The night sky welcomed them back as Tanjiro and Giyu climbed out of the broken maintenance tunnel. Cold air brushed against their faces. The city looked peaceful from afar, yet Tanjiro could sense the quiet suffering hiding behind every window.
People lived in fear. Humans never understood why the darkness swallowed some streets whole, why certain neighborhoods were abandoned, or why strange disappearances happened every night.
They never realized that the demons had returned.
And this time, Muzan ruled over the world.
Giyu wiped water from his hair and took one steady breath. "The next demon will be much stronger. You must prepare your heart, Tanjiro."
Tanjiro nodded and looked toward the distant horizon. "Who is next?"
Giyu tightened the strap of his sword.
"Upper Moon Six."
The air around them seemed to still.
Tanjiro's eyes sharpened. "So soon?"
"Yes. And this one is cruel. His name is Kazaen. He was created from a warrior who fed on hundreds of humans. His abilities revolve around vibration. Loud or soft, any movement in the air becomes a weapon under his control."
Tanjiro listened carefully, memorizing every detail.
"Vibration. So even breathing can be sensed."
Giyu nodded. "You must be careful. Even raising your sword can trigger his attacks."
Tanjiro smiled. "Good. Then it means I get to test my soul in a real fight."
Giyu stared at him for a moment. "You have changed."
Tanjiro blinked. "I have?"
"You are no longer the boy who only cried for others. There is strength in your eyes now. Resolve too."
Tanjiro lowered his gaze for a moment.
"It is because this world has no one left to protect it. I already lost everything once. I refuse to lose again."
Giyu said nothing. He simply placed his hand on Tanjiro's shoulder, a rare gesture of warmth from the Water Hashira.
"Then let us go. Kazaen hides in the old industrial zone. There are abandoned factories and power plants there. The perfect hunting ground."
Tanjiro stepped forward. The path ahead was bleak and broken, yet his heart felt somehow bright.
Sakura's voice echoed in his mind.
Come back safely.
He promised her that he would return.
He promised everyone.
---
The Industrial Deadlands
Hours later, the pair entered the abandoned district. The streets were empty. Rusted cranes towered like skeletons. Broken windows glinted under pale moonlight.
Not a single living soul was in sight.
But Tanjiro could smell something.
A sickening sweetness mixed with iron.
The scent of blood.
Tanjiro's eyes narrowed. "It is close."
They moved cautiously between ruined buildings. Every step stirred echoes that bounced strangely around them, as if the air carried sound differently here.
Without warning, Giyu held up his hand.
"Stop."
Tanjiro froze.
A faint sound drifted from one of the factories.
A slow tapping.
Then a soft humming melody.
Tanjiro listened carefully. The melody felt wrong. Too calm. Too slow. Too empty.
Then he smelled something worse.
"This is human blood. A lot of it."
"Correct."
A voice answered from within the shadows.
A soft vibration rippled through the air, shaking the broken beams above them. Metal groaned. Dust fell like dead snow.
From the darkness, a figure stepped out.
He wore a sleeveless robe torn at the sides, exposing skin covered in faint markings that pulsed rhythmically. His eyes glowed with a deep crimson color that seemed to beat like a slow drum.
A kanji symbol floated in his eyes.
Six.
Kazaen smiled gently. "What a lovely pair of guests. You smell nostalgic. One smells like Muzan. And the other smells like reincarnation."
Tanjiro clenched his fists. "You have killed so many. Why?"
Kazaen shrugged. "Because life tastes interesting when it begs for mercy. Humans shake, tremble, and vibrate in fear. The sound is beautiful. The taste is even better."
Tanjiro felt his breath grow heavy. "You make me sick."
Kazaen let out a soft chuckle. "Hatred is also a vibration. I can feel it in the air. Delicious."
Giyu stepped forward, sword ready.
"Kazaen. We are ending your existence tonight."
The Upper Moon tilted his head. "Are you now? Then come. Strike me. But be warned. Each footstep you take will echo. Every echo will move the air. And I can use that air."
Tanjiro exhaled slowly.
A trickster. Like a musician who used sound as a blade.
He took one step.
The air shivered.
Kazaen flicked his fingers.
A shockwave ruptured the ground. Concrete shattered like glass.
Tanjiro dodged, but the vibration sliced a shallow cut across his shoulder.
Giyu moved instantly.
"Water Breathing. Third Form. Flowing Dance."
Giyu rushed forward, moving like water sliding across stone. But every time his feet touched the ground, the Upper Moon flicked his fingers and triggered vibrations that disrupted Giyu's rhythm.
"Your breathing forms are beautiful," Kazaen said calmly. "But the sound is too predictable."
Giyu gritted his teeth.
Tanjiro jumped high above them, swinging his blade.
"Sun Breathing. First Form. Clear Blue Sky."
Flames whirled around him in a circular arc.
Kazaen lifted his hand and clapped.
A single clap.
The shockwave shattered the air itself.
Tanjiro was thrown back, crashing into a steel beam. The metal bent from the impact. He gasped but pushed himself upright.
Kazaen walked closer.
"Your cut was amazing, Tanjiro Kamado. Truly. A blooming sun. But it lacks foundation. Your heart is still trembling."
Tanjiro looked up, eyes burning.
"You really think so?"
Kazaen nodded. "Yes. You are strong, but your spirit is still young."
Tanjiro lowered his stance.
Then he smiled faintly.
"That is why you will lose."
Kazaen blinked.
Giyu looked over, surprised by Tanjiro's sudden calmness.
Tanjiro inhaled softly. His breathing steadied. His heartbeat slowed. His body trembled less.
He closed his eyes.
And then he remembered Shunsui. The long centuries of training. The strange lessons. The lazy lectures that hid wisdom.
Shunsui's voice floated in his mind.
Listen to the air. Feel the truth beneath noise. Do not fight vibration. Become part of it.
Tanjiro opened his eyes again.
They were calm like a still lake.
Quiet like an old mountain.
"You use the world's sounds to attack," Tanjiro said softly. "Then I will hear the sound of your fear."
Kazaen's smile faded for the first time.
Giyu whispered to himself. "His presence just changed."
Tanjiro stepped forward lightly.
The air did not tremble.
Kazaen widened his eyes. "What…?"
Tanjiro stepped again.
Still no vibration.
Tanjiro smiled gently. "I trained for two thousand years. Do you think I never learned how to hide my sound?"
Kazaen's eyes widened in realization.
Tanjiro charged.
"Sun Breathing. Eighth Form. The Solar Dragon."
Flames erupted behind him in the shape of a roaring dragon. He swung with the weight of an ancient sun.
Kazaen tried to clap again but Tanjiro reached him before the sound formed.
The blade sliced through the Upper Moon's arm, severing it cleanly.
Kazaen screamed as flames crawled across his flesh.
Tanjiro stood calmly, sword lowered.
Giyu moved beside him. "You have shaken him."
Tanjiro nodded. "Now we finish him."
Kazaen, trembling and furious, regenerated his arm and glared at them with burning hatred.
"This will not be easy anymore. You have angered me."
Tanjiro raised his sword.
"Good. Then show me your full strength."
The battle had only begun.