Records of the Heavenly Demon King (Demon Slayer/Multiverse)

Inside a lavish Japanese-style house, in a brightly lit room, a deathly pale teen who appeared to be no more than 13 years old was going through a shocking revelation of his life.

""…What is this?…I was/am a fictional character?"" Two distinct voices resonated inside the young teen's mind.

Two different minds: one who aspired to perfection and another who embraced imperfection...

Soon, the right side of the teen's face morphed and gained demonic traits. His right eye took on a darker plum-red hue. The depth of this eye invoked a cold feeling, sparkling with murderous intent.

Meanwhile, the left side retained the former human features of the face, but the left eye showed more humanity than the right eye.

It was as if both sides of the face belonged to different individuals.

"You dare steal my body? Human?!" the right side said.

"..Go fuck yourself, rip-off Michael Jackson!" the left side retorted while maintaining a calm demeanor.

The right side was able to access all of the left's memories, while the left could access the right's memories as well.

"So what does it feel like to have wasted centuries and yet unable to realize the truth of the Blue Spider Lily? How slow are you? For fuck sake, you have five fucking brains!"The left insulted the right with all of its frustrations.

The right side remained silent, while being in a state of shock.

The right spent centuries searching for the Blue Spider Lily—the mythical flower responsible for his demonization and apparent weakness to sunlight.

According to the new memories the right side gained from the left, it realized how fickle fate could be.

Apparently, the flower he put all of his effort into searching for only bloomed two to three days a year and only during the daytime.

There was no way for him to find the Blue Spider Lily if he couldn't go out in the sunlight in the first place.

Hundreds of years of effort spent searching were now for naught.

However, the right did not let itself submerge into the sea of depression.

Depression was nothing but a sign of weakness for the right side.

He hated weakness and so it was natural for right to overcome depression easily.

With this newfound information and several key memories from the left, the right one felt confident in attaining all of its desires.

Conquering the sun? Becoming a perfect being? Information about the so-called protagonist? The demon slayer corps?

The right could recall everything what left knew about this world.

The right no longer saw any use for the left one and decided to end the left once and for all.

Naturally, the right one was none other than Muzan Kibutsuji, the immortal demon king who hated the very concept of change.

In contrast, the left one was a soul from another reality. The identity of this soul was Makkari Aozaki, a man gifted with incredible good looks. Naturally, Makkari abused his looks to get what he wanted in life.

Makkari was also a devoted anime/manga lover. And he knew quite well whose body his soul was trying to take over.

After all, it was a character he liked but was very disappointed in the actions of this character named Muzan in entirety of Kimetsu no Yaiba.

Till the end, Muzan remained a coward who hid behind his demons.

He had several opportunities to end Tanjiro's life.

He also had access to  fucking broken cheat power that was Nakime.

Her blood demon art was literally a limitless dimensional fortress.

If Muzan used the full potential of this  blood demon art before the events of  Infinity Castle Arc, he could have won.

Makkari was very frustrated with Muzan's actions. And if he took over Muzan, he would try to do better than this failure of a demon.

Now coming back to reality…

'Shit! Shit! Shit! If I don't do anything, this demon fuck will kill me soon! Think, Makkari, think!!!'

At that moment, Makkari had an idea. He was not sure if it would work, but he had to take the risk.

One thing in his favor was that if he wanted, he could take control of the body for a short amount of time before Muzan fought back to counter his control.

And Muzan was unaware that Makkari could take over his body even if it was just for a second.

'A second will be more than enough! It is now or never!'

With that, Makkari focused all of his being into gaining control over the body. He succeeded; the right side was no longer demonic, and the body resembled the former appearance before his soul tried to take over Muzan's body:

'Burst!'

Just then, the teen's body exploded in a violent mess. Blood splattered all around the room. Invoking a horrific scene with iron-like stench of blood all over the room.

Makkari had just attempted to rupture the whole body of Muzan.

Why did he do this if he knew Muzan could just regenerate back?

Since Muzan was the most powerful demon king, wouldn't he regenerate back quickly?

That should have been the case, but what Makkari did was simply abuse this aspect.

The body is the soul, and the soul is the body.

Sounds familiar?

It's a concept Makkari remembered from the Jujutsu Kaisen world.

But why would Makkari do this, given Jujutsu Kaisen was a different anime and its logic should not apply here right?

Well, it was all a gamble.

After Muzan's body ruptured, it started regenerating back almost instantly.

However, the being who was now standing was neither Muzan nor Makkari. It was somebody else entirely.

A young man who appeared to be in his late twenties, standing six feet two inches tall, with messy white hair, plum-red eyes, deathly pale skin, and a devilish grin adorning his beautiful face.

"The body is the soul and the soul is the body? Brilliant, Makkari! You genius fool! Unfortunately, I am not you or Muzan!"

"I am better than both of you."

What Makkari did was harm Muzan's body, and this would inevitably affect the soul as well. Since the body and the soul are equal (according to his hypothesis), and exists in different realms at the same time, both could affect each other in one way or another. And so by harming Muzan's body to a bloody pilp, it would temporarily cause his soul to be damaged before his regeneration kicks in. And according to him, the time interval for Muzan's soul to recover was enough for Makkari to override Muzan's soul information over the body, effectively giving Makkari full control over the body.

It seemed that Makkari underestimated Muzan. What he did not know was that Muzan's  regeneration was way faster than the overriding process so his plan ultimately failed.

However, this ended up casuing a new change into their beings.  What happened was that Makkari's soul merged with Muzan's, causing the emergence of this new being.

The synergy was perfect. Both souls fused to become one, giving birth to a new being.

This 'new' Muzan looked at the moon  with his beautiful plum eyes which appeared brighter than before. 

"Demon, huh? This will be fun."

"I can't wait to meet you my dear…Nezuko…" said 'Muzan' with a massive grin.

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Author's Note:

He's refering to Nezuko's ability to conquer the sun, that's why he said he can't wait to meet her so he can absorb her and become immune to the sun. He's not a disgusting person you imagine. God why do I even need to clarify this.