The successful demonstration of "The Apollo Flash" changed everything. Ren was no longer just a guest; he was the Chief of the newly formed "The successful demonstration of "The Apollo Flash" changed everything. Ren was no longer just a guest; he was the Chief of the newly formed "Logistics and Development Division." The Ubuyashiki family had granted him a private wing of the estate, stocked with every material he requested—from rare ores to fine glass.
But Ren wasn't satisfied. He looked at the reports of slayers dying because they were ambushed by demons hiding in the soil or lurking in the rafters.
"The current method of detection is purely biological," Ren muttered, pacing his room while he adjusted his cooling-fan-equipped glasses. "Scent, sound, or instinct. It's too subjective. I need a constant. I need a frequency."
TP Spent: 2,000 (Remaining: 500)
Ren entered the Lab. He wasn't looking for weapons this time. He was looking for Sensory Tech. In the center of the Lab, a massive holographic blueprint of a demon's cellular structure flickered in ice-blue light.
"Every demon emits a unique electromagnetic disturbance due to the Muzan cells," Ren reasoned. "It's like a radio station that only plays 'Murder FM.' I just need the right antenna."
The Core: He used 1,500 TP to synthesize a "Bio-Magnetic Resonator." It was a tiny, vibrating crystal grown from refined Nichirin dust and wisteria sap.
The Interface: He didn't want a screen—that would be too suspicious for the Taisho era. Instead, he built a brass pocket watch.
The Engineering: When the "watch" was opened, it wouldn't show the time. It would show a circular glass plate with a glowing needle.
[Item Created: The Aoki-Radar (Mark I)]
Range: 500 Meters.
Specialty: Ground-penetrating sonar. It detects the unique "vibration" of demon flesh, even through 10 meters of solid earth or stone.
He spent the final 500 TP on "The Voice-Box Module." He didn't want to keep looking down at the watch, so he integrated a tiny, steam-pressurized speaker.
To prove the device worked, Ren requested a "low-risk" field test. The Corps sent him to a village where people were vanishing from their beds at night, despite no doors or windows being broken.
Accompanying him was Shinobu Kocho. She was there as his bodyguard, but also because she was the only one who could tolerate his "science rants" without getting a headache.
"So, Ren-kun," Shinobu said, poking the brass watch in his hand. "You're telling me this toy can see what my eyes cannot?"
"It's not a toy, Shinobu-san," Ren squeaked, his face already pink from her standing within his personal bubble. "It's a... a localized seismic-biometric analyzer. Logically, it's far more reliable than your 'gut feeling'."
Suddenly, the watch emitted a crisp, mechanical voice:
"TARGET DETECTED. DEPTH: 3 METERS. BEARING: SOUTH-SOUTH-WEST."
Shinobu jumped. "It... it spoke?!"
"Underground," Ren said, his eyes turning cold and sharp. "The demon isn't in the houses. It's in the drainage pipes under the street."
He pressed a button on the side of the watch.
"BIO-SIGNATURE MATCH: MULTIPLE LIMBS. AGGRESSION LEVEL: HIGH."
Before the demon could burst out for its nightly meal, Ren pointed toward a specific stone slab on the road. "There. If you strike now, you will sever its primary nervous system before it can surface."
Shinobu didn't hesitate. "Insect Breathing, Butterfly Dance: Caprice!" She plunged her stinger-like blade through the gap in the stones. A muffled, gurgling scream erupted from beneath the earth. Black blood seeped up through the cracks. The demon had been killed before it even knew it was being hunted.
As Shinobu wiped her blade, she looked at the village. "We need to send a Crow to report this. It will take hours for the message to reach the Master."
Ren watched the crow fly away. His logical brain clicked.
Information lag is the greatest enemy of any military organization.
"Crows are... slow," Ren muttered. "And they are prone to being eaten by hawks. If a slayer is in trouble, they shouldn't have to wait for a bird to fly across a mountain."
He pulled out his notebook. He started sketching a "Signal Repeater"—small devices that could be hidden in wisteria trees across Japan. If he could create a hand-held radio (disguised as a charm or a locket), the Demon Slayers could communicate instantly.
"Ren-kun!" Shinobu suddenly beamed, leaning in so close their noses almost touched. "Do you realize what you've done? No demon can ever hide from us again! You're like a little blue-haired god of tracking!"
She grabbed both of his hands and squeezed them tight.
Ren's internal systems suffered a catastrophic failure.
Tactile input: 100%.
Social pressure: Overload.
Result: Total System Crash.
Ren didn't just turn red; he turned a shade of neon purple. Steam actually hissed out from his collar.
"The... the signal... the wave-lengths... mother..."
Ren's eyes rolled back. He fell backward like a felled tree. Shinobu caught him, laughing. "Oh dear, I did it again. He's so powerful against demons, but one 'thank you' and he's out for the count."
The Butterfly Estate was unusually quiet. Most of the Hashira were out on coordinated "Daylight Sweeps." Ren was in the courtyard, testing the Aoki-Radar Mark II.
"SCANNING... RADIUS 1KM... ALL CLEAR," the voice-box droned.
"Ren-kun! You need to eat!" Shinobu called out, walking toward him with a tray. She was smiling, her usual sharpness softened by the recent string of victories.
Suddenly, the Radar in Ren's hand didn't just beep. It shrieked.
"WARNING. MASSIVE BIO-SIGNATURE DETECTED. DEPTH: 0. VELOCITY: IMMEASURABLE."
"Shinobu-san, move!" Ren yelled, his voice cracking with a rare, raw terror.
The ground didn't crack; it disintegrated.
A blur of pink and silver erupted from the earth. A shockwave of pure physical pressure sent Shinobu flying back and shattered the stone tiles of the courtyard. Standing in the crater was a man with basketball-patterned tattoos, pink hair, and eyes that glowed with a terrifying golden light.
Upper Moon Three: Akaza.
"The Master was right," Akaza said, his voice a low, vibrating hum of battle-lust. "There is something new here. Something that smells of cold metal and strange fire."
His eyes locked onto Ren. "You. You're the source of the 'miracles' that are killing my kind in their sleep. You don't have the spirit of a warrior... you have the soul of a tool-maker."
TP Spent: 8,000 (Remaining: 2,000)
Ren's brain didn't freeze this time. The presence of an Upper Moon was so overwhelming that his "Social Anxiety" was replaced by "Survival Calculation."
'I cannot run. Shinobu-san is dazed. I have 10 seconds before he moves.'
Ren triggered the System. "Enter Research Lab! TIME DILATION MAX!"
Inside the white void, Ren didn't go to the assembly line. He went to the Forbidden Prototype Rack. He had 10 minutes of real-world time compressed into his mind.
"I need a deterrent," Ren screamed at the interface. "Not a sword! A physical denial of space!"
The Item: The Gravity-Anchor Mine. (Uses 5,000 TP). A device that, when triggered, creates a localized 10G gravitational well.
The Weapon: Nichirin-Liquid Nitrogen Sprayer. (Uses 3,000 TP). To freeze the cells of an Upper Moon, slowing their regeneration to a crawl.
Ren emerged back into reality just as Akaza shifted his weight to dash.
"Destructive Death: Compass Needle!" Akaza leaped, his fist aimed not at Ren's heart, but at his head—he wanted to take the genius back to Muzan alive.
"Calculation... complete," Ren muttered.
He slammed his foot onto a hidden pressure plate he'd installed in the courtyard days ago.
THOOM.
The Gravity-Anchor activated. The air around Akaza suddenly turned to lead. The Upper Moon, mid-air, was slammed violently into the dirt, the ground beneath him cratering from the artificial weight.
"WHAT IS THIS?!" Akaza roared, his muscles bulging as he tried to fight the 10G force pinning him down.
Ren didn't wait. He pulled the Nitrogen Sprayer from his sleeve—a sleek, futuristic cannon. He pulled the trigger. A jet of absolute-zero blue mist engulfed the Upper Moon.
The sound was like breaking glass. Akaza's skin, his blood, his very cells began to crystallize. His legendary regeneration hit a wall; the frost was spreading faster than his flesh could knit.
"Shinobu-san! THE NECK! NOW!" Ren screamed, his face pale from the effort of holding the heavy weapon.
Shinobu, recovering with a gasp, saw the opening. She didn't use a butterfly dance; she used everything she had. "Insect Breathing: Centipede Zigzag!"
She became a blur of purple light, aiming her stinger straight for the frozen giant's throat.
Akaza was an Upper Moon for a reason. With a roar of pure willpower, he shattered his own frozen limbs to break the gravitational lock. He spiraled away just as Shinobu's blade grazed his neck, injecting enough Wisteria poison to kill a hundred normal demons.
He landed on the estate wall, his body smoking, his left arm missing, and his eyes wide with a mix of fury and respect.
"Incredible," Akaza hissed, his arm already beginning to grow back—though slowly, hampered by the frost. "To think a child and a weak girl could draw my blood. Ren Aoki... the Master will have your head for this."
He vanished into the trees just as the other Hashira, alerted by the explosion, came screaming into the courtyard.
The danger was gone. The courtyard was a wreck. Ren stood there, his high-tech sprayer hissed as it ran out of fuel.
Every Hashira—Rengoku, Tengen, Gyomei—was staring at the crater, then at the frozen fragments of an Upper Moon's flesh, then at Ren.
"You... you fought an Upper Moon Three?" Tengen whispered, his voice trembling with disbelief. "And you won the exchange?"
Ren looked at the crowd. He looked at the smoking machinery in his hands. He looked at Shinobu, who was covered in dust but alive.
The logic-gate in his brain finally flipped back to "Social Mode."
"The... the thermal expansion... of the... I... I broke the tiles..."
Ren's face went from white to a brilliant, glowing crimson. He looked at the terrifyingly impressed Hashira surrounding him.
"Please... don't look... at the... results..."
THUD. Ren Aoki fainted so hard he did a literal backflip before hitting the grass. He was the first person in history to drive off an Upper Moon and then immediately lose to a group of people staring at him.
________________________________________
Dear readers if you like my fanfic please give me a review and you can recommend it to others.