Transparent World.
It was a world, or rather a state, that could only be seen after striving with all one's might, enduring pain, and making one's mind transparent.
People who entered this state could concentrate and shut down unnecessary senses, making their bodies appear transparent and allowing them to see through the condition of the other's body.
They could observe the flow of blood vessels in other's lungs, the contraction of muscles, and the activity level of organs, thus predicting and sensing other's actions, significantly enhancing their own speed of movement, prediction of attacks, and ability to evade.
In simple terms, it was an ultimate sublimation of perception, control, and extension.
Generally, those who mastered the Transparent World could not only hide their fighting spirit according to their own needs but also accelerated their perception, making their enemy's movements appear slow, and their enemy's actions almost had no secrets in their eyes.
This was what the Upper Three, Akaza, referred to as the highest domain, known as the 'Selfless State' or the 'Domain of Supremacy', the final domain that masters of swordsmanship and martial arts could only see after honing their abilities to the limit.
Riezel, in his three months of relentless efforts, had not only awakened the 'Transparent World' but had also unknowingly arrived in the highest domain known as the 'Selfless State'.
As a result, once Riezel concentrated and shut down unnecessary senses, focusing all perception of the outside world on what was right in front of him, he could easily make his mind clear, see through the body of people in front of him, and effortlessly observe the signs of life in them.
He knew that the Transparent World was quite useful for him.
After all, his perception ability was not that outstanding; at most, he became more sensitive to the flow of wind and atmosphere when using 'Ariel', or sensing the flow of magical energy.
Now that he had awakened the Transparent World, his perception would be greatly useful in battle, making fighting easier and more efficient.
Truthfully, this was also a pleasant surprise even for Riezel, which put him in a good mood.
Meanwhile, Kyoujurou and Shinobu, who had no idea that they had been completely seen through by Riezel, were curious about his current strength.
'How strong has Tsugikuni become now?' Shinobu thought half-jokingly. 'Has he become unbeatable in this world?'
'Young Tsugikuni, who can easily kill the Upper Three, has now become even stronger...' Kyoujurou wondered inside his mind. 'Does Master expect something from this?'
At this moment, both of them recalled Kagaya's words before.
"I can already sense it. In our generation, Demon Slayers and demons may finally settle the score, putting an end to this millennia-old history of mistakes."
Remembering the rare excitement in Kagaya's words and face, Kyoujurou and Shinobu glanced at each other.
Now, perhaps they also understood and shared the same feeling as Kagaya.
After all, they had witnessed firsthand the astonishing phenomena that occurred just now after Riezel succeeded in his practice.
With Riezel's current strength, if there was indeed an opportunity, it was indeed possible to put an end to the millennia-long history of mistakes and finally settle the score between demons and Demon Slayers.
As they thought like that, the looks on Kyoujurou and Shinobu's faces changed as they looked at Riezel.
"Thanks for your help during this time." Riezel expressed his gratitude to the two. "If it weren't for you two, I might not have been able to succeed so easily."
"Nah, it's not much to do with us, Young Tsugikuni!" Kyoujurou completely declined any credit. "We didn't really help much. Everything was accomplished through your own efforts!"
"Yep." Shinobu nodded, but then her tone became somewhat resentful. "Tsugikuni doesn't need anyone to take care of him at my place either. Every time you collapse in terrible condition, you wake up looking completely fine the next day. Besides providing a bed, it's like I haven't done anything at all, you know?"
"Umu!" Kyoujurou nodded in agreement, playing along.
After all, all Kyoujurou did was put Riezel on the bed when he collapsed and send him to the Butterfly Mansion to recover; he didn't do anything else, at most providing a place for Riezel to practice.
Moreover, that kind of help was hardly worth mentioning in his eyes.
Of course, Riezel definitely didn't think that way.
"You both took care of me, that's a fact, no need to be modest." Riezel shook his head. "Anyway, I owe you both a favor. If there's anything you need my help with in the future, feel free to ask."
Upon hearing Riezel's words, Shinobu was somewhat intrigued.
"Then can I study your body?"
Finally, she revealed her ambitious side.
"Nope." Riezel didn't even hesitate, rejecting outright.
"Tch." Shinobu pursed her lips, looking disappointed.
"Please make normal requests, thank you..." Riezel sighed.
"Okay, duly noted." Shinobu replied with an unconvincing smile.
"So, Young Tsugikuni, do you still need to continue practicing?" Kyoujurou asked with raised eyebrows.
"I don't think so..." Riezel pondered for a while before continuing. "I've already achieved the breathing style I envisioned, so there's no need to continue... If there's anything else to do, it would be to try mastering 'Total Concentration - Constant'?"
"Total Concentration - Constant?" Kyoujurou nodded. "With your abilities, you'll master it quickly."
Riezel didn't refute his statement.
Truly, 'Total Concentration - Constant' wasn't particularly difficult for him.
Riezel's learning ability and growth speed were beyond human, to say the least. Moreover, as an adventurer, he had experience venturing into the dungeon which was useful for learning 'Total Concentration - Constant'.
When embarking on dungeon expeditions, it often took a dozen days or more. During this period, adventurers spent their time resting and sleeping in the dungeon.
Any adventurer knew that sleeping in the dungeon filled with monsters was in itself quite dangerous. It required not only calming one's mind and maintaining a strong mentality to sleep soundly but also staying vigilant at all times, as even amid slumber, one couldn't afford to let their guard down completely.
Even with party members keeping watch, adventurers needed to maintain a minimal level of vigilance toward the surroundings to avoid dangers catching them off guard.
Hence, all high-class adventurers had a mandatory skill—quickly falling asleep while remaining alert to the surroundings even in their dreams.
Finn had guided Riezel in this aspect during his first dungeon expedition.
For this reason, Riezel had to stay fully focused at all times, maintaining concentration even in his dreams, uninterrupted for twenty-four hours.
For him, mastering 'Total Concentration - Constant' was just a matter of adapting for a while before succeeding. Eventually, he would grasp it, always in a state of heightened breathing and constantly honing his body even in his sleep.
In this way, Riezel's initial idea of continuous growth over twenty-four hours, capable of tapping into his potential and enhancing his status even without practice, would come to fruition.
"Oh, by the way, Young Tsugikuni..." Kyoujurou suddenly remembered something. "The thing you asked me to handle before, I've already taken care of it."
Upon hearing that, Riezel was first puzzled but then quickly realized.
"Are you talking about the Scarlet Crimson Iron Sand and Scarlet Ore?"
His question was met with an affirmative response.
"Umu!" Kyoujurou grinned. "Do you want to go take a look?"
"Sure." Riezel nodded without hesitation.
"In that case, I'll head back first." Shinobu smiled as she turned away. "I'll report to Master about Tsugikuni completing his practice. He should be pleased."
"Gotcha!" Kyoujurou nodded. "I'll leave it to you, Kochou!"
With that, Shinobu left with a smile.
"Let's go, Young Tsugikuni."
Kyoujurou turned and quickly led Riezel into his residence's warehouse.
As for the courtyard, someone quickly came to put out the fire and clean up the mess.
===
"How? Is this enough?"
In a warehouse belonging to the Rengoku family, Kyoujurou confirmed with Riezel at his side.
Riezel didn't answer immediately, instead gazing at the scene before him, pondering.
Inside the sizable warehouse, piles of ore, shining like jade and steel, formed small mountains.
"Are these Scarlet Crimson Iron Sands and Scarlet Ores?" Riezel muttered to himself.
"Umu!" Kyoujurou exclaimed. "These are ores that can only be collected from the highest mountains closest to the sun. Bathed in sunlight year-round, they absorb ample sunlight energy, making them the primary materials for forging Nichirin Swords!"
Even without his explanation, Riezel could vaguely sense that these ores indeed emitted a gentle energy akin to sunlight. Although this energy didn't enhance the quality or structure of the ores, it had a special effect only on demons that were weak to sunlight.
When used in swordsmithing, swords produced from these ores were not fundamentally different from ordinary swords, except for their added effectiveness against demons.
Outside of the world of Demon Slayer or away from demons as enemies, swords forged from these ores would be entirely ordinary swords. Not only they couldn't slice through iron like butter, but even cutting through a rock would likely require the skill of a highly proficient swordsman.
However, with so many ores in his possession, they would still serve a somewhat great purpose.
"In that case, I'll take them. Thanks, Rengoku." Riezel thanked once more.
Shortly afterward, Kyoujurou departed, leaving Riezel alone to survey the warehouse filled with Scarlet Crimson Iron Sands and Scarlet Ores.
"Alright then, let's get started."
A surge of magical energy quickly rippled through his body.