Chapter 9: Infinite Castle. Kyōjurō Rengoku

After that single note of the qin sounded, Doumo fell into a strange space.

The houses within the space were tangled and interwoven, upside-down and disoriented; the atmosphere around was utterly eerie—this was the Infinite Castle.

The wounds carved into Doumo by Zero were slowly healing, but this did not stop him from maintaining his solemn, mocking grin.

He gazed at a woman kneeling on a high platform across from him, cradling a shamisen, her long straight bangs obscuring her entire forehead and eyes—she, too, was a demon.

“Oh my~ Sister Mei Nü, thank you for saving me! Otherwise, I’d have been killed by that strange white-haired boy—how sad everyone would be if they couldn’t see me again! Don’t you think so, Sister Mei Nü…?”

Doumo chattered on to himself, utterly unconcerned whether anyone was listening.

Qing!

Mei Nü plucked a string on her shamisen, and a wooden door materialized beside her; it opened, and a man in a suit stepped out.

His skin was deathly pale, his frame gaunt, his pupils blood-red and slit like a cat’s, his long black hair flowing—he was the progenitor of demons, Kyōjurō Rengoku! A demon who had lived for a thousand years.

Rengoku stood beside Mei Nü, towering above, his gaze descending upon Doumo from the platform, revealing no emotion whatsoever.

“Doumo, you really are a mess.” Rengoku’s voice carried no feeling, as if Doumo’s life or death meant nothing to him.

“Oh, my lord~ How can you say that~ I merely…”

Before Doumo could finish, Rengoku cut him off.

“Shut up, you useless waste!” Rengoku roared. “You could have killed the Demon Slayer Corps’ female Pillar outright, yet you failed to kill her—and then you were reduced to this pathetic state by a human who isn’t even a Demon Slayer! What use are you?!”

Calming his inner fury, he spoke again: “It seems even Upper Rank Two has grown weak! Not only do you have no leads on the blue Aneman flower, but even a human who isn’t a Demon Slayer has humiliated you!”

“My lord, my followers of the Wan Shi Ji Le Sect are diligently searching for the blue Aneman flower~ I’m certain we’ll find clues soon…”

“Did I give you permission to speak?”

At once, Doumo fell silent, yet his face retained its cynical, grinning, empty smile.

“Remember—I want results, not clues!”

“Mei Nü, send me back!”

“Yes, my lord!”

Qing!

After the qin note faded, a wooden door opened behind Rengoku; he stepped through and vanished from the Infinite Castle.

Seeing Rengoku leave the Infinite Castle and his wounds from Zero fully regenerated, Doumo said: “Sister Mei Nü, send me back too…”

Qing!

Before he could finish, Mei Nü sent him straight back to the Wan Shi Ji Le Sect—as if she had merely brushed off a speck of dust.

After leaving the Infinite Castle, Rengoku appeared in his study. Through Doumo’s memories, he had witnessed Zero’s battle—especially the red blade in Zero’s hand—and his body trembled involuntarily.

A figure surfaced in his mind—holding a blade that burned like fire, filling him with terror.

“That man… that man…!! Damn it! You damned bastard, even dead, you won’t let me rest! And that white-haired human—he’s not only powerful, but wields weapons I’ve never seen before…”

“No! I will not allow anyone comparable to Kyojuro Rengoku to stand before me and obstruct my plans! I will not!”

Forcing himself to calm, he transformed into a woman in a kimono, stepped out of the study, and wandered the night streets…

Time returned to several days later.

Zero awoke at dawn and left Butterfly House to get some fresh air; after being tormented all night by the Butterfly sisters and Mitsuri Kanroji, he had finally experienced what it meant to feel an hour stretch like a century.

Though less than an hour had passed, it had felt like a full century—agonizingly slow.

Throughout, the three women had taken turns dressing him up; the mental torment, combined with the physical “torture,” left him exhausted enough to fall asleep, while Xiao Yue had simply sprawled on the bed watching the spectacle.

Zero now understood what true humiliation meant—a being thousands of years old, humiliated by a few little girls, his entire millennium of dignity discarded in this otherworld.

“Sigh…”

Zero sighed softly and left Butterfly House, heading toward the Ubuyashiki estate. He needed to gather more information about this otherworld—the Demon Slayer Corps had endured so long, surely they kept records. He hoped Ubuyashiki Kagaya would agree.

Even if he refused, it didn’t matter—he could seek information elsewhere. After all, such matters were like secrets, meant to be protected.

Arriving at the Ubuyashiki estate, Zero explained his purpose. Ubuyashiki Kagaya did not hesitate—he immediately agreed and led Zero to his personal study.

Inside the study, shelves lined the walls, packed with books. Ubuyashiki Kagaya informed him that each volume contained the personal journals of every past head of the Demon Slayer Corps—he could read them freely.

Zero thanked Ubuyashiki Kagaya and began selecting a book to examine. As he left, Ubuyashiki Kagaya instructed Ubuyashiki Tsumugi to remain and assist Zero in any way he needed.

Thus, Zero read through the journals of each Demon Slayer Corps head—and learned some information about this world of Demon Slayers.

According to the journals, during Japan’s Heian period, a demon had appeared—the progenitor of demons, Kyōjurō Rengoku.

The journals recorded: before his birth, Rengoku nearly died due to his frail body, even being born as a stillborn infant.

Yet Rengoku, driven by an unyielding will to live, clawed his way back from death’s door—but still, before turning twenty, he was bedridden from an incurable illness…

The journal entry here was incomplete; details on how Kyōjurō Rengoku became a demon were missing.

“Kyōjurō Rengoku… I remember Xiao Yue and I encountered this name when we first arrived in this world, but I didn’t fully understand the details—I only knew he was the progenitor of demons, a demon who had lived a thousand years… Maybe I should try.”

Zero input a command into his Analysis Glasses:

【Search: Information related to Kyōjurō Rengoku!】

【Searching… No relevant data obtained!】

“Hmph… As expected, still no luck? If I could see Kyōjurō Rengoku’s face, my Analysis Glasses could analyze the data… Oh well, when the mountain blocks the path, the road will appear—take it one step at a time.”

He returned the book to the shelf and picked up another.

Soon he found another journal entry, which read:

Four hundred years ago, during the Sengoku period, a swordsman appeared who drove Kyōjurō Rengoku to the brink—Kyojuro Rengoku.

Kyojuro Rengoku, second son of the Kyojuro family, had a twin brother named Kyojuro Iwa. He was the creator and sole user of the Original Breathing Style—Sun Breathing.

Born with the innate ability to use Breathing Techniques and “Transparent World,” his swordsmanship was extraordinarily gifted, so much so that his brother called him “favored by the gods.”

He was the first swordsman to awaken the Demon Slayer Mark, born with it already present, and caused those around him—including his brother Kyojuro Iwa—to awaken it as well.

Due to his unique physiology, he became the only swordsman in over four hundred years to survive past age twenty-five after awakening the Mark—living to over eighty.

Sun Breathing inflicted massive damage upon demons, making Kyojuro Rengoku the only swordsman ever to drive Kyōjurō Rengoku into a corner…

Reading this, Zero furrowed his brow. The journal mentioned “Breathing Techniques,” “Demon Slayer Mark,” and “Transparent World.” He already understood Breathing Techniques.

He had previously fought several Pillars; his Analysis Glasses had analyzed their Breathing Techniques—methods that enhanced lung and heart function, flooding the blood with oxygen in seconds, temporarily boosting human physical capabilities to match those of demons in combat…

But he did not yet understand what “Demon Slayer Mark” or “Transparent World” meant.

“Let me try. Maybe I can find out.”

He activated his Analysis Glasses’ holographic display mode. Instantly, a one-meter-long, half-meter-wide 3D holographic screen appeared before him, and he input a command.

【Search: Demon Slayer Mark.】

【Searching… Search successful!】

“So it works. Let me see what this so-called ‘Demon Slayer Mark’ really is.”

【Demon Slayer Mark: When activated, it greatly enhances physical attributes. Anyone who awakens the Mark will, like resonance, cause others nearby to awaken it as well.

To activate the Mark, body temperature must reach 39°C and heart rate must exceed 200 bpm—placing extreme strain on the body.

Note: Those born with the Mark differ greatly from those who awaken it later in life. All later awakeners die before age twenty-five!】

“So that’s how it works. The activation conditions are… oddly similar to ‘overload operation.’ Machines increase output under overload, generating high heat—but also shorten their lifespan… Wait, shorten lifespan?!”

Zero suddenly realized something. The journal recorded that Kyojuro Rengoku was the only swordsman to survive past twenty-five after awakening the Mark.

But the Analysis Glasses stated all who awakened the Mark died before twenty-five. If overload operation was the analogy, then awakening the Mark was effectively trading one’s future lifespan for combat power!

“The journal says Kyojuro Rengoku had a unique physiology. The Analysis Glasses distinguish between innate and acquired Mark. So Kyojuro Rengoku must have been an innate case—no side effects—while acquired Mark carries the fatal cost…”

Realizing this, Zero took a notebook and pen from his nanospace and recorded everything.

After finishing, he continued searching.

“The journal also mentioned Kyojuro Rengoku’s older brother, Kyojuro Iwa. His brother called Kyojuro Rengoku ‘favored by the gods’—but why is there no record of Kyojuro Iwa himself? Oh well, maybe more will appear later…”

“Wait… Kyojuro Rengoku? Kyojuro Iwa? Demon Slayer Mark?” Zero suddenly thought of something.

The journal stated Kyojuro Rengoku was the only swordsman to survive past twenty-five—but mentioned no second.

Zero drew a triangle on his notebook: top-left, “Kyojuro Rengoku”; top-right, “Kyojuro Iwa”; center, a circle with “Demon Slayer Mark”; but he paused at the bottom corner…

He stared at the diagram, deep in thought—was there a connection between the two? Or was there a third party involved?

If a third party existed—who could it be?

After much thought, a name surfaced in his mind: Kyōjurō Rengoku!

And according to gathered data, Kyōjurō Rengoku was a demon who had lived a thousand years. If the third party was him, it was not impossible.

For example, during the Sengoku period, Rengoku encountered Kyojuro Iwa, saw him suffering from the fatal consequence of awakening the Mark, and offered him a deal: eternal life, greater power…

The condition? Become a demon.

Though this idea seemed fantastical, nothing in this world was truly impossible.

So he wrote “Kyōjurō Rengoku” at the bottom corner.

Zero formed two bold hypotheses in his mind:

First: Kyojuro Iwa, doomed to die before twenty-five after awakening the Mark, encountered Kyōjurō Rengoku in battle and was killed.

Second: Kyojuro Iwa accepted Kyōjurō Rengoku’s offer and became a demon—an extremely powerful one.

Either possibility was plausible, but Zero leaned toward the second.

Why? He made a guess: the Kyojuro family was a powerful clan during the Sengoku period. Because Kyojuro Rengoku’s unparalleled talent drew the family’s favor, his older brother Kyojuro Iwa grew jealous.

Zero imagined himself as Kyojuro Rengoku: Kyojuro Rengoku was someone who deeply valued emotion and respected his brother, always sharing everything with him…

Then he imagined himself as Kyojuro Iwa: Kyojuro Iwa saw his brother’s unmatched talent, the family’s favoritism, and slowly his heart filled with envy. In his eyes, every act of kindness from Kyojuro Rengoku was a lie…