Riezel's relentless efforts turned out to be longer than he had imagined, and more nerve-wracking than anyone else had anticipated.
After persisting in the courtyard for seven days, he finally collapsed for the first time.
It was Kyoujurou, who frequently came to check on him, who first noticed Riezel lying in the courtyard and hastily brought him back to the room, calling for a doctor.
Of course, the doctor called by Kyoujurou was none other than the Insect Hashira, Kochou Shinobu.
Aside from being a swordsman, Shinobu was also a proficient pharmacist. She also ran a treatment facility within the Demon Slayer Corps known as the 'Butterfly Mansion', which housed nursing personnel without talent in swordsmanship, specializing in providing treatment and rehabilitation for members injured on missions.
In the past, Tanjirou, Zenitsu, and Inosuke had suffered severe injuries after fighting the Lower Five and were sent to the Butterfly Mansion. With Shinobu's assistance and care, they not only recovered one after another but also mastered Total Concentration Breathing, greatly enhancing their strength.
Therefore, when Riezel had an incident, the first person Kyoujurou contacted was Shinobu, who managed the Butterfly Mansion.
Upon seeing Riezel, Shinobu's face immediately displayed a troubled expression.
"I really want to ask... what exactly has he done?"
"With such a pale complexion and lips almost devoid of color, it's clear he's extremely weakened. Plus, with his muscles and body twitching unconsciously, it's likely he's enduring inhuman pain, possibly even with internal organs rupturing, especially his lungs. I even suspect they might be damaged; otherwise, how could his breathing be like this?"
In her own words, this was already a severe condition where even immediate death wouldn't be surprising.
"Take him to the Butterfly Mansion. With his condition, it's basically impossible for him to recover without lying in bed for two to three months."
With that statement, Shinobu instructed Kyoujurou to send Riezel to the Butterfly Mansion immediately.
However, just when Kyoujurou thought Riezel would indeed need about two to three months to recover and was considering whether to visit him before going on a mission, he was shocked to find out the next day that Riezel had returned as if nothing had happened.
Upon his return, Riezel immediately rushed back to the courtyard and entered that strange breathing state once again.
At that moment, Shinobu followed Riezel and chased him all the way to the residence of the Rengoku family. When she saw Riezel enter the courtyard and fall into that strange breathing state, she remained silent for a long time.
"Monster..."
After muttering like that with a peculiar expression, Shinobu left.
And so, Riezel continued his arduous practice and efforts.
However, different from last time, this time, he persisted for ten days.
Ten days later, he was once again collapsed and sent to the Butterfly Mansion.
However, just like the last time, the next day, Riezel returned as if nothing happened again, prompting Shinobu, who was still following him, to say something to Kyoujurou.
"I can dissect him, can't I? I'm getting interested in his body! Maybe there is an impostor (demon) among us."
Shinobu's serious tone almost left Kyoujurou dumbfounded on the spot.
Despite all the chaos he caused, Riezel remained indifferent to it all, continuing with his own experiments.
With that, time flew by swiftly.
In the blink of an eye, three months had passed.
During this period, Kyoujurou had gone on several missions. Even the three main characters no longer stay at the residence of the Rengoku family and doing missions from the Demon Slayer Corps instead.
Sometimes they went out demon hunting, sometimes they came back to practice, sometimes they separated, and sometimes they set off together. From initially worrying about Riezel, they gradually grew accustomed to his bizarreness and unreasonableness.
Even at the Butterfly Mansion, under the direction of Shinobu, a bed was often prepared specially for Riezel. Strangely, this bed only got used for a day at a time, and the next day, the patient lying on it was nowhere, leaving directly.
At the same time, what was happening with Riezel was slowly entering the view of the Master of the Demon Slayer Corps and every Hashira.
Consequently, aside from Kagaya, who found it inconvenient to go out, every Hashira had secretly visited the residence of the Rengoku family over the past three months to observe Riezel for a while.
Some Hashira just took a glance and then left, like Giyuu and Muichirou.
However, some Hashira had been observing Riezel all along, such as Sanemi, Obanai, Gyoumei, Mitsuri, and Tengen.
Some observed for a whole day, and some even observed for several days before silently leaving.
As for Kyoujurou and Shinobu, they had become the Hashira who interacted with Riezel the most.
It goes without saying for Kyoujurou, as Riezel was in his home and their relationship was the closest. It could be said that Kyoujurou was the only one who had watched Riezel persist all the way through from beginning to end.
Shinobu, on the other hand, became curious about Riezel because of his occasional collapses and demonstrations of his inhuman recovery, so she started interacting with him and sometimes even personally cared for him.
Even Kagaya would often pay attention to Riezel's situation through Shinobu's reports, causing this family head to often show a meaningful expression.
And thus, three months later, on this day, Riezel's practice finally welcomed a change.
===
"Huuu... Haaa..."
It was a rather prolonged inhale, but swiftly completed within a short time.
Standing in the courtyard, Riezel remained in that familiar spot, as if he had stood there for several decades like a hermit, facing away from the entrance with his chest gently rising and falling.
At this moment, Kyoujurou and Shinobu stood at the entrance of the courtyard, observing Riezel's back.
Their gazes towards Riezel were akin to those directed at a divine being.
"What an extraordinary breath..." Shinobu whispered, her voice low. "How does he manage it?"
After three months, Riezel's breathing had undergone significant changes, no longer chaotic as before, nor disorderly as it initially began, it had become peculiar, almost wondrous, evoking an inexplicable sensation.
At least in Shinobu's eyes, Riezel's breathing before her at this moment seemed less like a human and more like a transcendent being, surpassing human comprehension.
To the untrained ear, the sound of his breath might seem insignificant, but to the Hashira who had honed their breathing styles to perfection, it resonated like the roar of a great dragon, the exhale of a bright sun, or even the breath of a god.
If Zenitsu were here, he would surely hear the thunderous breath hidden within the air, unmistakably not human.
Presently, Riezel was engaged in such breathwork, surpassing anything in rhythm and intensity compared to before.
"Hmm, I've never seen such breathing before..." Kyoujurou muttered with his arms folded. "Young Tsugikuni, have you created a new breathing style?"
Creating a new breathing style wasn't uncommon among the Hashira.
Breathing styles like Love Breathing, Serpent Breathing, Insect Breathing, Sound Breathing, Mist Breathing, and others were derived from the five fundamental breathing styles.
Current Hashira like the Love Hashira, Serpent Hashira, Insect Hashira, Sound Hashira, and Mist Hashira found the existing breathing styles not entirely suitable for them, so they innovated and modified the fundamental breathing styles to create their own derived breathing styles.
Throughout history, it wasn't rare for the Hashira to create their own breathing style.
Considering this, it seemed quite normal for Riezel, the great swordsman who slew the Upper Three, to have the ability to create a new breathing style.
However—
"I don't think his practice is as simple as that..." Shinobu sighed and paused for a moment before continuing. "Generally, when creating a breathing style, could one be forced to such extent, even several times to the brink of death?"
Upon hearing her reasoning, Kyoujurou remained silent.
"It's definitely not just a matter of creating a new breathing style, there must be some things we don't understand that have taken shape in that guy..." Shinobu stared fixedly at Riezel's back.
"Umu..." Kyoujurou agreed wholeheartedly, nodding heavily. "Young Tsugikuni remains an incredible youth. If my Tsuguko were here and saw the results of his hard work so far, he would surely feel greatly inspired!"
Surprisingly, the Tsuguko in his mouth was none other than Tanjirou.
Yes, Tanjirou eventually became Kyoujurou's Tsuguko, meaning he had accepted Kyoujurou as his mentor.
Moreover, thanks to this relationship, Tanjirou now always returned directly to Kyoujurou's place after completing missions, as if considering it his own home.
"Kamado, huh?" Shinobu muttered, remembering the kind-hearted boy.
She and Tanjirou were also old acquaintances, so upon hearing Kyoujurou's words, she smiled knowingly.
"That's a shame. I was considering whether to take him as my Tsuguko. After all, he seems to get along quite well with Kanao."
"Well, that's unfortunate!" Kyoujurou apologized, though there was no hint of regret in his face or voice, he exuded an air of openness and honesty.
Shinobu could only shake her head and smile wryly.
However, just as she was about to say something else, an anomaly suddenly occurred.
*BOOM!*
*Rumble—*
A rumbling sound erupted in the courtyard.
It was the sound of the air distorting.
It was the sound of the earth trembling.
A terrifying heat surged from the courtyard, as if capable of evaporating everything, causing the surrounding space to distort.
"What's happening?!"
"What?!"
Kyoujurou and Shinobu were both shocked, almost reflexively leaping back a considerable distance, lowering their bodies, and assuming a stance ready to draw their swords.
It was an instinctive reaction of their bodies.
Clearly, to have such a reaction meant that these two had sensed danger. Moreover, it was not just any danger, but a remarkably powerful one.
However, in the next moment, they found that the source of the danger was not from any enemy, but from the one they had been observing all along.
*Psssttt—*
At this moment, Riezel's body standing in the courtyard emitted an extremely fearful heat, making the ground beneath his feet sizzle and emit the smell of scorching as if it were being roasted by fire.