Taisho Era – Year 5 (1916)
Location: Kai's Secret Workshop, Azabu Outskirts
By the age of eight, Kai's life had become a masterclass in duality. To the merchants of Azabu-juban, he was the "Golden Boy," a culinary and medical prodigy who could fix a failing soup stock or a stubborn fever with equal ease. But to the silent woods on the edge of the city, he was something far more formidable.
Hidden beneath a trapdoor in his small shack was a cellar he had excavated by hand, reinforced with cedar beams and lit by flickering oil lamps. This was his sanctuary of science and slaughter. On the wooden tables sat glass vials, copper stills, and bundles of dried wisteria flowers—purple petals that held the key to a demon's demise.
'Great Sage, begin the molecular breakdown of the latest batch.'
[Notice: Wisteria concentration at 84%. Potency is currently 3x higher than standard Demon Slayer Corps grade. However, the toxicity is unstable. Rapid oxidation will occur if exposed to open air for more than 12 seconds.]
'Not good enough,' Kai thought, his brow furrowed as he carefully dripped a clear, viscous liquid into a bubbling purple decoction. 'If I'm going to save Shinobu and Kanae, I can't just rely on the poison they already have. I need something that can bypass the cellular regeneration of an Upper Rank. I need a catalyst.'
His hands were steady, his "Yoriichi-like Physique" providing him with a level of fine motor control that no ordinary surgeon could match. He was currently experimenting with a synthesis of wisteria and a rare, sun-soaked lichen he'd found on the peaks of Mt. Kumotori.
"Kai-kun? Are you in your 'stinky hole' again?"
The muffled voice from above made Kai smile. He quickly capped the vial and wiped his hands on a cloth. "It's a laboratory, Mitsuri-chan! And I told you, the smell is just the scent of progress!"
He climbed the ladder, emerging into the small shack just as Mitsuri descended. At eight years old, she was already tall for her age, her strength having reached a point where she had to consciously "think soft" to avoid crushing door handles.
"It smells like... a funeral for a flower," she said, wrinkling her nose. She was carrying a wrapped bundle of onigiri. "You've been down there for six hours. If you turn into a mushroom, I'm not going to be happy."
Kai laughed, taking the bundle. "A mushroom with very good hair, at least. Come, let's eat outside. The fumes in here are a bit much, even for me."
They sat on a flat rock overlooking a small stream. The spring air was sweet, a sharp contrast to the chemical bitterness of the cellar.
"Why do you work so hard on those medicines, Kai-kun?" Mitsuri asked, her green eyes watching him intently as he ate. "The apothecary says you're making things that don't even exist in the textbooks. He called you a 'scary genius.'"
Kai paused, chewing slowly. 'How much do I tell her?' He looked at her hands—hands that were soft but capable of shattering boulders.
"The textbooks are old, Mitsuri. They were written for a world that didn't know how to fight back," Kai said, his voice dropping into that serious tone that always made Mitsuri's heart skip. "I'm making a 'Gift for the Wicked.' A way to even the playing field."
"For the monsters?" she whispered.
"For the monsters," he confirmed. He reached into his pouch and pulled out a small, pressurized silver needle—a prototype of a delivery system he'd designed. "This poison isn't just to kill. It's to paralyze. To slow them down long enough for the sun to finish the job."
Mitsuri looked at the needle, then at Kai. "You're always thinking about the sun. Is that why you do those dances in the morning? The ones that make the air hot?"
Kai nodded. "The sun is the source of all life, Mitsuri. But for some, it's the ultimate judge. I'm just trying to be the sun's assistant."
He turned to her, his gaze softening into a flirtatious grin to lighten the mood. "And besides, if I'm busy being a genius, I need a strong lady like you to make sure I don't forget to eat my rice balls. It's a very important job. Very high stakes."
Mitsuri blushed, a vibrant pink that matched her hair. "You're so silly! But... if you're the sun, then I guess I'll be the forest. The forest needs the sun to grow, right?"
Kai felt a surge of warmth in his chest that had nothing to do with breathing techniques. "The best kind of forest. One with lots of fruit and zero scary shadows."
Later that night, after walking Mitsuri home, Kai returned to his cellar. The playful boy was gone, replaced by the architect of a new era.
'Great Sage, play back the memories of Shinobu Kocho's final battle. Analyze the composition of the poison she used against Doma.'
[Processing... Memory data retrieved. Shinobu Kocho used a massive quantity of wisteria (37kg) to saturate her own body. Efficiency: Low. Success rate: High, but fatal to the user.]
'Fatal. That's the part I'm erasing,' Kai thought, his eyes flashing with a cold, burgundy fire. 'I'm going to create a concentrated serum. Something that can be delivered via a blade or a projectile, with a potency that doesn't require the user to become a sacrifice.'
He began to grind a new set of ingredients. His "Unique Skill: Great Sage" allowed him to run millions of simulations in seconds, testing how the poison interacted with demon blood samples he had managed to "procure" from a low-level demon he'd hunted and captured a month prior.
[Notice: Integration of 'Sun-Soaked Lichen' and 'Concentrated Wisteria' has reached a stable state. New Substance Identified: 'Solaris Venom'.]
'Properties?'
[Effect: Inhibits cellular regeneration instantly. Causes 'False Calcification' in demonic tissue. To a demon, it will feel like their blood is turning into liquid sunlight.]
Kai held up a vial of the glowing, pale gold liquid. It was beautiful. It was lethal. It was the first of many "Fate-Breakers" he would create.
'Now,' Kai thought, leaning back against the cool stone wall of his lab. 'I have the medicine. I have the physique. And my Sun Breathing is nearing the point where I can execute the 'Burning Sun' form without collapsing.'
He looked at his hands—small, yet carrying the weight of a future he refused to let happen. He thought of the Kocho sisters and the bells Mitsuri had given them. He thought of the smile on Mitsuri's face when she talked about the "forest and the sun."
'Just wait, Muzan,' Kai whispered into the darkness of the cellar. 'You've had a thousand years to play god. I've had five years to learn how to kill one. I'd say the odds are finally getting even.'
[Warning: Heart rate increasing. Adrenaline levels peaking. Suggestion: Sleep is required for optimal muscle recovery.]
'Sleep,' Kai muttered, a predatory smile on his lips. 'I'll sleep when the sun rises over a world where my friends don't have to die.'
He extinguished the lamp, the glow of the Solaris Venom the last thing to fade into the dark.