Morning light washed over the Butterfly Mansion's garden as Erika wandered through the greenery, her steps slow, her thoughts scattered. It had been two days since she and Akira returned with the blue spider lily, but her mind was still stuck on… everything that happened.
Akira had been busy since they got back planning, researching, doing Akira things…so aside from the nights when she had to drink his blood, they barely spoke. And even though she didn't need to drink his blood every day… she still wanted to. It was selfish, she knew, but she couldn't help it.
Was this the same girl who hated drinking blood, but she also knows she could never go back, drinking his blood always made her happy.
Yet despite that, she had been avoiding him.
Not because she disliked him.
If anything… it was the opposite.
Those recent incidents, waking up beside him, hugging him in her sleep, drinking his blood unknowingly had all tangled up inside her chest, leaving her unsure how to face him normally.
When they were children, they were fairly close. They even took baths together sometimes in hot springs, and sometimes they even slept in the same bed. Back then sure, it was a little embarrassing sometimes, but they were small. Innocent.
But now they were older. And doing things like that now…
…just thinking about it made her face heat up.
Part of her wanted to go back to how things were before. But another part, an annoyingly loud part, refused to go back to being "just childhood friends." Except she had no idea how to move forward either.
She didn't have anyone to ask.
"I'm stuck…"
Then suddenly.
Selena's voice echoed telepathically in her mind. "Don't worry, Erika-chan. Just take your time deciding what you want to do. There's no need to rush."
Erika thought back, "I just don't know what he's thinking. Or how he feels…"
"Well," Selena replied gently, "I can't tell you anything yet. But you should know this, he's changing too. So don't worry, alright?"
Erika sighed. "Fine. I'm just… unsure how to act around him."
"You don't need to act differently around him", Selena said. "Just be yourself. If you act differently, he'll feel weird too. So don't worry too much".
"I'm not worrying", Erika insisted.
"Erika-chan", Selena giggled, "you've been walking really slow for a while now. Aren't you supposed to bring him to the meeting?"
Erika blinked.
"…Oh. I almost forgot."
Her face flushed with panic and she hurried her steps toward the shed behind the Butterfly Mansion, Akira's makeshift workshop.
She opened the door and called out his name.
"Akira…"
Then froze in stunned silence.
Because Akira was standing in the middle of the room, illuminated dramatically by lamplight, laughing like an actual mad scientist.
"Hahahahahahahaha! Finally, my new creation is complete! With this, I will…." He abruptly stopped mid-evil monologue and looked back. "...Oh, Erika. What are you doing here?"
Erika snapped out of it and asked, "What is that?"
Akira proudly gestured to the massive metal contraption behind him.
"This? Something I created that can induce Demon Slayer Marks. I'm going to introduce it in the Hashira meeting. Good thing I managed to finish it before then."
"You created WHAT?" Erika deadpanned. "Did you even check if it works?"
"Oh, it works," Akira said confidently. "Don't worry, I just tested it."
He walked over and pulled open the machine's large door.
A tired, slightly steaming Tanjiro staggered out.
"A-Akira-san... does it work?" Tanjiro asked weakly.
Akira nodded. "It works. Thanks for your help, Tanjiro."
Tanjiro sighed miserably. "For a moment, I thought I was going to be cooked alive…"
Akira laughed. "Hahaha, it's nothing like that."
Just then, a few Kakushi appeared at the door.
"Akira-sama, did you call us…? What is that?" one of them asked, staring at the machine.
"Oh, you guys are here. Take this thing to the meeting," Akira said, patting the machine, "and take Tanjiro back to rest."
"Yes sir!" They quickly mobilized, hauling the bizarre machine away along with the exhausted Tanjiro.
Once they left, Akira turned toward Erika.
"So, what is it, Erika?"
"Oh, I came here to get you for the meeting," Erika said. "They're waiting."
"Ah, right. I've finished everything here." Akira dusted off his hands. "Let's go together."
So they locked up the shed and went to the Hashira meeting place.
When they reached the room, everyone was already gathered.
Akira blinked. "Oh, are we late?"
Sanemi clicked his tongue. "You are late."
Akira gave a perfectly serious nod. "I'm sorry. I was on the way but had to help an old lady cross the road."
"…What?" Sanemi stared.
Akira broke into a grin. "Just joking, haha…"
STEP.
Erika crushed his foot under her sandal and whispered sharply, "Don't joke around."
Akira swallowed his scream.
Lady Amane gently addressed him, "Hoshino Akira-sama, is everything ready for the presentation of your machine?"
Akira straightened. "Yes, everything is ready. But before that, does everyone already know about the Demon Slayer Mark and its side effects?"
Lady Amane nodded. "Do not worry. I explained everything to them beforehand."
All the Hashira nodded in confirmation.
Sanemi snorted, arms crossed. "I don't care how long I live. I want to defeat Muzan."
Gyomei quietly added, "Though… I wonder what will happen to me. I am already twenty-seven. If marked individuals die at twenty-five… what would become of me?"
Akira answered calmly, "From my research, if the mark awakens during a battle, you'll probably die right after the fight is over."
Gyomei nodded with serene acceptance. "I see."
Akira clapped his hands together. "Well then, if there are no more questions… I will reveal the machine."
He snapped his fingers.
Two Kakushi dragged in a massive contraption to the ground outside the room, a metal cylinder with vents, pipes, and a treadmill sticking out of the front from an open door. It looked like half an oven, half a torture device, and half a steampunk sauna.
It was unclear why it had three halves.
The Hashira collectively leaned back.
"WHAT THE HELL IS THAT," Sanemi shouted.
Giyu said nothing, as usual.
"What a truly magnificent device," Rengoku declared, eyes blazing.
Tengen laughed. "This is wonderfully flashy!"
"This is horrifying," Shinobu said with a pleasant smile.
"Such danger…" Gyomei wept.
"I'm going home," Muichiro said flatly.
"Are you trying to kill us?" Iguro hissed.
"It looks like a giant rice cooker!" Mitsuri gasped.
Akira nodded proudly.
"This is the Demon Slayer Mark Induction Oven."
Everyone stared at him, stunned by the name alone. Erika quietly pretended she had no idea who he was.
Akira tapped the device and began explaining.
"This machine raises body temperature to thirty-nine to forty degrees by heating the outside and increases heart rate through controlled exercise. Combined with my new medicine, Vitality Boost Type 6, once it's activated, the treadmill will start, and a Demon Slayer will run inside. When they awaken the Mark, they'll be released. Otherwise, they'll be stuck in there…so eventually, a Mark should appear."
The room went dead silent.
Sanemi pointed at it, horrified.
"THAT'S JUST A DEATH TRAP!"
"Hm. So you created a human oven," Shinobu commented.
"I LOVE IT. IT'S FLAMBOYANT," Tengen declared.
Muichiro blinked slowly. "…I'm really, really going home."
Lady Amane, who had also gone pale, finally spoke in a gentle but firm voice.
"Hoshino Akira-sama… while your ingenuity is certainly… impressive… the Demon Slayer Corps will not be placing its warriors inside ovens."
Akira nodded, as if he had expected this.
"Don't worry, it's completely safe," he said. "I already tested it on Tanjiro."
Everyone reacted at once.
"Poor Tanjiro…"
"He did what to that boy…"
"That explains the smell of steamed charcoal…"
Several Hashira whispered in horror.
Akira continued casually, "Why don't I give you a demonstration? I can show you how effective it is. Any volunteers?"
Before anyone could stop him, Rengoku clapped a hand on Akira's shoulder, eyes burning with passion.
"EXCELLENT!! I SHALL TEST THIS MARVELOUS CONTRAPTION!"
Everyone instantly panicked.
"Rengoku-san, wait…!" Shinobu said.
"Please don't! You'll burn alive!" Mitsuri cried.
"Rengoku-sama, it is not required…" Lady Amane tried to stop him.
"YES, LET HIM…WAIT, NO…" Sanemi shouted, confused with himself.
But Rengoku had already placed a proud hand over his heart.
"If this machine can strengthen our comrades, then my body shall be its first flame! I will set my heart ablaze!"
Akira nodded respectfully. "Please step inside."
Obanai whispered to Himejima, "We are about to witness a man boiled alive."
Himejima wept softly. "May the gods have mercy…"
The test begins as Rengoku stepped into the oven.
Akira closed the iron door and activated the device.
Click.
Steam began filling the pipes. The treadmill started at a gentle pace.
"Begin Mark Simulation, test take one," Akira said.
Inside, Rengoku started jogging cheerfully.
"HAH! HAHAHAH! A DELIGHTFUL WARMTH!"
Temperature gauge: ↑ 90°C.
"Akira-kun, is that okay? That's TOO HOT…!!" Mitsuri panicked.
"Don't worry. I'm sure the Flame Hashira's flame tolerance is high," Akira replied calmly.
Psssshhh!
Steam hissed louder. The treadmill speed increased.
Rengoku's voice boomed from inside.
"MAGNIFICENT! MY BODY TINGLES WITH POWER!!"
Temperature gauge: ↑ 110°C.
"He's cooking, is this science?" Sanemi, who now smelled the burnt smell, muttered.
"This is not how science works…" Shinobu said, eyes narrowed.
Akira flipped another switch. The chamber glowed red.
"Is this really okay…?" Erika whispered.
"If the Mark activates, yes," Akira said. "If not… well, we can treat heat stroke."
Inside the box.
Fwooom!
A powerful wave of heat surged.
Rengoku's laughter echoed.
"ROAR, MY HEART! BURN BRIGHTER!!"
Temperature gauge: ↑ 130°C.
The machine began shaking violently.
And then.
Fzzzh…!
The device powered down.
Akira opened the chamber.
A wave of steam rolled out as the heavy chamber door creaked open.
Rengoku stepped forward, his skin red, steaming, heroic, veins standing out, eyes blazing with renewed power.
"AH! I FEEL REBORN!
TRULY, THIS MACHINE IS A FORGE FOR HUMAN POTENTIAL!"
On his cheek, a flame-shaped pattern burned brilliantly, different from Tanjiro's but unmistakable.
A Demon Slayer Mark… fully awakened.
Akira nodded proudly.
The rest of the Hashira… looked utterly traumatized.
Shinobu muttered, "…I cannot in good conscience recommend this for anyone."
Iguro added flatly, "This will kill normal people."
Muichiro, expression blank, said, "Absolutely no one else should use it."
Sanemi jabbed a finger at the machine. "This should be illegal."
Mitsuri, meanwhile, thought quietly to herself:
But Rengoku-san looks amazing!!
Lady Amane gently cleared her throat.
"Thank you, Akira-kun. Rengoku-sama has demonstrated its effectiveness.
However… for safety reasons… only Hashira may use this device."
She paused, then asked:
"Is there a way for normal Demon Slayers to gain the Mark without this device?"
Akira replied, "Well, the more traditional method is hard exercise, but this one is faster and easier."
Rengoku, still glowing, threw a thumbs up.
"LET THEM TRY IT!"
Everyone collectively shouted:
"NO!"
Akira exhaled lightly. "Well, good thing everything went well," he muttered.
Then, without drawing attention, he quietly scribbled notes on a small clipboard.
Success rate: 1/1.
Potential casualties: 97%.
Further refinement needed.
Good thing no one saw it.
Except Erika, who stood behind him with a deadpan look, deciding she absolutely needed to talk to him about this later.
Rengoku beamed triumphantly, his newly awakened Mark glowing like a burning crest.
"I AM REBORN!" he declared with fiery pride.
Lady Amane let out a gentle sigh of relief.
And the rest of the Hashira, after everything they had just witnessed, silently agreed on one thing:
The Demon Slayer Corps was absolutely not ready for Akira's inventions.
Thus, for the non-Hashira, a far safer alternative was decided:
Hashira Training.