Autumn in the Taisho era already carried a sharp chill in the wind.
More than a month had passed since that quiet, warm outing in town.
During this time, the Demon Slayer Corps experienced a level of peace unseen in a century.
There were no reports of Upper Rank demons appearing. Even lower-level demons had become far less active, as if they had all retreated into hiding overnight.
Rin understood the reason clearly.
Muzan, that old scheming bastard, was most likely holed up in the Infinity Castle, either frantically strengthening himself or preparing something even more sinister.
But while the outside world remained quiet, the backyard of the Butterfly Mansion had grown livelier by the day.
Bang!
With a heavy thud, Zenitsu Agatsuma was sent flying in a smooth arc before splashing straight into the koi pond, water spraying everywhere.
"Forty times! Still too slow! Thunder Breathing focuses on a single decisive strike, but your footwork is unstable! If you encounter a Blood Demon Art with wide-area defense and fail to finish the fight in one blow, you'll become a sitting target!"
A powerful voice rang out across the training ground.
Standing at the center was the Flame Hashira, Kyojuro Rengoku, his long hair blazing in shades of yellow and red.
"Next! Tanjiro-kun! Show me how much you've improved this past month!"
Rengoku gripped his wooden sword with both hands, his eyes burning with enthusiasm.
"Yes! Rengoku-san, please guide me!"
Tanjiro took a deep breath. Heat shimmered faintly around him as he pushed Hinokami Kagura to its limit and charged forward, wooden sword in hand.
On the nearby corridor, Rin sat cross-legged on a cushion, holding a cup of steaming roasted tea.
Watching the intense training in the courtyard, he let out a satisfied yawn.
Since the battle at the Swordsmith Village, Rin had come to a clear realization..
Trying to track down Muzan across all of Japan on his own was no different from searching for a needle in a haystack.
Since Muzan had chosen to hide, the overall strength of the Demon Slayer Corps needed to be raised to another level. Otherwise, in the inevitable final battle, even the Hashira might be cut down the moment they faced him.
Because of this, Rin had taken the initiative to propose a "Hashira Special Training Program" to Ubuyashiki Kagaya.
The idea was simple.
All available Hashira would rotate through the Butterfly Mansion as sparring instructors. On one hand, they would train promising newcomers like Tanjiro. On the other, through constant sparring—and even taking a few beatings from Rin—they would attempt to reach the realm known as the Demon Slayer Mark.
Kyojuro Rengoku had been the first to volunteer.
Clang!
The sharp sound of wooden swords colliding snapped Rin out of his thoughts.
In the training yard, Tanjiro's wooden sword was deflected at a precise angle by Rengoku. In the next instant, a clean front kick sent Tanjiro stumbling back several steps before he fell onto the ground, breathing heavily.
"Excellent! Tanjiro, your transitions between Breathing Styles are much smoother than before! However, there is still a slight hesitation in your movements! You must let the power of the flames flow through every heartbeat!"
Rengoku's voice rang out with unwavering energy.
"Thank you for your guidance…" Tanjiro wiped the sweat from his face. Though exhausted, his eyes shone with determination.
"Next! Bore-boy, your turn!"
"I'm not some bore boy! Remember my name—Inosuke Hashibira!"
Inosuke roared as he charged forward, swinging his twin wooden swords wildly.
Watching the lively scene, Rin let out a quiet chuckle.
At that moment, the sliding door nearby opened.
"Rin-san, a letter from Master Ubuyashiki."
Kanao Tsuyuri stepped out, her hair tied in a side ponytail. She no longer needed to rely on flipping a coin to make decisions. Though her gaze remained calm, there was now a hint of liveliness within it.
She handed him a sealed bamboo tube, then quietly knelt beside him and poured more hot tea into his cup.
"Thanks."
Rin took the tube, removed the stopper, and slid out a rolled parchment.
After giving it a quick read, his brow lifted slightly.
The letter had been written personally by Ubuyashiki Kagaya. It was brief, yet packed with information.
The message was simple:
The Kakushi had discovered several abandoned villages deep within remote mountain regions. Every resident had vanished without a trace—no blood, no signs of struggle.
Judging from the faint demonic aura left behind, the method was precise and efficient. It didn't resemble the mindless slaughter of ordinary demons feeding on humans, but rather a deliberate, organized "harvest."
"Disappearances without bloodshed…"
Rin tapped his fingers lightly against the table, a thoughtful glint passing through his eyes.
Was Muzan stockpiling food, or using living humans for some kind of large-scale experiment?
Combined with the fact that the Infinity Castle had been sealed off, Rin leaned toward the latter.
Muzan was most likely researching a way to overcome sunlight—or creating expendable forces to buy himself time.
"So, he doesn't plan on showing himself anytime soon."
Rin crumpled the parchment and let a faint black flame flicker in his palm, reducing it to ash in an instant.
"What's wrong? A troublesome mission?"
Kochou Shinobu walked over from the corridor, having just finished preparing medicine. She caught sight of the burning letter.
Dressed in her light purple casual attire, she sat down beside him.
"Nothing serious. Muzan's just started playing hide-and-seek."
Rin took a sip of tea.
"The Kakushi found a few empty villages in the mountains. He's likely using living people for experiments, preparing something big."
At this, Shinobu's gaze turned cold.
"Another one of his disgusting methods. Does Master Ubuyashiki want us to investigate?"
"No need. Searching for something deliberately hiding in the mountains would be a waste of time."
Rin turned to look at her.
"If he wants to prepare something, let him. Once he thinks his shell is strong enough, he'll come out on his own."
"What we need to do now…"
He pointed toward Inosuke, who was locked in a fierce exchange with Rengoku Kyojuro in the courtyard. "…is sharpen our blades."
"Let's go stretch a bit."
Rin stood up, then leapt down from the corridor, landing lightly in the center of the training ground.
"B-Boss is stepping in himself?!"
Zenitsu, who had just crawled out of the pond, took one look and immediately scrambled behind a tree in terror.
Rengoku halted his movements, his eyes lighting up as he looked at Rin.
"Excellent! Rin-dono! You're finally willing to guide us!"
He tightened his grip on the wooden sword, his fighting spirit surging to its peak.
Having once been defeated by a single strike of lightning in the Ubuyashiki estate, he had accepted it wholeheartedly.
But over the past month, he had trained relentlessly. His strength had improved, and he had long wanted to see just how wide the gap between himself and this towering figure truly was.
"Guide is too generous a word."
Rin walked to the weapon rack and casually picked up an ordinary wooden sword.
"Don't you all want to know how to awaken the Demon Slayer Mark?"
He held the wooden sword loosely in one hand, its tip angled toward the ground. His posture was relaxed, yet it carried an overwhelming pressure.
"A heart rate over two hundred. A body temperature above thirty-nine degrees. That kind of near-death state can't be reached through simple training."
"It requires external pressure."
Rin raised his head, a hint of madness flickering in his dark eyes.
"Rengoku-san. And you three brats."
"Come at me together."
"As long as you can make me take even half a step back—or touch the edge of my clothes—you win."
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