The "Mu" in Muzan stood for "useless"

(A little introduction to the world; those who have read the original can skip this.)

This world was the world of an anime Kanzaki Kumo had watched in his previous life.

It was a world where humans fought against demons.

The story began a thousand years ago. An ancestor of the Ubuyashiki Family had suffered from a terminal illness in his youth, and a doctor who claimed he could cure him came to his home to treat him.

But after repeated doses of medicine, his condition showed no signs of improving. Instead, his body underwent strange changes, making him like a monster. In a fit of rage, he killed the doctor, cutting short the unfinished treatment.

Afterward, the man became powerful and vicious. Not only did human food disgust him, but humans themselves aroused a ravenous appetite in him, and the torment of his illness disappeared.

Had that been all, he could have accepted it. But he discovered that he could not touch even the slightest ray of sunlight. The legitimate son of a great family could not even stand in the sun.

Already suffering from psychological problems brought on by his illness, he went completely mad.

He discovered that devouring humans could make him stronger, while his blood could turn others into beings like himself under his control. Neither he nor those he transformed would age.

He then named the immortal beings he transformed "demons," while he himself was the king of all demons, named Muzan Kibutsuji.

From then on, demons began their rampage.

Having obtained great power and a long life, Muzan had only one goal-to overcome sunlight.

From the notes left behind by the doctor he had killed, he learned that the final treatment he had never completed had lacked a medicine called the Blue Spider Lily. If he could obtain it, he could become a truly "perfect being" with no fear of sunlight.

For the next hundred years and more, Muzan operated without restraint at night, turning people into demons at will and sending them out to search for the Blue Spider Lily.

Because a demon as evil as Muzan Kibutsuji had been born into their ranks, the Ubuyashiki Clan was cursed, and its men died young one after another. Only after generations of intermarriage with a family of priests who served the gods did the men of the direct line live to thirty, but even that was the limit.

To make amends for their mistake, lift the family's curse, and save humanity, the Ubuyashiki Family founded the Demon Slayer Corps, determined to fight demons, with killing the Demon King as their ultimate goal.

At first, the Demon Slayer Corps was almost entirely overwhelmed by demons.

Although the Ubuyashiki Family had found ore containing the power of the sun and trained smiths capable of forging it into weapons, humans remained at an enormous disadvantage against demons for a long time.

Even the most ordinary demon was extremely difficult to kill.

That was because a Nichirin Blade containing the sun's power could only destroy a demon by severing its neck, whereas the neck was far from the human body's only vital point. A demon only needed to protect its neck, while a human had to protect far more places to remain capable of fighting.

Combined with the enormous gap in physical strength between humans and demons, this made it extremely difficult for humans to defeat demons.

Only when that man appeared did things begin to improve.

Yoriichi Tsugikuni created Breathing Styles that could greatly strengthen the human body, fundamentally increasing the Demon Slayer Corps' combat power. Though the Corps still suffered many casualties afterward, it could at least hunt and kill most demons effectively, rather than throwing lives at them as it had in the past.

It was also he who, upon encountering Muzan Kibutsuji by chance, slashed a lifetime of psychological trauma into this supposedly invincible Demon King, etching it into his very genes. Afterward, Muzan lay low for over a hundred years, and even when he emerged again, he kept an extremely low profile.

One moment of bravery bought him a lifetime of cowardice.

Only during that period in hiding did Muzan begin truly building up the forces under his command. The Twelve Kizuki system was established then.

But Muzan's so-called efforts to build his forces amounted to creating the Twelve Kizuki, a group of "high-grade workhorses." He gave them more blood so they could become stronger, and had them continue searching for the Blue Spider Lily and eliminating the Demon Slayer Corps for him.

At the same time, he forbade the demons under him from gathering together.

Because when Yoriichi had gravely wounded him, one demon had taken advantage of his weakened state to escape his control. That feeling of losing absolute control over something filled him with disgust and even fear. He feared that one day, demons gathered together might break free of his control and turn on him, their king.

To Kanzaki Kumo, everything this Demon King did, from beginning to end, displayed the pristine innocence of a mind untainted by knowledge.

Take the most basic example.

Despite possessing the means to grant immortality and overwhelming strength, he had never thought to approach people in high places and have them use their influence to search for the Blue Spider Lily. Wouldn't that be far faster than running around the world himself?

After all, how would anyone know that becoming a demon meant falling under Muzan's control unless he told them? Even the present-day Demon Slayer Corps did not know exactly how Muzan controlled demons.

Wasn't the Ubuyashiki Family doing everything it could to suppress information about demons precisely because it feared those in power might get dangerous ideas?

Not to mention all the absurd things Muzan had done in the original story. No wonder so many people in the world of Kanzaki Kumo's previous life had joked that "the 'mu' in Muzan stood for 'useless.'"

Aside from strength that no one alive could match, this Demon King had absolutely nothing worth praising.

And even that strength, fundamentally speaking, was thanks to that doctor back then, with little credit due to Muzan himself.

"No, I can't keep thinking about this." Kanzaki Kumo hurriedly reined in his thoughts. If he kept going, he would start letting his guard down around Muzan. Whatever else might be true, Muzan was currently invincible in this world, an enormous threat to everyone, Kanzaki Kumo included.

And not just Muzan. To Kanzaki Kumo as he was now, any demon at all was a threat.

So the most important thing right now was to study his Breathing Style diligently and grow stronger as soon as possible.

Only with sufficient strength could he survive well in this dangerous world and save those whose fates had left him unable to find peace in his previous life.

Kanzaki Kumo's thoughts ended there.

Just then, the carriage came to a stop as well.

They had arrived at Mount Momiji.