From the moment Gyokko first encountered Oboro, Muzan had become fixated on two words that held the key to his torment.
He sensed with growing certainty that the mystery surrounding that infuriating man was tied to the soul itself.
Why can that bastard ignore the control of my blood after transforming someone into a demon? This question had haunted Muzan for decades, gnawing at his pride like a festering wound.
While others wondered about Oboro's methods of creating demons, Muzan was consumed by this singular, maddening anomaly. If not for this bizarre exception to his absolute dominion, every inheritor on Earth today would be like the demons under his command: mindless slaves bound to his will by blood.
The past few years had been hell for the Demon King. The Inheritors relentlessly hunted him like rabid dogs while the Demon Slayer Corps scoured every shadow and crevice for him. The constant threat forced him into the most humiliating existence for a king: reduced to cowering in the darkness and never daring to show his face.
But above all else, it was Oboro he feared most.
The Ubuyashiki clan had long since deciphered fragments of Oboro's true intentions, and Muzan had witnessed their growing understanding with mounting dread. For over a century, he had methodically collected every scrap of information, every whispered rumor, and every trace of evidence related to that accursed man.
Today, finally, his patience had borne fruit.
Within the twisted geometry of the Infinity Castle, three figures of immense power had gathered: Kokushibo, whose six eyes gleamed with ancient malice; Akaza, whose muscles were coiled like steel cables; and Doma, whose perpetual smile masked depths of cruelty. Beside them sat Nakime, her biwa against her shoulder, watching with silent intensity.
The four demons observed Muzan slowly and deliberately insert his razor-sharp fingers into the skull of a captured inheritor.
Blood trickled from the victim's brow as violent tremors wracked his body. Muzan's eyes rolled back as his consciousness dove deep into realms beyond flesh and bone.
In the deepest recesses of his mind, Muzan once again confronted the maddeningly familiar, blurred silhouette: the soul fragment that lingered like poison in the Inheritor's blood.
"How pathetic," he mused, studying the ghostly remnant before him.
The hazy shadow radiated soul pressure so overwhelming that even this mere fragment sent waves of primal terror through Muzan's being. Though he had experienced this crushing spiritual weight countless times before, it still threatened to shatter his composure.
But this time was different.
This time, he did not flee back to reality or sever the mental connection in panic.
After a hundred years of agonizing trial and error, Muzan had finally achieved the once-impossible: He could sense his own "consciousness," the very essence of his soul.
The journey had been torturous. Initially, he could barely glance at Oboro's soul without his fingers trembling like a frightened child's. Now, he could meet that piercing spiritual gaze with steady resolve. Before this breakthrough, the concept of a "soul" had been nothing more than abstract philosophy to him, perhaps related to personal spirit or willpower, something to theorize about, but never truly feel.
Without the ability to perceive the soul's true form, he was powerless against Oboro's overwhelming spiritual dominance.
Through countless grueling attempts, he had finally forced open the mysterious door to transcendence. Unlike in the past, when soul contact manifested as mere "images" in his mind, Muzan could now gather his scattered consciousness, forge his spiritual essence into a weapon, and transcend the limitations of physical perception to reach life's deepest dimension.
He had spent the past century in relentless pursuit of this power. He scoured the world, researching every occult practice, biological theory, and whispered legend about strengthening the human spirit.
Though he had yet to create a systematic cultivation method, he was no longer stumbling blindly through spiritual darkness.
Most importantly, despite the excruciating agony, each contact with Oboro's soul had taught him invaluable lessons about this transcendent force.
The deeper his understanding grew, the more obsessed he became. The soul's power was magnificent, far more sublime and terrifying than his blood demon arts could ever be.
A chilling realization began to crystallize in his mind: soul power seemed alien to this world.
Souls had always existed as intangible phantoms, but no being in this realm had ever truly learned to harness them. Muzan had lived longer than Yoriichi Tsugikuni, with over a thousand years of accumulated knowledge and observation. If anyone could claim expertise about the origins and evolution of this world, it would be him.
His curiosity about Oboro had evolved into a desperate obsession.
How did that man appear in this world? Who or what created such impossible power?
"I've had enough of your condescending stare," Muzan snarled. His scarlet eyes blazed with centuries of accumulated hatred as he faced the shadow.
Though the face remained frustratingly obscured, those familiar, judgmental eyes made veins bulge across his forehead like twisted roots.
But this time, he could finally strike back.
The nails on his right hand elongated into gleaming talons. With a vicious swipe, he slashed directly at Oboro's spiritual remnant.
Swish.
His claws tore through the soul fragment like tissue paper, and Oboro's will scattered into wisps of fading smoke.
"HAHAHAHAHA!"
The sound of Muzan's wild, triumphant, and maddened laughter echoed through the mental void. For the first time in over a century, genuine joy transformed his usually grim features into something almost human.
"Even if you created methods to cultivate soul power and jealously guarded those secrets, you should never have challenged me," he whispered, his voice trembling with ecstatic excitement. "What I can accomplish transcends your pathetic imagination. From the perspective of biological evolution, I am this world's apex, the most perfect being to ever exist."
Oboro underestimated me, Muzan thought with savage satisfaction.
He was the Demon King, the ultimate life form that had surpassed every natural limitation. Just because others lacked the capability didn't mean he was bound by their weaknesses.
Even without formal training, he had touched the essence of the soul and gradually mastered its mysteries through sheer determination and superior evolution.
"It's time for you to pay the price for your arrogance."
His consciousness snapped back to his physical body like a rubber band. In the Infinity Castle, Muzan's eyes flew open, and he slowly withdrew his blood-soaked fingers from the inheritor's skull.
The inheritor collapsed lifelessly before hitting the ground.
Muzan glanced down with casual interest. A moment later, the corpse's flesh began to expand like an overinflated balloon, then exploded in a shower of gore and bone fragments.
With Oboro's will completely eradicated, the inheritor had become nothing more than a puppet whose strings had been cut. A single thought from Muzan could cause the cells to multiply infinitely until the body tore itself apart from within.
The assembled Upper Rank demons watched with varying expressions.
Akaza's eyes widened in genuine surprise. He had invested considerable effort in capturing this particular inheritor, even sustaining injuries in the process. The Inheritor's power level had been formidable, presenting a real challenge even for an Upper Rank demon.
Moreover, as members of the Twelve Demon Moons, they were all intimately aware of Muzan's century-long struggle against the Inheritors. Until now, that struggle had seemed utterly hopeless.
The balance of power had just shifted dramatically.
"Something very interesting is about to happen," Doma observed, his perpetual smile widening with anticipation.
As time passed and their physical forms fully regenerated, both Doma and Kokushibo finally broke through the blood demon art that Oboro had used to scramble their memories.
Muzan slowly turned to face his three most powerful subordinates. Even Kokushibo, ancient and battle-hardened, found himself holding his breath under that penetrating gaze.
For a split second, Kokushibo's enhanced hearing picked up the roars of countless tortured souls, an auditory hallucination born from the sheer mental pressure radiating from their master.
Akaza and Doma fared even worse. Their limbs locked rigid as stone sculptures and their thoughts froze mid-formation under the weight of Muzan's transformed presence.
Lord Muzan had become something else entirely.
"The 'God of Swordsmen,'" Muzan said, his lips curving into a predatory smile as he spoke the legendary title with mock reverence. "He's still breathing somewhere in this world. I very much want to meet him face to face. He possesses something that has captured my interest."
While outsiders might puzzle over the identity of the God of Swordsmen, the Upper Ranks knew exactly who their master meant.
Without warning, massive, flesh-colored tumors erupted from the shadows surrounding Kokushibo, Akaza, and Doma. Writhing tentacles shot upward like striking serpents and pierced their necks with needle-sharp precision. Then, they began pumping torrents of Muzan's enhanced blood directly into their systems.
"I am giving you my spiritual power and evolved cells," Muzan declared in a menacing whisper that seemed to echo from the castle's walls. "Be reborn from ashes and flame, then go find him."
As the blood transformation continued, the three Upper Ranks' eyes glazed over, and their faces contorted into masks of agony. Akaza and Doma convulsed violently as their bodies underwent a rapid transformation.
Networks of bulging blood vessels appeared beneath their skin, like a roadmap of pain. They writhed and pulsed with unnatural life.
However, Muzan's ambitions extended far beyond merely claiming Oboro's secrets about soul cultivation.
What he truly craved, what had driven him to the brink of madness for centuries, was the blue spider lily he had sought for over a thousand years.
Oboro undoubtedly knew its location.
Soon, that knowledge would belong to the Demon King.