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{This is my first time writing a story. I never wrote one for someone else to read. I took inspiration from many different stories. I'm not claiming all the ideas i will use in this story as mine, maybe they have already been used in a much better way. Conclusion, I'm a novice. I'm not claiming that you will like this story. Hope you give constructive criticism.}
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{Oh and English is not my first language. You may find a few errors but I will try to convey the story in a way that most people can understand, atleast I hope so.}
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'How did all of this even happen?' Muzan thinks to himself while walking on tatami mats in a small wooden hut in the deep mountains. He'd prepared this place with extreme care, since who knows how long he will be out? Anyway, his thoughts were spiraling into the events of the past few months, as if trying to narrate his own story up till this point.
Why he felt the need to do so?
Who knows.
{{Narrator Note: We need it for the PLOT!!!}}
'To be honest, it was a pretty straightforward deal,' he thinks while sitting down on his comfortable wooden chair. He got this from one of the residences his human identity was the owner of. It was a nice chair.
'I died in my original world. And that's it. That's all that was supposed to happen.' He starts to inspect a few papers. The sheets were full of scientific formulas and hastily scribbled notes.
'Death is supposed to be the end of our story, but my insignificant, ordinary self was somehow selected to become one of the chosen few.' He inspects a few vials of liquid. The vials varied in color, but one of them was a vibrant red, and the liquid seemed to stir by itself, as if it were alive.
'In short, I was getting reincarnated, and it was a classic case of isekai. And fortunately, it was a world I was extremely familiar with: Demon Slayer.' He picked up and set up a few more vials and added the red liquid to all of them.
Muzan inspected a few more apparatuses and then got to work again.
"This is the last chance, or I'll have to go back again. Sigh," he said to himself, and with an exhausted sigh, he focused on the experiment. This was truly the last chance, because while going back was no trouble, the Blue Spider Lilies were already finished, and he also had a very small chance of surviving to the point when they reemerged.
'After the whole process of meeting a randomly overpowered being and piquing enough of their interest that they reincarnate you with some cheats—well, cheat is a big word. The best I got is a quick exit or escape route.'
The experiment continued, and Muzan stayed sitting, without caring how much time passed. It wasn't that he was not aware of time passing, but that he didn't care right now. The only thing keeping him going was sheer will, because if one could see through his body, they would be able to easily notice how his body was starting to break down, organs continuously failing, blood continuously clotting, and the pain in his brain—oops, sorry, brains!
'What I got was a multiverse traveling system, and while it's definitely not worthless, as any advantage is better than no advantage, it offered no direct growth or combat ability.' He added a few other liquids to the vials before finally setting them down onto the heater.
'Of course, I was willing to grit my teeth and grind my way up. Besides, with a multiverse system, I could escape whenever I wanted. Unfortunately, the system only allowed temporary travel, but resources from whole different worlds were available to me, so I wasn't that disappointed. Still, something like a gamer system or a magic or skill system would have been better.'
'But just as such thoughts began taking shape in my head while I was in the process of reincarnation, my system suddenly activated and issued a high-end quest. Now, this wasn't a bad thing at all. It means that even this multiverse system can generate quests for me.'
A slight bluish-green liquid traveled through a thin glass apparatus and dropped slowly into four different vials. As the drops fell, the liquids changed their colors. Some even changed in texture, getting more viscous or generating grains of solid substances in them.
'Missions always bring achievements and rewards, something I will need to survive in a world filled with man-eating demons, but honestly, I underestimated the mission.'
A painful grunt seemed to come out of Muzan's mouth as the pain got worse and worse, but he kept his focus on the boiling liquid in the four vials.
'Apparently, Muzan, the main antagonist of this world, had ingested some kind of poison. It was created by him himself, of course. He thought it was the cure. He had fully believed that it was the cure to his only weakness, the sun, but while it did help with that, it also started breaking his body down. His regeneration, his strength, his vitality—everything started slowing down, and Muzan realized he was dying. Doesn't this sound a lot like the poison Tamayo and Shinobu made during the last battle?'
Finally, one of the vials seemed to have reached its limit and exploded, but before it could damage anything, it was encased in a box made of flesh. The explosion was contained and extinguished before it could even burn.
'Frantic and desperate, he went mad. For someone who is extremely afraid of dying, it was like the Grim Reaper was standing right beside him, showing him the last sights of his life before slicing his head off, and honestly, I think his reaper must've looked like a certain Sun Breathing user.'
Muzan let out a sound that was a mix of pain and amusement. The memories playing in his mind seemed to greatly amuse him.
'But anyway, to extend his life, Muzan, the original Muzan, started doing all sorts of things, things he never would have done before, like appearing in open sunlight and going to look for the Blue Spider Lily himself.'
'Apparently, according to Muzan's knowledge, the last experiment should have completely cured him, but this failure was once again only because of the one resource he did not possess: the Blue Spider Lily. Sigh. He truly went mad, and finally, he summoned all the demons—yes, I mean all the demons—throughout Japan and dissolved them.'
'He ate them.'
One by one, the vials seemed to give out, and either the liquid in them started to simply turn a blackish gray or explode, when finally one of the vials, the one with the light bluish-green liquid, turned a complete green.
Muzan's eyes widened, not in pain but in pleasant surprise.
"I really thought I was going to fail again!" He picked up the vial, slowly taking it up to his lips and drinking it all in one go.
'Muzan ate all the demons, and yes, that included the Upper and Lower Moons. This had some effect on him. His strength was greatly restored, and his regeneration kicked back in once again, but he knew he was only delaying the inevitable. Finally, even after another month of searching and not finding the Blue Spider Lily, his mind couldn't take it anymore, and he just collapsed.'
After drinking, Muzan set the vial down and waited for the effects to kick in, hoping nothing would go wrong.
'Honestly, what a dumb idea he had. He actually ate even the Upper Moons. Honestly, if it were me, I would have kept the Upper Moons alive.' He sat there for a few minutes, and then the effect from drinking the liquid started.
"Argh!!" Despite his mental preparation, a pained grunt escaped him. The pain in his body was only intensifying, just that this time, it was in the opposite direction.
A burning heat originated from Muzan's belly and started burning his insides, but this was a good kind of burn. A burn that made the new Muzan realize he'd done it. He's finally free from impending doom.
And just as he was starting to celebrate, a pop-up from his system appeared in front of him.
[Notice: It appears the host has completed the S-rank emergency quest without higher assistance from the system.]
[Notice: Mission rewards are being allocated!]
[Notice: Major function: Multiverse Travel has been fully unlocked!]
[Notice: All your debuffs are being removed! Full restore under process!]
[Notice: The host is entering a restoration sleep in 10 seconds!]
"Wait, now?! No! At least give me 10 minutes! I can't be sleeping out here where someone could easily find me!" Muzan said to the system.
[Notice: Host's wish is recognized. The waiting time has been extended to 10 minutes!]
As soon as he heard the confirmation from the system, he ran toward the mountains, and after going under one of the harder-to-find ridges, he dug out a small hole into the underside of that ridge, laying down inside. It was honestly very easy. Self-biology manipulation, or biokinesis, is Muzan's Blood Demon Art, and it's extremely versatile. His natural strength and regeneration completely offset any kind of problems that could arise.
'This restoration sleep was definitely going to be like a coma. I'll be out for at least a few days, if not more! I won't be staying in a place where I can be easily found! I'll just sleep here in this hole, and after waking up, I'll leave directly! I want to leave this world as soon as possible!'
He was already getting a bit dizzy, so he lay down on the ground directly.
'I already knew that I was going to be sleeping for a while, since I knew what the reward for the quest was going to be like. With a little inquiry, I got to know I'll be going into a deep sleep for an extended period of time, during which all my body's ailments and any other problems will be relieved and my body will be restored to its peak. Of course, I also double-confirmed that my powers won't be just "cured" too.'
He lay down and started counting the seconds until sleep came.
"Oh right, I didn't complete the story, did I?" Muzan paused, startled by his thought.
"Wait, why was I thinking about all this again?" he thought to himself, but continued anyway.
'Anyway, after Muzan's mind collapsed, I was given the mission to take over his body, keep him alive, and complete the whole Demon Slayer story. Well, no. I was given the mission to complete the story. It wasn't that I had to follow the story down to the letter. That's just dumb. So I took things into my own hands. I had all of Muzan's memories and knowledge anyway. Only the soul was different. I narrowed down the Kamado family's residence through the clues Muzan already had, went there, and sought out Kie Kamado, and had her procure some Blue Spider Lilies for me. It wasn't hard at all. I just told her that I was a researcher and that I wanted to find the damned plant. She very kindly helped, and in exchange, well, of course, I didn't kill her.
'I even gave her some money, nothing too much, of course, since that would just make her refuse or be suspicious or whatever. I didn't want to deal with any of that, so I just gave her 100 yen. Nothing much.'
'She, of course, was still saying that she didn't need the money, but I handed it to her anyway. She also informed me that this flower only blooms sometimes during the year and that I was just in time. How convenient, isn't it?'
The timer ticked down to only two minutes.
'After that, it was only about experimenting and succeeding in restoring myself. On that thought, I also went out and cured the Ubuyashiki family, and finally ended the 1000-year-old feud. Demons are already gone, and I'll be leaving soon anyway, so I guess the system acknowledged the results in advance and gave me the rewards. I mean, I did cure myself on my own. Or well, on Muzan's knowledge, but it's mine in the end anyway.'
Finally, as the last three seconds ticked away, a final thought emerged from Muzan's mind.
'A whole new adventure, all across the multiverse.'
And so, the Great Demon King, Muzan Kibutsuji, finally rested and fell into his first deep sleep since forever.
.
.
.
.
.
.
'Wait, where am I?' Muzan questioned as he materialized in a completely white space.
[Notice: It appears that the host's body has consumed over 600,000 human lives and over 17,000 demon lives over the course of its life.]
[Karma cleansing has begun. The host is advised to prepare himself for confrontation.]
'Oh my God, so many lives.' Muzan felt sick, and yes, it was because he could actually remember all the times the real Muzan Kibutsuji had consumed people. He really wanted to vomit, but he realized that he only felt bad and didn't actually feel sick. What the hell? But he threw those thoughts away for now. Something more important was going on right now.
'Confrontation? Oh, that does not sound good,' Muzan thought. His thoughts echoed as if he were speaking, and he realized that whether he was moving his head or not, he could hear himself. So this was like his soul? Or perhaps his mind?
'Hey, System! Didn't you say this was some restoration sleep or whatever?! Then what the hell is this confrontation about?!'
No answer came, only the notice that the first confrontation was going to begin.
He wanted to complain, but in the end, he sighed and readied himself anyway.
'It's going to happen anyway, so let's just get this ove—' but before he could complete his words, a sword strike, faster than he could react, struck his body, slicing cleanly through him, and yet it did not stop there.
The sword rotated, weaved, and cleaved him until he was just pieces on the ground.
'What?!' Muzan shrieked, and yet even that sound didn't come out cleanly, because his face was already in ten different pieces, and it burned.
'Argh!!! What is this?! Fire aspect?!' He realized that he couldn't heal fast. His regeneration was being dampened. Those burn injuries were the cause of that.
'You sick son of a—' but once again, his breath caught and he couldn't complete his words, but this time it was not because he was being attacked again. No, this time it was because he finally saw the face of the attacker.
'Oh no, no, no! How is it YOU?!?!'
"We meet again, Kibutsuji Muzan." A calm voice, yet carrying a deep chill, a chill that seemed to burn Muzan's skin, drifted from the attacker's mouth.
This man, the single greatest demon slayer and swordsman to ever live on the face of this earth, a monster acknowledged even by a demon like Muzan, and the creator of the original breathing style, Sun Breathing, and also the very concept of breathing styles,
"I ask you again, what is the value of a life to you?"
Yoriichi Tsugikuni.