Kamakura Period.
On a night of the red moon.
A forest in the middle of nowhere.
"I know you're there. Come out," Muzan commanded.
A man dressed in a kimono stepped out from the shadows.
Muzan narrowed his eyes. "You are?"
"It's been a while, brother! Hahahaha!" the man said joyfully.
Muzan's eyes widened slightly. "Fujin?"
"Yes, yes! I'm your little brother, Muzan-niisan!"
"What brings you here?" Muzan asked, his tone flat.
"Eh? If I remember correctly, you brought me here. I just followed you."
"Hah... Why are you following me?"
"I should be the one asking that," Fujin replied. "Why did you leave?"
Muzan remained silent.
"Do you remember our plan? To conquer the heavens and become an Omnipotent God?"
"...What do you mean?"
"Oh? You don't remember?"
"What are you talking about?"
"Puhahahaha! To think you've lost your memories."
"So what if I have?" Muzan said coldly.
"It's so bad, you know? I even started sharing my blood with others so I could create a legion of demons under your command!"
"I don't care about that."
"Oh? Why not?"
"I never wanted to kill anyone to begin with."
"Pfft... Hahahaha! The 'God' who killed hundreds of gods, now claiming he dislikes killing?"
"The 'God' you're talking about... Are you referring to me?"
"Of course, dear Demon God—or should I call you by your other title?"
Hmm? So, he sees me as the Demon God. So, Fujin might be the reincarnation of that Demon King.
"No need. There's no point in saying that anymore." Muzan answered.
"I see..."
Silence. A wind of awkwardness drifted between them.
"You're too cold, brother."
"Shut up."
"Anyway..." Fujin's tone suddenly shifted. He became serious.
"Why can't I smell any human blood from you? Did you find a way to hide that scent?"
"I told you already. I dislike killing."
"What? You're joking, right?"
"Joking? The hell are you talki—"
Before he could finish, Fujin's punch connected with Muzan's face.
"Are you trying to destroy our plans? Your plans?"
I'm not the Demon God he's talking about anyway, Muzan thought. I'm just a college student who got reincarnated into this world.
Even if I inherited a fragments of the Demon God's memory, I am not him. I don't even know his plans. Since the memories I received from him are only about 'Devour' and how he ruled the Demons.
"I already lost my memory of the plan you're talking about, so it's useless to me."
Muzan entered the state of Nirvana and conjured a katana from his blood.
"Blood Demon Art: Blood Transformation."
In the blink of an eye, Muzan sliced Fujin into thousands of pieces.
"You bastard... Are you the Sun God?" Fujin asked, his voice strained, remembering how the Sun God killed him.
"I'm not. But I'm also not the Demon God. I'm just... a person who exists in this world."
"Heh. But still..." Fujin smirked. "I created a Demon Art to counter the Sun God's attacks. Blood Demon Art: Reform."
The scattered pieces of his body began pulling together, restoring him to his human-like form.
"Your sword..." Fujin muttered.
"What about it?"
"Is it a Nichirin sword?"
"No."
"Then how does it carry the Sun's essence?"
"It's just one of my Blood Demon Arts."
"That should be impossible. Only the swords of the Demon Slayers should have that."
"Demon Slayers?" Muzan asked. (I already know about them, but I should pretend I don't to avoid suspicion.)
"You didn't know? Do you remember the branch family of our clan, the Ubuyashiki family?"
"Yes."
"After we became demons, they received a curse. In response, they formed a group of Demon Hunters to slay us. But don't worry-they're far too weak for us."
"Oh? You're saying that they got cursed because of us. But I doubt the curse came after you devoured a human."
Fujin froze, as if realizing something.
"You..." His voice was low. "I don't see you as our ruler anymore. You're our enemy."
"Eh. Don't care."
"You think I'm weak? Imagine how many humans I've devoured over the past three hundred years!"
In an instant, Fujin appeared in front of Muzan and struck.
"Blood Demon Art: Thousand Destructive Strike!"
He unleashed a thousand punches per second. Muzan was overwhelmed.
So fast... Even if I use Nirvana again, I'll lose consciousness before this fight ends.
"Combat Form!" Muzan roared.
His body transformed. From his back sprouted bow-like bone whips. He launched his right hand at Fujin—fast, but not fast enough.
"Blood Demon Art: Infinite Blood Slash!"
A barrage of slashes tore through the air. Muzan tried to dodge but failed. His arms were severed.
"What...?"
"Pfft. To think I once praised someone as weak as you."
"So you became this strong after devouring hundreds of humans?"
"Correction—I've devoured tens of thousands."
Kneeling, Muzan trembled.
"You bastard... Those people could've lived full lives."
"Like I care."
"Die!"
Muzan whipped at Fujin, but Fujin sliced the bones cleanly. Muzan coughed up blood.
"A golden blood? So you've already conquered the sun."
"What...?" Muzan gasped. How does he know?
"I've finally found the demon who conquered the sun."
"...."
"You are not our God. So it's fine if I kill you. I'll just devour you, conquer the sun, and find our God's true reincarnation."
Fujin approached.
But before he could get close, Muzan activated his secret ability.
"Devour."
A swirling black hole-like energy surrounded him, absorbing Fujin's blood and rapidly regenerating Muzan's arms.
"How?! Why do you have our God's ultimate ability?!" Fujin shouted, panic creeping in.
"Ultimate Blood Demon Art: Demon King Series – Demon Slave Swap!"
Fujin vanished from the vortex, replacing himself with a demon bearing the mark "Upper Three" on its eye.
"That should be the end," Muzan muttered.
Boom.
Muzan's blood vessels burst. His body lightened. He felt twice as strong-but his thirst for blood and flesh surged violently.
"Damn it... If this continues, I'll end up killing innocent people..."
He left the forest behind.
---
Two months later.
After enduring and resisting his growing urges, Muzan finally found a place to meditate. He climbed a mountain and found an abandoned fortress.
"I'll stay here for the time being... and master the state of Nirvana. I need to last longer without losing consciousness."
---
Two Hundred Years Later...