his mysophobia. It is revealed that he's lying on a ground of what seems to be a forest edge. The lower half of the malevolent being that tried to eat him is seen lied motionless beside him.

'Now then…. Where am I exactly? Am I supposed to be dead?' He looks his surrounding after he finishes cleaning the filth from his body and found out that he wear his trademark plague mask. "Judging from the fresh air, seemingly I'm in the forest of Japan or something… Huh?" He then looks at the being who tried to attacking him. 'Who or what is that man? He said he tried to eat me? Maybe he had Quirk like Tabe?' He thought remembering one of his Eight Expendable Bullets. "Well, at least he won't bother me." He then walk away from the scene, tried to find out where he is exactly.

To say Kai was confused is beyond that. He's clearly shocked by what he seen.

Recently after walk away from the forest, he discovers a town, or village to be precise. He's clearly shocked by its lack of technology and the clothing of its people that seems very backward.

Heck, even asphalt path and concrete rock hardly exists.

'What's the meaning of this?' Kai thought in confusion. The air, the scenery, all seems backward. But, the existence of electric pole to light up the house means one thing…

'Could this probably…. Japan around early 20th Century?' Kai's mind quickly deduced. He finally remembers it due to his Pops tells him about the time when the Hero started appearing and pushes Yakuza to the shadows. 'That's impossible isn't it? Is it possibly that I being thrown to this backward world? That means this time… Just a few years before the sickness of heroes and villainy is born?'

However, he soon draw the attention of the villagers.

"Look at that man."

"Who's he exactly?"

"His clothing seems strange to me."

"What's with that strange mask?"

'Ugh… This sick people.' Kai thought in frustration. However, he already push his mysophobia and killing intent for now to appear civil enough not cause mayhem.

'For now, I think the best option is to find a library in this small village.' He thought before walking toward a random female villagers. "Excuse me Ms, can you show me the way to the library?" He asked in kind tone.

The female at first is scared because of Kai's menacing mask, but finally agreed and escort him to the closest script building. "Y-yes. It is that way. I'll escort you!"

"Thank you very much." Kai thanked while making his facade smile that of course unseen because of his plague mask.

Kai later is escort to the closest script building that's available on this village-like city. In there, the first thing that he takes is about the history of Japan. He then reading the scroll for an hour, before he widens his eyes. 'This is….'

He really found something interesting. There's no telling about Quirk at all. What tells on the history is all about the history of Japan and there's nothing about those vile sickness. Heck, even no records about All for One. But there are two history records that interest him. The first record is about Yakuza. Seemingly on this era known as Taisho, Yakuza is a collection of numerous thugs with brotherly bond that mainly an outcasts, not really different from his time. Except on this time, they're still able to grow without the intervention of heroes. Even he reads that some Yakuza is humanist enough to help civilians that reside on the district that they rule.

But it's the second thing that seemingly picks his interest.

The myth about Oni. It is told in the script that every night, the malevolent creatures known as Oni will kill and eat any unsuspecting humans who wander alone. It is said from the script that they become flourished or more exactly recorded after Sengoku Period.

"Well… That was interesting enough." Kai commented to the content. "So in this era, there no such thing as hero sickness, so Yakuza can thrive. Pops principle really fit for this period." Kai mused to himself remembering his Pops principle about the old tradition of Yakuza. If Yakuza in here still thriving although with surveillance from the government, he doesn't need to use that vile plan. Using the old tradition method is more than enough. "But at the same time, this Oni picks my interest. So in other words, their leader is the master of the night. And that left me wonder, who's the master of the day? What kind of heroes' sickness in here?" He mused to himself.

"Oh, I see that the script of history is occupied." Kai then look up from his reading to see a young woman that's maybe around Mirio's age. She had pale skin and long, straight hair of a very dark gray color that reached just below her waist, worn down with two shorter clumps over her shoulders, and arched, ear-length bangs on either side of her face. Her eyes were a pale violet and slanted towards the sides of her face, bright and bug-like, with many notable white sparkles reflecting light off of them. Her body is very slender but Kai's notice that behind this slender body, she's a fighter. "No need to worry Mr. You can read it while I search for something else." She said in a very kind tone.

"No. I already finished reading it." Kai replied as he put the script on the table. "By the way, from what Yakuza's group are you?" He asked since he clearly interest in this woman's outfit. A black uniform with a butterfly wing-pattern haori and a katana on her waist. Truly fit a Yakuza.

"No need to worry Mr, I'm not Yakuza." She assured. "I'm just a passerby on this village that wants to go to the script building. However, that's strange mask you wear there." She commented about Kai's plague mask.

"No need to worry about my mask. I'm just a little sensitive about the dust you see." Kai replied with a fake smile, unseen because of his mask. 'Truly, what a de javu. Someone's asking the same question as you, Lemillion.' He thought.

'This man. I don't know why…. But I feel the strange thing about him.' The young woman thought in confusion. She then closes her eyes, smiled. "I see. Sorry for asking something personal. By the way my name's Kanae Kocho."

"It's okay Ms. Kocho, now if you excuse me." Kai then stand up from his seat, walk out from the building.

As he walks out from the building, he finds another woman waiting on the outside. Kai quickly suspects that she's must be the woman's younger sister due to their almost similar appearance. She was a petite girl with pale skin and large compound-looking eyes that do not possess pupils, only a haze of gradient, purple iris that becomes darker, the lower it goes, which made her eyes appear similar to those of bugs. She had chin-length hair that faded from black into dark purple, that usually were tucked in,. Her outfits is the same as Kanae with darker shade, except she only wear a white haori and also carrying a katana on her waist, although being unseen by a fabric that wraps it.

'Not that I care.' Kai thought as he walks away from the scene. 'But that means… In this world, I can truly become the supreme ruler of the night. The only thing to do now I how will I search some valuable underling since this world had no Quirks and how to get rid of this Oni leader to fulfill my plan?'

The time later changes from noon to almost sunrise. Kai is spent his time reading about battle techniques script that he stole from the building on one of the house roof of the village. After the matter of dealing with Shigaraki, Kai finally had enough time to think and realized some of his flaws. His mysophobia is unavoidable, but the other is his biggest mistake.

The first is his overconfidence. Due to his overconfidence that his previous plans using Eri will be effective, he failed miserably. From now on, even though he has a plan, he seems must built it very carefully and keeps the key of the plan in hiding.

The second is his underlings' trust. While true maybe his Eight Expendable Bullets is extremely loyal to him except for few person, his brother-in-arms who's more like to follow Pops teaching never saw him as leader but more as an usurper. It also because of his lack of care for his underlings that make two useful pawns from the League of Villains, Toga Himiko and Twice, betray him. Maybe if he not killed that magnet bastard who attacking him, the League of Villains will view him as allies and he'll become the next ruler. The acts of Tomura proves that he cares deeply for his comrades of villains.

"But not what I wanted if it became like that." He once again mused to himself. He read this script for one purpose: practice the moves to improve his fighting style that too over-relied to his Quirk.

Suddenly, Kai's sees a building not far from the place he's sitting is emitting a dust. Clearly there are some battles going on.

Could it be one of Yakuza's gang from this age? Surely they so full of spirit isn't they?

Curious enough, he jump down from the house, while momentarily looking inside the house. "Thanks for the seat." He said as he walks away, revealing that he kills the owner of the houses using his Overhaul. His lower body is the one that only remains, while a splatter of blood is seen on the floor.

Kai later walked toward the sites of carnage, hand on his pocket. He clearly interested in the scene that currently unfold. He then sees the sky and it already showing sunrise.

"Tch, never thought I wasted my time for so long." He uttered, probably because what he usually do during night is experimenting on Eri's DNA to create his Quirk-destroying Bullets. However, he then stop walking after seeing someone in front of him.

It was Kanae Kocho, the girl that he encountered on the script library yesterday. She appears to hold her chest, which is revealed to have a huge slash attack that seemingly scratched her heart and damaging her right lung. She support her body using her katana.

"You…" Kanae raspily said, recognizing Kai before falling down face first to the ground because of her injuries.

Kai sighed, before walking with no care passing the dying Kanae. However, a thought suddenly crossed on his mind. 'Maybe this woman can be useful to me for the information that I desired. After all, I can deduced that this woman maybe a samurai or some kind of that, meaning that she's lying for not being a Yakuza.' His head already filled with what use he had for Kanae Kocho. He then turn to the body of Kanae, who continuing to bleed from the wounds. "Don't make me regret touching your filthy body." He said as he prepared to turn Kanae body to heal her body from her injuries. However, just as he's about to touches the body of Kanae…

"B-big sister!" Kai turned to see the same woman that he encountered on the script building entrance. She looks panting, maybe because of running from somewhere else. Her face also shown panic. "Y-you!" She readying her hands on her katana. 'So this man is the one that Big Sister talking about! A man with strange clothing, wearing white gloves, and a strange bird beak-like mask!'

Kai at first tried to use his kind-facade nature. "No need to worry little girl. I just suddenly pass by and found her badly injured. If you want, I can treat her."

"I-is that true!?" The girl clearly panicked but at the same time had a hopeful tone. "Please help my sister!"

"Well not here." He then stand up and walk toward the girl. "At least, I can't risk any witnesses." He then unleashes his killing intent, making the girl wary.

"After all-" Kai later touches the girl head at a speed that surpasses Rappa speed, one of his Eight Expendable Bullets that had a fist that as fast as bullets from gun. He then disassembles some part of her head to cause unconsciousness with extremely fast timing. "-If I'm not keeping you alive, I cannot talk properly with the subject that I desire."

On the place that seemingly a collections of rooms of tatami that located randomly.

"Is that true what you said, Douma?" Ask a man with red-blood eyes. His physical appearance is like that of a man in his late twenties with an unnaturally pale skin which can be easily confused as being unhealthy and sickly, curly black hair. He also wears a white fedora with a red ribbon and a somewhat extravagant outfit.

"Well in a reality. Then yes!" The rainbow-eyed man known as Doma said. Doma took the form of a young man with a moderately toned and muscular body, who is of a notably pale complexion and possessed unusually long, pointed nails that appeared to be stained a pale blue. His hair could appear either silver or a pale golden blond, and he wore it parted to his left, the shorter parts around his face seeming to flare out to either side around his head, curving backwards with one strand left down his face to fall between his eyes, while the longer parts were left to drape centrally down his back in a thinning spiral. His eyes are rainbow in color. His outfit consists of a blood-red turtleneck of a design that made it appear that the section between his neck and upper chest is covered by a black substance that looks like it's dripping slightly down his body, this same design repeated at his wrists and down his hands. Below this, he wore a pair of straight, tan-colored pants of a pinstriped design, their cuffs visibly loose, which he secured with a pale green-tinged golden belt, its buckle a bright silver. His trademark, Doma is also wearing a black, crown-like hat which is lined with gold around its frontal plates and has flowing black ribbons dangling from each of its sides."During that night before I battling the current Flower Pillar, I found a minor Oni that only its lower half that remains. And when I touch it, It was revealed already dead. Seemingly his core is destroyed." He informed.

Kibutsuji Muzan clearly not expecting this. Not that he cared about the death of his expendable minions, but the description of how this minor Oni death is what that disturbing him. What kind of being that able to target an Oni's core that only as small as gravel? Maybe Demon Slayers can because of their Nichirin Blade that specifically targeting their neck. But, this kind of attack is something he had never heard of from his almost thousand years old lifespan.

But once again, the one that equalize himself as an almost perfect being and natural disaster dismissed the possibility of threats. The only known person who he considered as his greatest fear is only one.

"Are you sure about this information Douma?" He asked his Upper Moon Two with a little threatening tone. He hated if this information is merely a ruse.

"Pretty sure!" Doma said in his carefree tone.

He then think about it for a while, before spreading his thoughts into his underling.

Investigate this mysterious case. The one that manages to find it will be allowed to Enter the Twelve Demon Moons. For Lower Moons, they can challenge the Upper Moon.

He'll sure will caught this mysterious person who dares to challenge his supremacy as the Ruler of the Night.

Back to the street, Kai finally turn Kanaes' body to make her back face the ground, exposing her grievous injuries. But, the fact she's still breathing is amazed him.

'She's still alive after a wound like that! Once again a de javu.' Kai thought as he using his Overhaul on Kanae body. In slow process, The front of Kanae's upper body is destroyed or exploding into a splatter of blood before magically reassemble itself and after all of that is finished, her fatal injuries is all but gone. Such is the power of Overhaul, a Quirk that gain the Wakagashira of Shie Hassaikai a complete control over matter. However, Kai have only one thought in mind after that.

"Tch. Disgusting." He said as he clean his arm from contamination. After finishing cleaning the so-called contamination from his arm, he then look toward the unconscious body of the two females.

That's for chapter 1! What do you think so far?

By the way, this story take places two years before the massacre of Kamado family.

As the Idea of Oni's core, I get the reference from UM 6: Gyuutaro and Daki. Since they said they can separate their cores to avoid death by decapitation, I think all Oni had a core on some places in their body. If the core is destroyed, they as good as dead.

Also I want to ask the opinion of reader. How would Overhaul fare against Oni and Demon Slayers? Did you think Muzan and Overhaul personality is the same?