Fog-enshrouded mountain a few kilometers west of Asakusa, bringing a sense of eeriness into the vicinity. To any passerby at this moment, this mountain felt no different from hell itself. Its famous tourist destination, the Kirigato waterfall, plummeted downwards just as usual, but with the scent of blood. The water, which was painted red, further gurgled downwards from the hut located at the peak of the mountain.
At this very moment, heinous crimes were being committed at the hut. Muzan Kibutsuji, who was casually passing by, had kindly spotted this hut. The demon king wanted more subordinates, useful ones that would not be swatted around like flies by the demon slayers. Despite not having any hope, he still decided to take a chance—one that he would later on remember as the best decision he had ever taken.
A few minutes and the house lay in ruins. The demon king had ensured that blood was smeared across the walls, limbs scattered, and hearts and intestines clawed out and tossed aside like disposable plastic bags.
"Please spare me. I am willing to do anything," a boy screamed.
Muzan thought about it carefully. One thing was sure: the boy had talent.
As Muzan decided whether or not he wanted to transform the boy, an inexplicable transformation took place within the boy's head.
"Huh, where am I? The boy murmured.
"Holy what the f*ck! Why is there blood smeared on the walls? Is that Muzan? Am I in Demon Slayer?
His contemplations soon came to an end as he felt a piercing pain on his forehead. A black haze enveloped him, taking away his consciousness.
A few hours went by, with the boy waking up in a narrow, candlelit passageway. Opening his groggy eyes, a wave of dizziness enveloped him. He smelled something appetizing, exquisite, and mouthwatering. Little did he know that the delicious smell had slithered into the cracks of his mind.
Feast.
A voice echoed in his head. It wasn't a mere word but a feeling—one that pounded on his head like a hammer.
Kill.
Another voice resounded within. This time, though, it was more animalistic, bringing a sense of excitement from within. An excitement that made every muscle in the boy's body tense up. Evident by the infinite drool pouring out of the boy's mouth.
Rip. Tear. Feed. EAT.
A cacophony of voices kept on ringing inside his head. "Stop!" he gasped aloud, slamming his palms to his temples. "This isn't me. I'm not—!"
They're weak. Soft. Bleed easy. Just a bite. Just a taste.
The shadows pulsed. He could feel his fingernails digging into stone, carving it like butter. His jaw twitched. Teeth… had they always been that sharp?
"No. No, no, no—" he stumbled back, scraping his shoulder against the wall.
You were born to devour.
"Shut up!" he screamed. But the echo of his own voice sounded inhuman now—layered, distorted, like two voices trapped in one throat.
Just eat!
A final voice rang inside his head before the boy lost control. In a flash, like a wild animal, he disappeared towards the lingering smell—dead little children.
The boy's fingernails dug into each child's chest, removing a muscular red organ. He held every organ in his hand before crushing it into a pulp. The objects, which were all hearts, had now turned into a horrid amalgamation of veins. The boy licked it clean and then feasted on every piece individually.
Despite feasting on every single organ and every piece of flesh, the insatiable hunger within the boy only worsened. It was at this moment that a voice in his head sounded. Good job, Saeto! You have passed my first test."
As the voice disappeared, Saeto experienced another splitting headache.
"Who am I? Why am I committing such heinous acts? Did I just consume innocent children?
"Screw you, Muza...." Just as he said that, the veins inside Saeto's body bulged with blood gurgling and spraying. He learned his lesson that saying Muzan's name out loud or even within his head was not a good idea. The screams continued, with the torture ending after nearly an hour.
A heavy silence followed the screams, broken only by the gentle footsteps of a man walking towards him. A man with crimson eyes, a black cloak, and a white hat. The man was Muzan, standing a few paces away, arms crossed, observing with an unreadable expression.
"Ara--Ara. How fascinating," Muzan murmured, tapping a slender finger to his chin. "Your mind... it is unlike any I've seen before."
Saeto's body slumped against the wall, chest heaving, eyes wide with confusion and fear. His hands, which were soaked in red, trembled with disbelief. His lips, glossy with gore, muttered.
"Kamados... the... mark... what mark?"
Muzan's smile widened.
There were the fragments of images, memories, and names. Names the Demon King had never heard, but now they swirled in front of his mind like glimpses of a broken prophecy. A scarred boy with a blade of sunlight. A Hashira cloaked in mist. A girl with a bamboo gag.
To Muzan, these felt real.
T "These... aren't hallucinations," Muzan whispered. "You know things, don't you, Saeto?"
But Saeto couldn't respond. The pain was returning.
His eyes glazed, and as he stumbled to his feet, he caught sight of a glint beneath him. A shallow puddle of water, dyed red, reflected the flickering candlelight—and his face.
He froze.
His reflection blinked back at him—barely human.
Porcelain-white skin, now etched with thin, black veins that pulsed like roots. His sclera had darkened, irises now a glowing, hateful gold. And worst of all, his mouth… it was still chewing. Muscle memory. A piece of someone's throat hung from one fang.
He screamed.
"That's not me. That's not—!"
The puddle rippled as a reflection of him tilted its head... and smiled.
"You are me, Saeto."
The voice was no longer external. It lived in him now.
His own.