The night grew heavier.
Wind stopped. The moon dimmed behind thick clouds. Even the rusted metal beams around them seemed to hold their breath. Tanjiro and Giyu stood ready as Kazaen trembled, his body flickering like a lantern about to burst.
The Upper Moon lowered his head.
Silence filled the industrial yard.
Then his lips curled into a wide, unnatural smile.
"So this is fear, is it?" Kazaen whispered. "My heart is trembling. How interesting. I have not felt this in centuries."
Tanjiro tightened his grip. "That feeling means you are still alive."
Kazaen raised his head slowly. The markings on his body glowed with an intense crimson light. His heartbeat echoed like a massive drum.
Thump.
Thump.
Thump.
With each beat, the air around him vibrated violently. The metal ground cracked. Dust rose in spirals. The abandoned factory pillars groaned.
Giyu's hand tightened on his sword. "His aura has changed."
Kazaen stretched out his arms as if inviting the world to watch.
"Let me show you the true music of the Upper Moons."
The vibrations around him burst outward in a dome-shaped shock. Tanjiro and Giyu both jumped apart, landing on different sides of the field.
Kazaen's voice deepened. "I no longer need to use delicate sounds. Now the entire world will be my instrument."
A deep hum surrounded them, almost too low for human ears. Tanjiro's skin prickled. His bones vibrated painfully.
Giyu grimaced. "He is using resonance on the entire ground."
Tanjiro steadied his breath. "If we move carelessly, the vibrations will shatter us."
Kazaen began to walk, each footstep creating ripples through the earth.
"You learned to hide your sound," he said calmly. "Admirable. But can you endure when the world itself shakes?"
He slammed his fist onto the ground.
The earth rippled like water.
A wave of pure vibration rushed toward Giyu.
Tanjiro shouted. "Giyu, jump."
Giyu leaped high. The ground where he stood exploded in a burst of concrete shards.
Kazaen flicked his finger toward Tanjiro next.
Tanjiro braced himself.
The air in front of him wavered, then sliced forward like an invisible blade.
Tanjiro stepped aside with precision.
But even so, the air scratched his cheek like a knife.
Kazaen smiled softly. "Your body remembers pain very well."
Tanjiro wiped the blood with the back of his hand.
"It remembers enough to know how to fight."
He exhaled, calming his senses.
His ears tuned in to the smallest details. Wind brushing metal. The beat of Kazaen's heart. The faint tremble in the ground before each attack.
He listened.
He felt.
He understood.
Then he moved.
Tanjiro dashed low, barely touching the ground, his sound hidden perfectly. Kazaen immediately sensed the shift and flicked his wrist. Thousands of tiny vibrations shot forward like needles.
Tanjiro swung his blade.
"Sun Breathing. Fourth Form. Burning Bones, Summer Sun."
A scorching wave of heat erupted from his strike. The vibrations lost their direction, bending through the heated air.
Tanjiro broke through.
Kazaen's eyes widened. "So you can distort air itself. Clever."
Tanjiro readied his next strike, but Kazaen placed his palm on the ground and pushed vibration downward.
The earth beneath Tanjiro softened.
Then collapsed.
Tanjiro fell into a sinkhole, only to use the falling momentum to flip back out, landing beside Giyu.
Giyu spoke quickly. "Tanjiro, we need to destroy his rhythm. As long as he controls these vibrations, the entire battlefield is his weapon."
Tanjiro nodded. "Then I will attack from above. You attack from below."
Giyu stepped forward, sword drawn.
"Kazaen. The Water Hashira is here."
Kazaen raised an eyebrow. "You have grown weaker, Giyu Tomioka."
Giyu answered with calm resolve. "Yet strong enough to stand beside Tanjiro."
He moved first.
"Water Breathing. Ninth Form. Splashing Water Flow."
Giyu rushed forward with fluid speed, his movements unpredictable.
Kazaen smirked. "Perfect. Predictable unpredictability."
He sent a pulse into the air. Sound waves shot outward, curving mid-air to strike Giyu.
But Tanjiro was already above him.
"Sun Breathing. Second Form. Rising Scorching Sun."
His blade descended with blazing force.
Kazaen spun, clapping his hands once.
The clap shattered the air around him, creating a dome of destructive force.
Tanjiro was thrown back again, flames scattering like sparks.
Giyu slid backward across the ground but stayed on his feet.
Kazaen exhaled slowly. "Your timing is good. But you both still lack unity."
Tanjiro stood up. "No. Our timing is perfect."
Giyu nodded once.
Kazaen paused.
Tanjiro raised his sword.
Giyu raised his as well.
Their breathing matched.
And in that instant, their presence harmonized.
Giyu moved like water.
Tanjiro moved like fire.
The ground beneath them cracked from the pressure of their synchronized breathing.
Kazaen's eyes widened. "You combined your rhythms."
Tanjiro's voice was steady. "You control vibrations. Then we will create a rhythm your body cannot react to."
Giyu whispered. "Now."
Both of them launched at the same moment.
Kazaen tried to sense the vibrations.
But their combined movement created two opposing rhythms that interfered with his perception.
Left. Right. High. Low.
Fire and water blended into a single overwhelming force.
Tanjiro swung first.
"Sun Breathing. Eleventh Form. Solar Wind Halo."
A blazing vortex spiraled around Kazaen.
Giyu followed instantly.
"Water Breathing. Tenth Form. Constant Flux."
Water coiled around Tanjiro's flames.
The combined strike burst forward.
Kazaen tried to clap again, but his timing slipped for the first time.
His vibration barrier cracked.
Their attack hit him directly.
Kazaen's body was sliced in half, flames and water tearing through him. Burning steam filled the air.
Kazaen screamed, his form dissolving into ash.
Even as he died, he whispered, "Your rhythm… is beautiful."
Tanjiro and Giyu lowered their swords.
The battlefield fell silent.
Giyu exhaled slowly. "He was strong."
Tanjiro nodded. "And Muzan will be even stronger."
Giyu looked at him with a serious expression. "Tanjiro. You must not let your heart break again. The world needs you."
Tanjiro's eyes were steady.
"I will not break."
He looked toward the distant city lights.
"Sakura. I will return."
The night carried his promise across the ruins.
The hunt for the Upper Moons had only begun.