Isekai Adventure in the Dimensional Cube

Known as an exceptionally cowardly person afraid of death by Tamayo, Kibutsuji Muzan had a great obsession with immortality.

This demon's progenitor was born in the distant Heian period, and before his birth, his heartbeat had stopped while in his mother's womb, being born into the world as a stillborn. Ultimately though, he survived, but he was diagnosed with a terminal illness that confined him to bed, proclaiming he wouldn't live past 20.

He should have died a very, very long time ago.

However, a doctor, in an attempt to prolong his life, formulated a medicine that changed Kibutsuji Muzan's entire life.

Yes, Kibutsuji Muzan was originally a mortal human susceptible to sickness and death.

It was not until he consumed the medicine from that doctor that Kibutsuji Muzan underwent a transformation, not only regaining his health but also gaining immortal and formidable strength.

From that moment, a new monster appeared in the world—a demon.

The so-called Demon King, the Ancestral Demon, was thus born.

After becoming a demon, Kibutsuji Muzan acquired a body that would recover from any injury and would never age or die.

Yet, he still pursued perfection, intending to overcome his weakness to sunlight to become a truly immortal being.

Thus, in his quest for immortality, his quest for a perfect life, he abandoned his family, and his former identity, constantly moving throughout the world.

Tamayo became a demon because Kibutsuji Muzan wanted to utilize her expertise in medicine to develop a drug that could overcome sunlight.

As someone well-acquainted with the original story, Reiger was well aware of Kibutsuji Muzan's obsession with immortality and knew what he intended to do by wandering the world for a thousand years.

"Blue Lycoris."

Reiger then revealed this secret known by very few.

"Kibutsuji Muzan researched the formula of the medicine initially developed by that doctor and discovered that the drug was actually an unfinished product, requiring a plant called 'Blue Lycoris' to be completed."

Therefore, Kibutsuji Muzan had always been searching for traces of the Blue Lycoris to complete that unfinished medicine and make himself the strongest being, truly immortal.

Knowing this, right when the jade flask was about to be destroyed, Reiger whispered into his ear.

"The Blue Lycoris is in my hands."

Reiger could be certain that these words had reached Kibutsuji Muzan's ears.

Kibutsuji Muzan could give his blood to others, transforming them into demons and using this to completely control them.

Those who became demons with Kibutsuji Muzan's blood not only had their thoughts read by him when near but also had their directions and locations sensed, and even shared telepathic communication and visions, allowing Kibutsuji Muzan to know everything happening with them to the greatest extent.

Although, this control was limited, such as when distances were too great accuracy would drop, or when Kibutsuji Muzan himself was in poor condition, which would significantly reduce his control over the demons, generally speaking, unless there was an accident, all demons were under Kibutsuji Muzan's control, with every action known to him.

Therefore, Reiger could affirmatively tell everyone.

"Kibutsuji Muzan definitely heard my words at that time," Reiger stated, "So, he will definitely come for me."

In the face of his obsession with immortality, Kibutsuji Muzan, no matter how cowardly, couldn't just do nothing and hide again like he did hundreds of years ago.

For the Blue Lycoris, he had been searching from the Heian period to the present, for a full thousand years, all to no avail; now suddenly there was news. No matter what, he would be driven mad by it.

Even if he feared and dreaded Reiger's strength, was shocked by Reiger's knowledge about the Blue Lycoris, or even suspected deception, for immortality, for a perfect life, he would inevitably take action.

This was his sole obsession, and Reiger didn't believe he could control this thousand-year-long craving, not trying even once before confronting Reiger, and simply hide away.

"It can be said that from the moment I uttered the name 'Blue Lycoris' beside the ear of the Upper Moon Five, Kibutsuji Muzan and I were inevitably destined to confront each other."

In the room filled with tatami mats, Reiger declared this before the high-ranking members of the Demon Slayer Corps.

"Perhaps, we will have a battle soon."

Upon these words, the entire room fell silent.

Everyone present, including Ubuyashiki Kagaya, started to breathe slightly faster in that very moment.

There was no helping it.

If all this were true, it meant one thing.

The Kibutsuji Muzan they had always dreamed of finding and exterminating, who they had never been able to locate, was about to appear.

When that time comes, if they kill him, all the demons in the world will die.

No more people would be eaten by demons.

No more people would suffer the pain of losing their loved ones.

No more people would despair.

No more people would lose a happy life.

The Pillars present, more or less, all had their original lives tragically altered by demons, losing their loved ones and thus harbored deep hatred and enmity towards demons, wishing to eradicate them all.

Especially Kibutsuji Muzan, it was because of him that the demons who killed their loved ones appeared in this world.

In other words, the tragedies they bore were all due to Kibutsuji Muzan's actions.

Now, the main culprit was about to appear, how could the Pillars present remain calm?

At that moment, an astonishing aura filled the room.

Some were surrounded by whirlwinds.

Others were engulfed in flames.

Ripples flowed around some of them.

Others caused the air around them to display bursts of aberrations, either sounding like sonic booms or snake-like shadows rising behind them, which looked extremely terrifying.

"Cough cough..."

Even Ubuyashiki Kagaya coughed violently due to the severe emotional fluctuations, coughing blood onto the tatami.

Instantly, Kagaya's wife and daughters panicked, quickly rubbing his back and feeding him medicine.

Reiger watched all this, frowning without saying a word.

But by now, even if he remained silent, no one present could ignore him anymore.

"Is everything you said true?" Shinazugawa Sanemi, with bloodshot eyes, stared at Reiger and squeezed out the words through clenched teeth, "Kibutsuji Muzan is coming after you? Are you absolutely certain?"

Every pillar's gaze suddenly focused intensely on Reiger, filled with various fierce and impassioned emotions.

Although Reiger had anticipated that revealing this information would excite the Demon Slayer Corps, he hadn't expected their reaction to be this strong.

After all, he was not one of the Demon Slayers who, like those here, had experienced despair, pain, the nightmare and hell of having their loved ones taken by demons. He knew about the pillars' tragic pasts through the original work, but knowing was one thing; resonating was another.

Thus, Reiger felt compelled to remind them.

"You all seem to think this is good news, but I don't see it that way."

Reiger promptly splashed cold water over them.

"Regardless of what you think, once you face Kibutsuji Muzan, there is only one outcome."

"Complete annihilation."

This cold shower truly extinguished the fiery emotions within the pillars.

Only a few pillars, who harbored deep hatred for demons, were completely reckless by now.

"If it means killing Kibutsuji Muzan, what does complete annihilation matter?" Shinazugawa Sanemi roared, "Even if it means death, I will drag him to hell with me!"

"I'm afraid you might not even manage that," Reiger said mercilessly. "If you can't even defeat the upper rank demons, even if all of you attack together, without unlocking the Mark, without entering the Transparent World, and without Nichirin Swords, you can't even harm a single hair on him."

"What did you say?"

Shinazugawa Sanemi stood up.

Reiger glanced indifferently at Shinazugawa Sanemi, then averted his gaze, looking as if he didn't want to discuss it further.

This caused Shinazugawa Sanemi to lose his temper.

"Sanemi!"

Just as Shinazugawa Sanemi was about to charge and grab Reiger by the collar, Ubuyashiki Kagaya timely intervened.

While coughing, he stopped Shinazugawa Sanemi.

"Do not disrespect Tsukiguni Yoriichi. He saved me, he saved the entire Ubuyashiki family and the Demon Slayer Corps."

With just a simple sentence, Shinazugawa Sanemi calmed down.

It was then Iguro Obanai, who, like Shinazugawa, had a past conflict with Reiger and was not very friendly, who spoke up.

"What exactly are the 'Mark,' 'Transparent World,' and 'Nichirin Swords' you mentioned?"

Iguro Obanai's question successfully redirected everyone's attention to this point.

"...I remember, Lord Tsukiguni also mentioned these three names before."

Kochou Shinobu suddenly brought up this matter.

"Ah, that seems right."

Kanroji Mitsuri recalled it as well.

"What exactly are they? They sound quite magnificent!"

Uzui Tengen couldn't help but speak up.

Tomioka Giyuu, Tokitou Muichirou, Himejima Gyoumei, and Rengoku Kyoujurou all showed expressions of interest.

Seeing this, Reiger thought for a moment and finally decided to explain.

"There's no harm in telling you," Reiger said. "The so-called 'Mark,' 'Nichirin Swords,' and 'Transparent World,' refer to three types of powers."

"The 'Mark' refers to a pattern that can appear on the body, significantly enhancing physical abilities."

"Nichirin Swords refer to a technique that can greatly increase damage to demons, even reducing their regenerative abilities."

"The 'Transparent World' is a state, a realm, known among martial artists as the Supreme Domain. Once attained, it significantly improves sensory abilities, allows for the perception of human bodies, and enhances the speed of movements, predictive capabilities for attacks, and dodging abilities."

Reiger thus informed them.

"These three powers, once someone successfully possesses any of them, can effectively enhance strength and the threat level against demons."

"Should someone possess all three, even if facing a high-ranking demon among the upper ranks, one would have the power to fight."

"Even the lowest-ranking Swordsman, equipped with these three powers, could pose a threat to upper-rank demons."

This was what people in the previous world called the "Demon Slayer's Three-piece Set."