Ren felt as though his entire physical form had been rewoven by the hands of a celestial tailor. Every muscle fiber felt lighter, more efficient, and humming with a rhythmic vitality he had never known. As he exhaled a long, turbid breath, a puff of white vapor that carried away the last of his bodily impurities, he marveled at the sheer weightlessness of his limbs.
In this world, oxygen was the ultimate currency, and Ren's lungs were suddenly very, very wealthy. Every breath he drew was fuller and more powerful than the last, fueling a metabolism that was now working at several times the rate of a normal human.
Curious about his sudden surge in power, Ren called up the system interface.
[Host: Ren]
[Race: Human]
[Rank: Kanoto]
[Comprehension: High]
[Aptitude: High]
[Talent: Enhanced Hearing]
[Skills: Bandaging (Mastery), Thunder Breathing First Form: Thunderclap and Flash (Mastery), Thunder Breathing Second Form: Rice Spirit (Mastery), Thunder Breathing Fifth Form: Heat Lightning (Mastery), Total Concentration Breathing: Constant (Proficient)]
"I jumped two whole ranks? Is this the raw power of 'Constant' breathing?"
Ren was genuinely astonished as he looked at the "Kanoto" rank. In the Demon Slayer Corps, there were ten ranks: Mizunoto was the tenth and lowest, while Kinoe was the first. Ren had skipped over Mizunoe entirely and landed squarely at the eighth rank, Kanoto. Furthermore, he felt a faint, buzzing sensation in his chest, a sign that he was already nearing the cusp of the seventh rank, Kanoe. At this rate, he would reach the upper echelons of the Corps before he even officially joined.
His stomach suddenly let out a thunderous growl, reminding him that high-level breathing came with a steep caloric price. He retreated into the wooden house and began devouring rice balls. Usually, three or four would leave him stuffed. Today, he stubbornly polished off more than twenty before his body finally signaled that it had enough fuel to sustain his new, hyper-accelerated state.
Refreshed, Ren stepped back into the courtyard. His eyes were fixed on the wooden practice sword. He realized that with his new "High" comprehension and aptitude, he no longer needed Gramps to painstakingly walk him through the remaining forms. For a year, he had watched Zenitsu and the senior disciple, Kaigaku, practice these moves. Back then, the details were a blur of confusion. Now, those memories were like a high-definition recording playing in his mind.
He closed his eyes, visualizing the Third Form: Thunder Swarm.
The key to this form was high-speed, centrifugal rotation. One had to circle the enemy with such velocity that they left behind ripples of lightning, creating a buzzing sound that mimicked a swarm of angry mosquitoes.
Ren moved. He didn't just step; he spun. His body became a blurring top of golden light, his blade tracing jagged arcs in the air. The courtyard was suddenly filled with a high-pitched, electric hum, the unmistakable vzzzzzt of a thunder swarm.
He came to a halt, staring at the sword in his hand. "That... was it? I learned the Third Form on the very first try?"
It was almost laughable. The move that had once felt impossible was now intuitive. He realized that the various forms of Thunder Breathing weren't isolated skills; they were branches of the same tree. His absolute mastery over the First, Second, and Fifth forms provided the foundational logic he needed to "solve" the others.
Over the next few hours, he moved on to the Fourth Form: Distant Thunder and the Sixth Form: Rumble and Flash. The Fourth Form, a long-range streak of multi-pronged lightning, came easily. The Sixth Form was more complex, a series of rapid, multi-stage strikes that required immense stamina. Even with "Constant" breathing, using the Sixth Form twice in a row left Ren's lungs burning.
"Ren!! Help me! I'm going to die! I'm already dead! You're looking at a ghost!"
Zenitsu's characteristic wail shattered Ren's concentration. He opened the courtyard door to find Zenitsu sprawled face-down in the dirt, looking like a discarded yellow rag. His face was a messy cocktail of tears, snot, and mountain dust.
"Ren! I'm finished! Gramps is a tyrant! I'm going to expire right here on your doorstep!" Zenitsu shrieked, crawling forward to grab Ren's ankles with a death grip.
"Caught again?" Ren asked, his forehead creasing with a few black lines of exasperation.
"He was waiting behind a tree! A tree, Ren! Who does that?! He hit me so hard I think my soul actually left my body for a second. Look at this bruise!" Zenitsu pulled up his sleeve to reveal a small purple mark that he treated like a mortal wound.
Ren sighed and turned to get his ointment, but Zenitsu wouldn't let go. He clung to Ren's leg as he was dragged across the floorboards.
"Ren, if I don't make it through tomorrow... you have to live for the both of us," Zenitsu sobbed, his voice muffled by Ren's trousers. "Marry a beautiful girl... have three, no, five children. Name the loudest one after me. Don't let my memory fade into the cold, dark night!"
"Let go, Zenitsu," Ren muttered, trying to shake him off.
"I can see the light, Ren! It's so bright! Is that a giant peach in the sky?!"
"I said, let go."
By the afternoon, Gramps had finally managed to peel Zenitsu off the floor for one last training session. This time, Ren joined them. While Zenitsu was sent to the back mountain to practice his "suicide-prevention" sprints, Ren followed Gramps to a high ridge overlooking the valley.
Throughout the afternoon, Ren demonstrated his progress. Gramps watched in stunned silence as the boy who had been a "cripple" twenty-four hours ago executed the Third, Fourth, and Sixth forms with increasing fluidity. By sunset, Ren had reached "Mastery" in almost every form.
Gramps stood at the edge of the ridge, his white hair whipping in the wind. He looked at Ren, who stood tall with a practice blade strapped to his back.
"The selection begins the day after tomorrow," Gramps said, his voice unusually soft. "You and Zenitsu must leave at dawn."
Ren nodded. He had been waiting for this. The "safe" portion of his life was ending.
"The world beyond Peach Mountain is a cruel place, Ren. Do not underestimate the demons. They don't have hearts, and they don't have mercy. You must..."
"Gramps! Is training over? Can I stop? I think I've developed a fatal case of leg-cramp!" Zenitsu's voice drifted up from the forest below.
Gramps' eye twitched. The moment of gravitas was gone. He looked at Ren with a weary, knowing expression. "Look after him, Ren. He has the heart of a hero, but the brain of a frightened rabbit. Don't let him get eaten."
"I will," Ren promised. He knew Zenitsu's true strength would eventually emerge, but for now, he was happy to be the shield his friend needed.
"And Ren..." Gramps paused, placing a heavy, calloused hand on Ren's shoulder. "You must be careful too. You are a genius I never expected to see. Do not let that genius be extinguished in some dark corner of Mount Fujikasane."
Ren felt a rare pang of emotion and nodded. "I'll come back, Gramps. We both will."
A few minutes later, Zenitsu came huffing up the trail. He froze when he saw the serious atmosphere. "What? What's going on? Are you guys planning my funeral? I knew it! I knew I was dying!"
He lunged at Ren, hugging him around the waist and sobbing into his belt. "Ren! Don't let them bury me in a cheap casket! I want silk! Lots of silk!"
Gramps cleared his throat loudly. "Zenitsu! There is no training tomorrow."
Zenitsu's sobbing stopped instantly. He looked up, his eyes wide and hopeful. "Really? No training? Gramps, are you serious? Have you finally seen the light? Am I free?!"
"You are leaving for the Final Selection tomorrow," Gramps said bluntly.
Zenitsu's face went from pure joy to absolute terror in a fraction of a second. "The Final Selection?! You mean the place with the demons? The mountain where they keep the man-eaters behind the wisteria flowers?! No! No, no, no! Ren and I will die! Especially Ren! He hasn't even trained! He's just a regular guy! You're sending a lamb to the slaughter!"
Gramps looked at Ren, then back at the hysterical Zenitsu. He didn't bother explaining. If even Ren couldn't pass the selection, then the Demon Slayer Corps might as well close its doors forever.
"Pack your bags," Gramps said, turning away. "The lightning doesn't wait for cowards."