In the central hall of the Infinity Castle, black mist churned like a living thing.
Kibutsuji Muzan floated in midair, his crimson eyes filled with suppressed rage.
Below him, a trembling lesser demon knelt on the floor, its voice shaking with fear.
"Ma… master, I've confirmed it. The red-haired boy wearing the sun earrings is still alive. He seems to be repairing his Nichirin Sword right now."
"Still alive?"
Muzan's voice was cold as ice. His ink-black hair drifted without wind.
He had thought Mushiki's Moon Breathing would have crushed Kamado Tanjiro completely, yet the boy not only survived but dared to repair his weapon and prepare to fight again.
The black mist gathered in Muzan's palm, forming a sharp blade of air that instantly pierced the lesser demon's skull.
The body turned to ash and scattered, leaving only Muzan's burning fury within the vast hall.
He closed his eyes, letting his awareness spread outward, and soon located the aura of Upper Moon Five, Gyokko.
"Gyokko, come to the central hall immediately."
Moments later, Gyokko slithered in, dragging along his water-filled pot, his tone dripping with flattery.
"Master Muzan, what do you command of your humble servant?"
"Go and find someone for me."
Muzan opened his eyes, the red gleam locking onto Gyokko.
"Find a red-haired boy named Kamado Tanjiro. He wears sun earrings and can use Sun Breathing, which suppresses a demon's regeneration. Bring me his head—and while you're at it, destroy every Nichirin Sword and every swordsmith in the Swordsmith Village."
Excitement flickered in Gyokko's eyes as he bowed quickly.
"At once, my lord! I shall not fail you!"
He loved torturing humans, especially those with special powers like Tanjiro. Their blood made perfect nourishment for his pots.
"Don't disappoint me," Muzan said with a wave of his hand. "Go quickly and return quickly."
"Understood!"
Gyokko bowed deeply, then turned into a surge of water and vanished into the air.
Muzan watched the direction of his departure, his gaze still cold.
He had a feeling that Tanjiro's existence was a threat—a shadow of Tsugikuni Yoriichi lingered within that boy.
He had to be eliminated as soon as possible, or he would one day become an obstacle to Muzan's eternal path.
Meanwhile, morning had already begun in the Swordsmith Village.
Kamado Tanjiro, Agatsuma Zenitsu, and Hashibira Inosuke sat together at the stone table outside a wooden house. In front of them were big bowls of rice, golden grilled fish, tender stewed potatoes, and fresh green vegetables—the aroma was irresistible.
"Whoa! This fish is amazing!" Zenitsu exclaimed between bites.
"It smells even better than the food at Butterfly Mansion!"
Inosuke was too busy to talk. He held two rice balls in hand, his mouth stuffed with rice and fish, cheeks bulging like a hungry squirrel. He finished his own portion quickly, then reached for Zenitsu's bowl.
"Oi, blondie! You're too slow! Give me one of your potatoes!"
"Hey! That's mine!" Zenitsu yelled, clutching his bowl tightly as they started tugging at each other.
Tanjiro laughed softly and sped up his own eating. The Swordsmith Village's meals were truly delicious and hearty—every bite full of warmth and flavor, reminding him of his mother's cooking when he was a child.
"Take your time eating—there's plenty more," Kotetsu said, carrying over a bamboo steamer filled with buns and setting it on the table.
"In our village, everyone forges iron every day. Without enough strength, we can't work. So our food's always made to be filling and nutritious. My dad always says: 'Good blades need strength to forge, and strong bodies need food to grow.'"
"Well said!"
Inosuke dropped the stolen potato, grabbed a bun, and stuffed it into his mouth.
"Then I'll live here from now on! Eating this good every day sounds awesome!"
Kotetsu laughed. "When you become the strongest swordsman, you're welcome anytime! But for now, after breakfast, you're coming with me for morning training. My dad said it'll take half a month to repair the Nichirin Sword. You can spend that half-month however you want."
The three exchanged looks and nodded together.
After eating, Kotetsu led them to an open space behind the village. On the ground lay several iron blocks of various sizes, each weighing over ten jin.
"These are our usual training weights. Try them and see what fits."
Tanjiro pointed at the iron blocks.
"Forging swords needs endurance, and so does fighting demons. Running with weight strengthens our legs and stamina."
Inosuke immediately dashed forward, hefted two large iron blocks onto his shoulders, and sprinted toward the mountain path.
"This weight's nothing! I can run ten laps!"
Zenitsu groaned, lifting two as well and trudging after him.
"If I'd known morning training would be this hard, I'd have eaten another bowl of rice…"
Tanjiro laughed, picked up his own iron blocks, and followed behind.
Running with weights was exhausting. After just one lap, sweat streamed down his forehead, and his legs began to ache—but he didn't slow down.
He knew that only through constant training could he one day have enough strength to protect himself and his comrades when facing Mushiki or any other Upper Moon again.
By the time Kamado Tanjiro reached his fifth lap, he saw two people arguing in front of Haganezuka's master's hut.
As he got closer, he realized it was Hotaru Haganezuka himself—still wearing his fierce-looking mask, holding an iron hammer, and shouting loudly at Tecchin Tecchikawahara.
"Master! Tanjiro's Nichirin Sword must be repaired by me! I was the one who forged it for him, and now that it's broken, it's my duty to fix it!"
Tecchin frowned, his tone strict. "Haganezuka, I know that you're responsible for his sword. But Tanjiro's strength has already reached the level of a Hashira, though he hasn't been officially appointed yet. He needs the best Nichirin Sword—one made with the finest Scarlet Ore and Tamahagane. Your skill is good, but it hasn't yet reached the level needed to forge a top-grade Nichirin Sword. Let me handle it."
"I can do it!"
Haganezuka swung his hammer passionately. "I've been practicing high-grade forging this whole time! I can definitely forge the best sword for Tanjiro! Besides, I promised him that if his sword ever broke, I'd repair it myself!"
Tanjiro watched them argue, a warm feeling rising in his chest. He remembered the first time he received his Nichirin Sword—Haganezuka had looked terrifying behind that mask but carefully explained every detail of how to use it. Later, when Tanjiro accidentally damaged the blade, Haganezuka had threatened to kill him in anger—yet still worked overnight to forge him a new one.
That man, who always seemed so fierce, actually cared more deeply than anyone about his blades and the ones who wielded them.
"Master, Haganezuka-san," Tanjiro said with a smile as he stepped forward. "I think it's best if Haganezuka-san repairs my sword. I trust his skill—and besides, this sword has been through so many battles with me. I want it to stay by my side."
Tecchin paused, glancing at Tanjiro. "Are you sure? My sword would be tougher and sharper than his."
"I'm sure," Tanjiro said firmly, turning to Haganezuka. "Please, Haganezuka-san. I'm counting on you."
Haganezuka froze, clearly not expecting Tanjiro to speak up for him. Then, with a quick motion, he removed his mask, revealing a flushed face. His voice was hoarse but steady. "You can count on me! I'll repair your sword and make it even sharper than before!"
Tecchin sighed helplessly, shaking his head. "Fine, since you've both decided, I'll let you handle it. But Haganezuka—if you slack off even a bit, I won't forgive you!"
"I won't slack off!" Haganezuka shouted, grabbing Tanjiro's sword and rushing straight into the forge. "I'll start right now!"
Watching his hurried figure disappear, Tanjiro couldn't help but laugh. Haganezuka-san really hasn't changed at all—still fierce on the outside but completely dependable inside.
…
The next two days passed in constant training for Tanjiro, Agatsuma Zenitsu, and Hashibira Inosuke.
Every morning, they ran five laps with weighted iron blocks, then practiced breathing techniques and swordsmanship on the training grounds.
Zenitsu's Thunder Breathing grew faster and sharper—his "Thunderclap and Flash" was nearly twice as quick as before. Inosuke improved his Beast Breathing as well, and his self-made technique, "Explosive Throwing Strike," hit targets with much greater accuracy. Tanjiro focused on the Hinokami Kagura, trying to channel more power into his strikes to increase both heat and force.
On the afternoon of the third day, after finishing their sword drills, the three sat resting on the open field.
Kotetsu ran over holding a strange-looking mechanical part, his face full of excitement. "Tanjiro-nii, Zenitsu-nii, Inosuke-nii! I've got something amazing that can help you improve your sword skills!"
"Something amazing?" Inosuke sat up at once, eyes wide. "What is it? Stronger than the master's hammer?"
"Not a hammer—a robot!" Kotetsu said proudly. "My dad worked with the Kakushi to build a training robot called 'Yoriichi Type Zero.' It can imitate all kinds of demon movements and attacks, and it adjusts difficulty based on the user's skill. Lots of Hashira have trained with it here and improved their swordsmanship fast!"
Zenitsu's eyes sparkled. "A robot? It can really copy a demon's attacks? That's perfect! I can finally practice dodging for real!"
Tanjiro looked intrigued. "Yoriichi Type Zero? Sounds incredible."
"Of course it's incredible!" Kotetsu dusted off his hands and grinned. "Come on, I'll show you! It's in my house's basement. Usually it's locked, and my dad only lets others use it with permission—but he went into the mountains today to find ore, so I can sneak you in for a look!"
Inosuke jumped to his feet instantly, grabbing Kotetsu's hand. "Let's go! I wanna see how strong this thing really is!"
Zenitsu scrambled up to follow, while Tanjiro laughed softly and joined them. A machine that can help Hashira-level swordsmen train… maybe this could even help me in my next fight with Giyu-san.
The four of them headed toward Kotetsu's wooden house, sunlight spilling over their shoulders in warm streaks. Tanjiro looked at his excited companions beside him, his heart full of anticipation.
None of them noticed that outside the Swordsmith Village, a surge of water was quietly approaching—carrying a deadly killing intent straight toward their direction.