"What exactly is a Bright Red Nichirin Sword? What is the Mark? And what is this so-called Transparent World?"
The moment Shinobu Kocho stepped inside, she fired off her questions in rapid succession, unable to hold back her urgency. "How do you awaken a Bright Red Nichirin Sword? How do you manifest the Mark? And how does one reach the Transparent World?"
"Don't rush, don't rush."
Soma took a sip of sake, his gaze calm and gentle as it rested on the visibly impatient Shinobu.
Only then did she realize how agitated she had sounded.
…He's probably looking down on me again.
The thought made her feel a bit embarrassed. Though she had already made a fool of herself in front of him more than once, and the initial awkwardness had faded, a faint blush still crept onto her delicate face.
Once Shinobu finally quieted down, Soma turned his attention to Sakonji Urokodaki.
"Urokodaki-sensei should know a little about this, right?"
Behind his mask, Urokodaki shook his head.
"I am aware of the names: the Bright Red Nichirin Sword, the Marks, and the Transparent World. These were the heights reached by the practitioners of the First Breathing during the Sengoku era. In those days, the initial swordsmen could even stand against Muzan Kibutsuji himself because of them. But these secrets have long been lost to time; I know them only by name."
He paused before continuing.
"As for what a Bright Red Nichirin Sword truly is, what the Marks look like, or what it feels like to perceive the Transparent World... I truly do not understand them."
Among the current members of the Demon Slayer Corps, very few likely knew of these concepts anymore. Even many Hashira were unaware of them. If there was anyone who might have known more, it would have been the former Flame Hashira, Shinjuro Rengoku, who had once delved into such knowledge.
And perhaps… it was precisely because he had learned of these things that he had ultimately lost his will to fight.
Soma nodded, inviting them to take a sip of sake before finally speaking under their expectant gazes.
"The so-called Bright Red Nichirin Sword is achieved by raising the temperature of your sword. This can be done through immense grip strength or through intense clashes between weapons. Once the blade heats up and turns crimson, its lethality against demons increases significantly—and the regeneration of any demon struck by it is greatly slowed."
He spoke calmly, explaining the principle behind the Bright Red Nichirin Sword as he understood it.
Urokodaki listened carefully, then drew his Nichirin Sword and gripped it tightly, channeling his strength into the hilt.
"You need to transmit your strength into the blade itself, raising its temperature," Soma added from the side.
As a former Hashira, Urokodaki's strength was formidable—but even so, it was not quite enough. He could feel his grip tightening, his strength surging, yet the blade did not heat up as quickly as expected.
If this were in the middle of battle, achieving a Bright Red Nichirin Sword would be extremely difficult. Perhaps it could be triggered at the exact moment of striking a demon's neck—but maintaining it consistently in combat would be nearly impossible, and might even become a burden.
"That's why, before achieving the Bright Red Nichirin Sword, it's best to first awaken the Demon Slayer Mark," Soma continued.
"Once the Demon Slayer Mark is activated, the body releases an astonishing surge of power. At that point, forming a crimson blade becomes much easier."
"And how does one awaken the Demon Slayer Mark?" Shinobu asked immediately.
She had just tried to create a Bright Red Nichirin Sword herself, but relying solely on grip strength was clearly impossible. Perhaps, she thought, weapon collisions might yield better results later.
"To awaken the mark, your body temperature must exceed thirty-nine degrees, and your entire body must remain in a state of intense heat. At the same time, your heart rate must surpass two hundred beats per minute. With Breathing Styles supplying massive amounts of oxygen, your heart accelerates, fueling an explosive increase in physical strength—"
Before Soma could finish, Shinobu interrupted him.
"But under those conditions, the human body can barely endure it. Especially at such high temperatures—it could be life-threatening."
As a physician, she understood all too well how fragile the human body truly was. Sustaining such extreme heat could easily push someone to the brink of death.
"Exactly," Soma replied calmly. "Awakening the Mark places you in an extremely dangerous state. Only by surviving that intense heat will the Mark manifest on your body. And once it appears, it acts almost like a resonance—those nearby may also begin to awaken their own Marks more easily."
He paused slightly before adding in a quieter tone:
"So, awakening the Mark is essentially the same as consuming your own lifespan. Those who manifest it… rarely live past the age of 25."
Hearing this, Sakonji Urokodaki seemed to finally understand why the knowledge of the Demon Slayer Mark and the Bright Red Nichirin Sword had been lost over time.
At that moment, Tanjiro Kamado suddenly thought of his father.
"So… the markings that suddenly appeared on my father's forehead… were because—"
"Yes," Soma replied calmly. "Tanjuro Kamado awakened the Demon Slayer Mark. That's why his life force faded so early."
Hearing this, Tanjiro finally understood.
His father had once been strong and healthy—yet he had suddenly fallen ill, growing weaker and weaker without explanation. Even after seeing countless physicians, no one had been able to determine the cause.
Now, the answer was clear.
Soma did not linger on Tanjiro's quiet grief and continued speaking.
"As for the Transparent World, its key lies in your breathing, the precision of your movements, and your ability to achieve maximum effect with minimal motion. When you reach that point, your mind gradually becomes 'transparent.'"
He spoke slowly, as if guiding them step by step through an unseen path.
"In the beginning, you must gather your movements and sensations together. As your five senses open further, you'll come to recognize every blood vessel within your body. This stage is extremely painful for most people."
"Once you've absorbed enough, you begin eliminating unnecessary movements, leaving behind only what is essential. Over time, the contraction and expansion of your muscles and blood vessels will become as natural and effortless as blinking."
"And when that moment comes—clarity will emerge. The path will open."
"When your mind becomes truly transparent, you will begin to perceive what is known as the 'Transparent World.'"
"…So that's how one reaches it?" Urokodaki murmured, almost to himself.
It felt as though a window had suddenly opened along a path that had long been shrouded in fog.
"Once you enter the Transparent World," Soma continued, "everything around you will appear almost transparent. Your ability to predict movements and evade attacks will improve drastically. You'll gain a much clearer awareness of your own muscle contractions, and you'll be able to see your opponent's breathing, the flow of their blood, the expansion and contraction of their vessels."
"With that, you can discern their movements—and their weaknesses—with absolute clarity."
"And beyond that, once the Transparent World is activated, your 'fighting spirit' will be concealed. Others will no longer be able to perceive your intent."
Sitting nearby, Tanjiro suddenly recalled something.
That night, when his father had placed a hand on his shoulder and guided him—he had briefly felt as though he had entered another realm entirely.
So… that had been the Transparent World.
…
Soma laid out everything he knew about the Bright Red Nichirin Sword, the Demon Slayer Mark, and the Transparent World.
On the surface, it all sounded simple.
But in reality, achieving any of it was anything but easy. For many, it would remain forever out of reach.
Yet that only applied to some.
For those with exceptional talent—those who had found the right direction—these things were not impossible.
At the very least, for the Hashira, aside from the Transparent World, the other two, though difficult, were still within reach.
The only real concern was the cost.
Awakening the mark placed one in a near-death state—and could significantly shorten one's lifespan.
…
The meal lasted a long time.
Even the sake ran out several times, forcing Tanjiro's mother to come in and refill it again and again.
After it was over, Urokodaki returned to his quarters and wrote a letter. As his Kasugai crow took flight into the night sky, the knowledge of the Bright Red Nichirin Sword, the Mark, and the Transparent World was carried to the headquarters of the Demon Slayer Corps.
…
When Kagaya Ubuyashiki received the news, he clenched his fists tightly, his usually calm expression finally breaking with visible excitement.
He had once been weighed down by despair after the death of Shinjuro Rengoku.
But now… it was as if he could finally see a path—a possibility—to defeat Muzan Kibutsuji.
Not long after, more Kasugai crows took flight, spreading the information across the land.
…
The Stone Hashira, Gyomei Himejima, lowered his head as he read the report.
The moment his eyes fell upon the section about the Bright Red Nichirin Sword, he immediately began to test it.
Gripping his weapon tightly, his immense strength surged through it. Though he could not see the blade change color, he could clearly feel its temperature rising rapidly.
A low growl echoed—
A demon lunged toward him.
Gyomei raised his weapon and struck.
The heated blade cut through the demon's arm faster and more smoothly than ever before. And more than that—the intense heat continued to spread from the wound, burning deeper into the demon's flesh.
"…So the Bright Red Nichirin Sword is already this powerful?"
Gyomei Himejima murmured under his breath. After crushing the demon's head with a single punch, he fell into deep thought, already considering how to awaken the Demon Slayer Mark and reach the Transparent World.
…
In Tokyo, the Flame Hashira, Kyojuro Rengoku, set down the letter in his hand. Without hesitation, he began to experiment with the Bright Red Nichirin Sword, carefully feeling the gradual rise in temperature along his Nichirin Sword.
…
The Wind Hashira, Sanemi Shinazugawa, also received the message soon after and immediately began his own attempts.
…
At the Butterfly Mansion—
Kanae Kocho quickly received messages delivered by Kasugai crows—one from her younger sister and another from the Master of the Demon Slayer Corps.
Both spoke of the Bright Red Nichirin Sword, the Mark, and the Transparent World.
However, compared to the Master's letter, which focused more on the knowledge and details of these concepts, her sister's letter repeatedly mentioned a particular name—
Soma.
As an older sister, Kanae understood Shinobu Kocho almost to her very core.
Even through something as simple as a letter, she could sense it—
Something about her sister had changed.
And she knew her sister far too well not to notice.
...
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