In the central hall of the Infinity Castle, thick black mist swirled like boiling ink, the air heavy with suffocating malice.
Suspended in midair, Kibutsuji Muzan's black hair trembled faintly with fury as his crimson eyes glared down at the empty seat once belonging to Gyokko.
"Useless trash!" His voice cut through the silence like a blade. "You couldn't even kill a few human brats—and got your head cut off! You've disgraced the title of Upper Moon!"
With a flick of his hand, the black mist condensed into a sharp energy blade that cleaved the jade table beside him cleanly in half. Shards scattered across the floor.
The remaining Upper Moons—Kokushibo and Akaza—stood in silence, none daring to speak.
"Please calm yourself, my lord," said Nakime softly, clutching her shamisen with both hands. Her tone was cautious, deferential. "Perhaps Lord Gyokko underestimated those boys' strength. That's why he failed."
Muzan cast her a frigid glance, his eyes devoid of warmth. Then his tone shifted. "You've followed me faithfully for years. Unlike Gyokko, you understand your place. Since Upper Moon Five's position is vacant, you'll take it."
Nakime froze in shock for a heartbeat before kneeling low. "Thank you for your mercy, Lord Muzan! I won't fail you!"
Muzan turned away from her, his gaze sinking into the endless darkness beyond the hall. His voice was calm, but heavy with menace. "Two matters take priority now. First—continue searching for the Blue Spider Lily. Not a single clue is to be ignored. Second—Kamado Tanjiro, the boy with the hanafuda earrings who uses Sun Breathing… must die. The sunlight within him disgusts me."
His crimson eyes gleamed cruelly. "Whoever kills him will be rewarded with more of my blood—and greater power."
The air stirred as the Upper Moons exchanged tense looks. Akaza's fists clenched tightly. He'd long wanted to fight that boy who bore the mark.
Kokushibo remained expressionless, though the six eyes across his face glinted faintly—curiosity flashing for an instant. Another practitioner of Sun Breathing… interesting.
"The meeting is over."
Muzan waved his hand, and the dark mist enveloped his body, swallowing his form into the shadowed depths of the castle.
"Do your duties. Don't disappoint me again."
The Upper Moons turned one by one to leave. Akaza strode out first, muttering under his breath, "I'll find that brat myself."
Kokushibo walked out more slowly, a black shogi piece turning between his fingers as he headed toward his training hall.
Just as Tomioka Giyu—now called Mushiki—was about to exit, a figure stepped in his path.
It was Upper Moon Six, Iguro Obanai.
He wore a black kimono, his jet-black hair hanging down like scales framing his pale face. His dark red serpent-like eyes burned coldly as he drew his twin blades—curved swords gleaming with a dim, sickly light. Black demonic energy coiled tightly around them.
"Tomioka Giyu."
Obanai's voice was low and edged with fury. "You killed Kyojuro, didn't you?"
Giyu stopped, his expression unchanging. His narrow, dark-blue pupils flickered faintly as he answered flatly, "My name is Mushiki. That Flame Hashira you mentioned… yes, I killed him."
The words struck Obanai like a spark hitting dry powder. His killing intent exploded, black mist erupting around him.
Giyu frowned slightly as a flicker of memory broke through the haze in his mind—a man with orange-red hair smiling as he handed him a rice ball, saying, 'Protect what's important to you.'
But the image vanished as quickly as it came, swallowed by Muzan's curse and the deep seals binding his memory.
(The memory suppressant injected by future Giyu has begun to take effect, though it will take time for his consciousness to awaken fully.)
He shook his head slowly. "He was a Hashira of the Demon Slayer Corps. My enemy. Killing him was my mission."
"Mission?"
Obanai sneered, his voice dripping with disdain. "You've really forgotten who you are, haven't you? Always acting so high and mighty—talking like everyone else is beneath you! I've wanted to put you in your place for a long time. Now I'll challenge you to a Rank Duel of Blood!"
A Rank Duel of Blood—an official death match between Upper Moons. The winner took the loser's position; the loser was annihilated. Muzan himself had created this rule to keep his elite constantly sharpening their fangs.
As soon as Obanai spoke, Muzan's voice echoed coldly inside both their minds. "A Rank Duel of Blood? Very well. I approve."
His tone was detached, almost amused. "But understand this—if one of you dies, the survivor must find a new demon to fill the vacant rank within three months. Fail, and you'll be punished equally."
"Yes, my lord!" Obanai answered immediately, his tone resolute. This was his chance—to avenge Kyojuro and rid Muzan's ranks of a dangerous foe.
He knew Mushiki's memory was gone. That made him the greatest weapon Muzan had—and the one Obanai most needed to destroy.
Giyu offered no reply, only tightened his grip on his black Nichirin Sword. Since inheriting fragments of his future self's combat memories, his style had changed. He no longer relied on complicated techniques. Each strike now carried the distilled essence of all his training—pure, lethal precision capable of blending multiple breathing styles into a single motion.
He wanted to see, for himself, just how strong he had become.
The two walked into the Infinity Castle's dueling chamber—a vast circular arena built of black stone, its walls scarred with deep slashes and dark stains of blood.
Clearly, it had hosted countless battles among the Upper Moons.
Obanai took his stance first. His twin curved blades spun in his hands as black demonic aura wrapped around them. Snake-like markings appeared on his arms—the sign of his Blood Demon Art activating.
"I won't let you die easily," he hissed.
Before the last word left his lips, Obanai sprang forward, his body twisting like a serpent as he lunged at Giyu. His twin blades slashed in a cross arc, dark energy trailing their movement.
"Blood Demon Art—Constricting Serpent: Heartbite!"
As Obanai moved, black demon blood surged from his body, forming five serpent-like entities that glistened with a metallic sheen.
The demon snakes writhed through the air, their bodies stretching and coiling with eerie grace before lashing out, binding Giyu's limbs in an instant. Their jaws opened wide, revealing venomous fangs dripping with faint purple fluid that shimmered dangerously in the dim light—one bite would paralyze him completely.
Giyu's eyes narrowed slightly. There was no trace of panic.
He twisted his wrist, his Nichirin Sword glowing faintly with rippling blue water patterns, then flickering with an overlay of fiery orange light. "Water Breathing, Third Form—Flowing Dance: Flame Variation!"
The blade's motion flowed like water, circling around his limbs as he cut through the serpents' bindings. The heat of his sword surged outward, transferring the fiery essence of Flame Breathing onto the demon snakes.
A sizzling sound filled the air as the serpents shrieked in pain, their bodies seared by the fiery aura before dissolving into black mist.
Obanai's pupils contracted—he hadn't expected Giyu to break free so effortlessly, nor to blend Flame Breathing into Water Breathing with such seamless precision.
Without hesitation, Obanai changed his hand seals. Black demon blood misted into the air, spreading into a dense purple cloud that drifted toward Giyu. "Blood Demon Art—Thousand Serpent Venom Mist!"
The poisonous fog carried a choking stench, corroding even the rocky floor it touched, leaving small holes that hissed and smoked. More insidious still, the mist contained traces of demon blood that disrupted a Breathing user's rhythm, distorting their concentration and sense of airflow.
Giyu took a deep breath. A flicker of memory—one from his future self—flashed through his mind. The best way to counter venomous fog… is to cut through it with speed.
He tightened his grip on his sword, the blade now glowing with a faint lunar blue. His speed spiked. "Moon Breathing, Fifth Form—Moon Spirit Calamitous Eddy!"
Curved blades of light shaped like crescent moons burst from his sword, slicing through the poisonous mist like a raging storm. The purple haze shattered, dissipating into harmless wisps that vanished into the air.
Using the residual motion of his attack, Giyu surged forward. His sword carried the flow of Water, the heat of Flame, and the calm precision of Moon—all converging into one.
Obanai reacted instantly, his voice sharp. "Blood Demon Art—Serpent Shadow Step!"
His body melted into shadowy serpent silhouettes, scattering across the arena as multiple afterimages darted through the mist. Using vibrations in the ground, he sensed Giyu's movements precisely, tracking even the faintest ripple in the air caused by breathing.
A moment later, he appeared behind Giyu, twin blades raised high. "This time, you won't escape!"
But Giyu moved as if he had eyes in the back of his head. His body shifted just enough, and his sword swept backward in a single motion. The blade shimmered in three colors—pale blue, deep blue, and burning orange. "Moon–Water–Flame Triform Slash!"
Their blades collided with a deafening crash. Black demonic aura clashed with the triple-colored energy, bursting into a violent shockwave that shattered the stone floor beneath them.
Obanai staggered back, his hands trembling from the recoil. His palms split open, black blood seeping from his grip.
"Don't—" he hissed, his voice breaking, "—don't joke with me!"
His eyes widened in disbelief. That one strike from Giyu had fused three distinct Breathing styles perfectly—each essence pure and untainted by the others. Even Kokushibo might not have achieved such balance.
Giyu didn't pause. His attacks flowed again, each movement fluid, measured, and deadly. His sword danced like a living thing—sometimes like water, smooth and evasive, slipping past Obanai's guard; sometimes like fire, fierce and scorching, searing through his demonic aura; sometimes like the moon, cold and sharp, aiming straight for his vital points.
Obanai was forced onto the defensive. His twin blades spun in desperate arcs, black energy swirling as he unleashed one Blood Demon Art after another—Constricting Serpent: Heartbite, Thousand Serpent Venom Mist, Serpent Shadow Step.
Each technique was countered flawlessly. Giyu deflected, redirected, or shattered every strike, his minimalist swordplay dissecting Obanai's rhythm.
Then—an opening. Giyu's blade cut across Obanai's arm, leaving a shallow wound that sizzled with burning pain.
Obanai's face darkened. He realized too late that he had severely underestimated his opponent.
The Giyu before him was no longer the same man—his movements carried the weight of countless battles, his strikes precise, efficient, and merciless. Every slash struck exactly where it hurt most.
"You… what happened to you?" Obanai panted, dark mist coiling tighter around him as his fury peaked. "You're nothing like the Tomioka Giyu I knew!"
Giyu gave no reply. He only tightened his grip, his sword's tricolor glow burning brighter and hotter.
He could sense Obanai's power rising—the demon's aura thickened, his energy surging. He was preparing to stake everything on one final attack.
The ground trembled. Dark mist and radiant energy twisted together into a violent vortex at the arena's center.
Their figures blurred, darting through the maelstrom—steel clashing against fang, sword light colliding with the roar of serpents. The battle reached its boiling point.
Obanai drew in a deep breath, crossing his twin blades before him as his demon blood gathered into a massive serpent-shaped phantom that towered behind him. It was his strongest technique—his final gamble.
At the same time, Giyu channeled every Breathing style he'd mastered into his Nichirin Sword. The blade flared like the sun itself, its brilliance cutting through the dark.
Neither man had a path of retreat.
In this Rank Duel of Blood, only one could live.
And the outcome of this clash would not only decide their fates—but could shift the balance of the entire war between the Demon Slayer Corps and Muzan Kibutsuji.