Riezel's plan wasn't entirely baseless.
Initially, Total Concentration Breathing already had the effect of enhancing physical ability, but it only strengthened the heart and lungs, as well as accelerating blood flow, thereby indirectly boosting the body's performance and physical prowess.
Akasha Continent's breathing technique, on the other hand, was merely a support for swordsmanship by stimulating and activating magical energy within the body, allowing it to grow in all aspects by absorbing the magical energy.
If this principle could be integrated into Total Concentration Breathing, it would expand the strengthening range of Total Concentration Breathing to the entire body, which was not an impossible feat.
If this feat were to succeed, it would allow the magical energy to merge with the accelerated blood flow, enhancing its intensity and effect, and would undoubtedly provide him with more enhancement and stronger power when using the breathing style.
Undoubtedly, this was a process of mutual acceptance, integration, and harmony. As long as the two were cleverly combined, Riezel's plan was not just empty talk.
For this reason, Riezel chose Sun Breathing as the foundation.
Sun Breathing could produce various breathing styles because it was the origin and foundation of all breathing styles, proofing its immense versatility.
Using Sun Breathing as the foundation, merging the breathing principles of two worlds, and the swordsmanship of two worlds, the success rate would greatly increase.
Each of these fundamental breathing styles had its own characteristics—some greatly enhance speed, while others greatly improve flexibility. By drawing inspiration from them, he may be able to expand the strengthening range of breathing style to every part of his body.
Coupled with his mastery of Breaking Steel Sword Style and incomplete Holy Dragon Sword Style, merging all the breathing of the two worlds into Sun Breathing would elevate the power of Sun Breathing to an unprecedented degree.
It was entirely feasible.
Becoming a sun was not just talk, as it was indeed Riezel's goal.
Drawing inspiration from the Holy Dragon Sword Style, which could hone one's body into that of a dragon, if he could make the new version of Sun Breathing also have the effect of honing one's body, then his body might undergo unprecedented changes.
By then, if practicing Holy Dragon Sword Style was to transform a human into a dragon, wouldn't practicing Sun Breathing be transforming a human into a sun?
Of course, all of that was currently just Riezel's wishful thinking and theory. Whether it could be achieved, he didn't know.
However, by integrating the breathing of two worlds into the Sun Breathing, Riezel was still somewhat confident with his theory.
Once this plan came to fruition, his gains in this world would be well worth it.
Just imagine, after transformation, not only could Sun Breathing strengthen every cell in his body, but it could even utilize his abundant magical energy, turning it into actual strength to reinforce his body.
How powerful would he be then?
Certainly not weaker than when using 'Ariel', right?
More importantly, Total Concentration Breathing had another advanced state.
In that state, even as he drifted into sleep, he could maintain full focus, and regulate his breathing, keeping his body in a constant state of strengthening—a state known as 'Total Concentration - Constant'.
Swordsmen who could proficiently master Total Concentration - Constant were among the top swordsmen in the Demon Slayer Corps, and mastering it was also the minimum requirement to reach the strength of a Hashira.
If Riezel's plan succeeded, then he could completely develop a new form of Total Concentration - Constant for the new Sun Breathing. In that case, he would be as if constantly using Sun Breathing and practicing swordsmanship at all times.
What did this mean?
It meant that Riezel would become stronger each second, even in his dreams, without actively practicing!
In this way, even if he couldn't hunt down large numbers of monsters inside the dungeon like in the world of Danmachi and constantly training himself, his status would still skyrocket!
Perhaps this wouldn't allow him to gain high-rank excelia for leveling up, but it could help him reach his current level's upper limit at the fastest speed, that was, SSS-rank in all basic abilities.
'Can I really succeed?'
Imagining was delightful, and Riezel also believed that his plan had no problems, but whether he could succeed in integrating the breathing principles of the two worlds with his current Sun Breathing to create a completely new Sun Breathing, he naturally didn't know.
'Well, I still have to give it a try.'
Various principles from the five fundamental breathing styles, Sun Breathing, Breaking Steel Sword Style, and Holy Dragon Sword Style began to emerge in his mind.
Soon, Riezel closed his eyes.
"Huuu... Haaa..."
Gradually, he started to regulate his breathing.
===
To Riezel, the process of fusion was honestly very painful.
Riezel had to constantly adjust his breathing, continuously trying various breathing techniques, borrowing from them, blending them, and even changing them, which had a significant impact on his body.
Sometimes, the completely disordered breathing made him feel like his lungs were about to explode, his trachea burning as if on fire, exceptionally painful.
During this process, there were often symptoms such as ringing in the ears, sore throat, chest tightness, blood vessels popping up all over the body, muscles tearing, and so on.
He had to admit that it felt a bit like enduring torture.
However, Riezel gritted his teeth and persisted, even disregarding the awful state of his own body.
In any case, with the top-class healing magic 'Dia Fratel' in hand, as long as his limbs weren't blown off and his heart and brain didn't explode, he could restore himself.
Hence, with the belief that 'as long as I am not dying, then I'll keep pushing myself to death', Riezel endured the pain and persisted.
He knew that whether Sun Breathing could successfully evolve according to his own plan had a huge impact on his future and strength.
At this point, if Riezel didn't persist, then Riezel wouldn't be Riezel.
'Isn't this also a kind of challenge?'
Riezel always liked challenges, otherwise, he wouldn't have pushed himself so hard before, constantly challenging the limits of his human body, or seeking all sorts of thrills and excitement in his previous life.
If enduring a bit of torment and pain could lead to success, then for him, it was definitely worth it.
And so, Riezel kept trying, kept persisting, combining various understandings and ideas about breathing styles in his mind. Eventually, he smoothly forgot about time, even forgot to eat and sleep, standing in the courtyard day and night.
"Huuu... Haaa..."
In the courtyard, the sound of breathing with various variations, changes in tone, and fluctuations continued.
During this time, Kyoujurou glanced over here, and the three main characters also came to take a look, but they didn't dare to disturb Riezel, letting him continue his practice.
One day.
Two days.
Three days.
In the blink of an eye, several days had passed, and Riezel still maintained his posture in the courtyard, not moving a step from beginning to end.
On this day, Tanjirou finally couldn't help but speak up.
"Mr. Rengoku, are you really not going to stop Mr. Tsugikuni?"
Tanjirou stood at the entrance of the courtyard, looking at Riezel's back as he spoke to Kyoujurou, who was standing beside him, deeply worried.
"He hasn't eaten or drunk anything for days... If this continues, his health will deteriorate."
"Umu, it's indeed a serious problem..." Kyoujurou responded, staring intently at Riezel. "However, we can't stop someone from advancing toward a higher domain."
"Advancing towards a higher domain? But even so, he still needs to eat and rest..." Tanjirou retorted. "Can't he eat well, rest, and then continue later?"
Tanjirou himself was a hardworking person.
In order to practice Water Breathing and become a Demon Slayer of the Demon Slayer Corps, he spent two consecutive years practicing in the mountains, his hands worn out countless times, and his body sustaining numerous injuries.
He knew the hardships involved all too well.
Moreover, after the events with the Lower Five, Tanjirou also underwent the practice of Total Concentration Breathing under the Insect Hashira, which was equally agonizing.
Yet, he persevered through it all.
Therefore, Tanjirou understood very well that the more effort one put in, the more rewards one could reap. Yet, even he wouldn't go to the extent of not eating, drinking, or sleeping to the point of neglecting basic needs for days.
It was too worrisome and Tanjirou couldn't help but speak up about it.
"I understand your concerns, Young Kamado. I'm worried too..." Kyoujurou nodded, patting Tanjirou's shoulder. "But take a good look at that young man's back. Do you really think you can rush up and stop him?"
Tanjirou instantly fell silent.
Riezel's back looked so desolate, so lonely, yet for some reason, anyone gazing at his figure could sense an unwavering determination, a refusal to compromise.
Tanjirou's sense of smell was keen.
He could smell how hard and painful Riezel's current situation was just from his body odor, otherwise, why would the scent of sweat and even blood emanate from him?
Riezel was definitely undergoing a practice that others couldn't comprehend and was extremely difficult.
"From his breathing rhythm, you can tell he's attempting something." Kyoujurou said with a serious expression. "It's a dangerous attempt. Just listening to his breath makes me uneasy. However, he's still persisting. I believe whatever he's attempting now must be important to him."
For this reason, Kyoujurou couldn't intervene.
"Have faith in him, Young Kamado." Kyoujurou suddenly smiled. "Young Tsugikuni is an incredible young man. I already felt that the first time I met him on the Mugen Train. Master must have seen something in him too, which is why he decided to help him without hesitation. Let's just trust him and wait here for his success."
Kyoujurou's words made Tanjirou turn his gaze toward Riezel.
After being silent for a while, Tanjirou slapped his own cheeks, his eyes full of determination.
"I need to work hard too!"
Tanjirou quickly turned and left to continue his training.
Kyoujurou, who watched it all, sighed.
"They're truly two fine youths... A pity they couldn't be taken in as my Tsuguko... No, maybe not Young Tsugikuni, but Young Kamado might be worth a try?"
Kyoujurou suddenly stroked his chin, remembering something.
Meanwhile, Riezel, unaware of all this, continued his relentless efforts without a moment's pause.