Isekai Adventure in the Dimensional Cube

Reiger's path of trial was longer than he had imagined, and even more horrifying than others had imagined.

After persisting in the courtyard for seven days, Reiger finally collapsed for the first time.

Rengoku Kyoujurou, who habitually checked in on him, was the first to discover Reiger collapsed in the courtyard and hurriedly carried him back to his room and called for a doctor.

The doctor called by Rengoku Kyoujurou was none other than the Insect Pillar, Kochou Shinobu.

In addition to being a swordsman, Kochou Shinobu was also a Pharmacist who excelled in pharmacology. She had also established a treatment facility called "The Butterfly House" within the Demon Slayer Corps, which housed some nursing staff without swordsmanship talent, specifically to provide wounded members with medical treatment and rehabilitation.

Tanjirou had been seriously injured in the incident with the Lower Five and, along with Zenitsu and Inosuke, was sent to The Butterfly House. Ultimately, with the help and care of Kochou Shinobu, they not only recuperated in succession but also mastered "Total Concentration: Constant", significantly increasing their strength.

Therefore, as soon as something went wrong with Reiger, Rengoku Kyoujurou's first contact was The Butterfly House, bringing Kochou Shinobu, who managed it, over.

Upon seeing Reiger, Kochou Shinobu displayed a troubled expression.

"I really want to ask, exactly what did you all have him do?" Kochou Shinobu said in a disheartening tone, "His complexion is so pale, his lips nearly devoid of color. It's obvious he's extremely weak. Looking at his muscle spasms and bodily tremors, it seems his body is enduring unbearable pain. His organs, especially the lungs, are likely to have ruptured. I'm even wondering if they've failed, otherwise, how could his breathing possibly be like this?"

In her own words, his condition was so severe that it wouldn't have been surprising if he had died on the spot.

"Take him to The Butterfly House. In his state, he won't be able to recover unless he rests in bed for two or three months."

With those words, Kochou Shinobu had the Rengoku Family send Reiger to The Butterfly House.

However, just when Rengoku Kyoujurou thought Reiger really needed two or three months to recuperate and was considering visiting him before his next mission, he was stunned to find that the very next day, Reiger came back full of energy.

Upon returning, Reiger immediately went back to the courtyard without stopping and once again entered that peculiar breathing state.

At that moment, Kochou Shinobu was following right behind Reiger, all the way to the Rengoku Family's residence. When she saw Reiger enter the courtyard and that shocking breathing state, she fell silent for a long time.

"Monster."

Thereafter, Kochou Shinobu uttered such an evaluation and turned to leave.

Reiger then continued to embark on his arduous training and attempts.

This time, he lasted ten days.

After ten days, he was once again sent to The Butterfly House.

Just like the last time, the next day, Reiger came back full of vigor, prompting Kochou Shinobu, who had still been following him, to say to Rengoku Kyoujurou,

"Can I dissect him? I'm beginning to take an interest in his body! He might be a demon hiding among us, don't you think?"

Kochou Shinobu's earnest words nearly caused Rengoku Kyoujurou to freeze on the spot in shock.

Yet Reiger ignored it all and continued with his trials.

And so, time flew by quickly.

In the blink of an eye, three months had passed.

During this period, Rengoku Kyoujurou went on several missions, and even the main trio no longer stayed at the Rengoku household, undertaking missions for the Demon Slayer Corps—sometimes venturing out to hunt demons, sometimes returning to train, sometimes splitting up and sometimes setting out together. From initially worrying about Reiger, they gradually grew accustomed to his reckless behavior.

Even The Butterfly House, under the instruction of Kochou Shinobu, often prepared a bed for Reiger, although typically, this bed was only used once every so often, with the patient invariably leaving the next day.

Meanwhile, what was happening with Reiger gradually caught the attention of the Demon Slayer Corps' leader and all the Pillars.

As a result, apart from Ubuyashiki Kagaya, who was indisposed from going out, all the Pillars secretly visited the Rengoku household in the past three months, covertly observing Reiger for a while.

Some Pillars only watched him briefly before leaving, such as Tomioka Giyuu and Tokitou Muichirou.

Others observed Reiger closely, like Shinazugawa Sanemi, Iguro Obanai, Himejima Gyoumei, Kanroji Mitsuri, and Uzui Tengen—some watched him for an entire day, some even for several days, before quietly departing.

As for Rengoku Kyoujurou and Kochou Shinobu, they became the Pillars who interacted with Reiger most frequently.

Rengoku Kyoujurou's case is self-explanatory, as Reiger was staying at his home and had the best relationship with him. It could be said that Rengoku Kyoujurou had watched Reiger persist all the way.

Kochou Shinobu, on the other hand, grew curious due to Reiger's frequent collapses and superhuman constitution, beginning to get in touch with him, and sometimes even taking care of him personally.

Even Ubuyashiki Kagaya often paid attention to Reiger's condition, prompting the leader to frequently reveal thoughtful expressions at home.

In this way, on this day, three months later, Reiger's training finally welcomed a change.

...

"Breath—Inhale—"

This was a remarkably prolonged breath that was crisply and swiftly completed in a short span of time.

Standing in the courtyard, Reiger had taken his familiar position every day for the last ten years, his back turned to the entrance as his chest rose and fell ever so slightly.

This time, the ones standing at the entrance of the courtyard, looking at Reiger in such a manner, were Rengoku Kyoujurou and Kochou Shinobu.

Both gazed at Reiger's silhouette, their expressions as if they were beholding a deity.

"Such incredible breathing," Kochou Shinobu murmured, "How on earth did he accomplish this?"

Three months had passed, and Reiger's breathing had greatly changed.

It was no longer disordered as it once was, nor was it erratic like in the beginning; instead, it had become peculiar, wondrous, with an indescribable quality to it.

At least, in the eyes of Kochou Shinobu, the person breathing before her seemed not to be human anymore, but an extraordinary being beyond human comprehension.

That breathing sound might go unnoticed by the average person, but to the Pillars who had trained in the breathing techniques and honed their own to perfection, it was like listening to a dragon's breath, a morning sun inhaling and exhaling, or even a god breathing life.

If Zenitsu had been there, he would surely have heard the thunderous breaths hidden in the air, breaths no human could emit.

Now, Reiger was breathing in such a way, whether in rhythm or scope, far beyond what he had managed before.

"Hmm, I have never seen breathing like this before," Rengoku Kyoujurou said with crossed arms, "Did the young disciple create a completely new style of breathing?"

Creating a breathing style wasn't a particularly rare occurrence among the Pillars of the Demon Slayer Corps.

The likes of Love Breathing, Serpent Breathing, Insect Breathing, Sound Breathing, and Mist Breathing were derivative techniques created by the current Love Pillar, Serpent Pillar, Insect Pillar, Sound Pillar, and Mist Pillar, respectively. They had felt that the five fundamental breathing techniques did not quite suit them and thus innovated upon the originals to create these new derivative techniques.

Among the generations of Pillars, those Swordsmen who had created their own breathing methods were not as numerous as carps crossing the river, but it wasn't considered odd, either.

With that in mind, it seemed quite normal for Reiger, a powerful Swordsman who could slay an Upper Moon Three demon with a single strike, to have the ability to create his own breathing technique.

However...

"I don't think this is as simple as creating a new breathing method," Kochou Shinobu sighed, "Is it typical for someone who creates their own breathing method to push themselves to this extent, even facing death on several occasions?"

Rengoku Kyoujurou remained silent.

"I have never heard or even imagined that such breathing could exist in this world," Kochou Shinobu continued, staring intently at Reiger's back, "It's definitely not as simple as creating a brand-new breathing method; there must be something we don't understand happening to that person."

"Yes!" Rengoku Kyoujurou responded without any objections, nodding vigorously, "The young disciple is still as unfathomable as ever; if my adopted son saw the effort he has put in, he would surely be inspired!"

The adopted son Rengoku Kyoujurou mentioned was Tanjirou.

Indeed, Tanjirou had ultimately become the adopted son of Rengoku Kyoujurou, in other words, his disciple.

Thanks to this relationship, the current Tanjirou always made his way back to the Rengoku Family after completing his missions, as if he considered it his own home.

"Kamado-kun?" Kochou Shinobu and Tanjirou were also acquainted, and upon hearing Rengoku Kyoujurou's words, she gave a charming smile, "That is a pity. I had thought of taking him as my own adopted son, especially since he seemed to get along so well with Kanao."

"Then I am really sorry about that!" Rengoku Kyoujurou apologized, yet his face bore no sign of remorse, instead radiating righteousness.

Kochou Shinobu could only shake her head and chuckle, about to say something more when suddenly, an anomaly occurred.

"Boom!"

In an instant, a roaring sound came from within the courtyard.

It was the sound of the atmosphere twisting.

It was the noise of the earth trembling.

A terrifying heat spread from the center of the courtyard, as if it could evaporate everything, seemingly distorting the very space itself.

"What is this...!?"

Rengoku Kyoujurou and Kochou Shinobu were both stunned, and they almost reflexively leaped backward a great distance, crouching low while preparing to draw their swords.

This was a natural bodily response.

When such a response occurred, it meant they both sensed danger—considerable danger at that.

But the very next second, they realized that the source of the danger wasn't from an enemy, but from the very person they had been observing all along.

"Ssss—"

Reiger, standing in the courtyard, radiated an alarmingly fierce heat from his body, hissing as it made the ground beneath him sizzle as though scorched by fire, giving off a burnt smell.