"My god! Nobody said it would be this intense!"
On a dense forest path in Mount Momoyama, Izumo Tatsuya's wails shattered the morning silence. He looked absolutely wretched—his once-clean training uniform was now caked in mud and grass, and a crushed leaf dangled from his temple.
"Master! Is this what you meant by a 'simple post-meal jog'? I'm going to die!"
This wasn't a "simple jog." It was a death trap filled with "surprises."
He had just narrowly ducked under a swinging log when the ground beneath him vanished. A snare hidden under dead leaves snapped shut, hoisting him into the air by his ankle.
"Wuaah!" Tatsuya dangled like a piece of dried meat, swinging back and forth against the tree trunk.
"Fool!"
Jigoro Kuwajima's powerful shout boomed from nearby. The old man stood on a protruding rock, leaning on his peach-wood cane like a strict foreman.
"Do you keep your eyes on the top of your head? You look forward but ignore your feet? A demon's attack can come from any direction! If you make the same mistake again, your dinner tonight is halved!"
"Master, no! You can ask anything of me, but please don't touch your dear disciple's dinner!"
As Tatsuya swung in the air, his mind screamed: This old man's traps are more devious than a demon's! Are we training to be swordsmen or ninjas?!
This was only the beginning. For the rest of the course, he had to dodge spring-loaded bamboo spikes, leap over cleverly camouflaged pitfalls, maintain balance on swaying logs, and stay alert for wooden balls wrapped in cloth that flew from every angle—hitting with enough force to leave bruises.
"Ow!" "Too slow! A snail crawls faster than you!" "Hah... hah...! Master, let me... catch my breath..." "Your breathing is erratic! Do you plan to suffocate yourself halfway through the run?"
A log swung horizontally, forcing Tatsuya to leap into the air. "I dodge!"
"Too much movement! A waste of energy! Are you dancing or slaying demons?"
Kuwajima's sharp critiques were like cold rain, soaking every mistake Tatsuya made. Tatsuya gritted his teeth, swallowing his complaints. Relying on a month of forged willpower and his extraordinary reflexes, he navigated the gaps between the traps.
His arms burned with pain where logs had grazed him, but he didn't dare stop. He knew stopping meant harsher punishment—or being beaten into a pulp.
When the day's training finally ended, Tatsuya practically crawled back to the small, simple shack on the mountainside. He hauled a bucket of ice-cold spring water and poured it over himself. The biting cold made him shiver, but it washed away most of the exhaustion.
He walked to a blurred bronze mirror and wiped away the water droplets. The person reflected in the mirror was a different boy from a month ago.
At fifteen, his frame had begun to fill out, shedding the softness of childhood. His shoulders were broad, his chest thick, and his limbs long and balanced. His once-slender body was now covered in a layer of smooth, powerful muscle—not bulky or bloated, but lean and explosive, like a leopard.
Water droplets slid down his firm abdominals and arm muscles, glistening in the dim light of the oil lamp. Most striking was his coloration. His short black hair was neat, but a few strands of brilliant gold were "dyed" into his sideburns and the nape of his neck, looking like sunlight piercing through dark clouds. His clear, sharp golden eyes were tired, yet they flickered with a defiant light. He was a handsome young man who had been through the fire and was beginning to show his edge.
"Tch, I barely recognize myself," Tatsuya muttered, poking his rock-hard abs. "I just wonder if this muscle can survive the old man's next round of 'affection'."
Life on Mount Momoyama consisted of day after day of systematic, brutal foundation training.
Every morning before dawn, Tatsuya had to strap heavy sandbags to his limbs and run up and down the steep mountain paths. His lungs worked like bellows, and his legs felt like lead. Every step was a battle against gravity. Kuwajima would follow him, moving with a lightness that defied his age, shouting: "Adjust your breathing! Imagine the air sinking into your core! If this weight makes you grimace, how will you ever wield a real Nichirin Sword?"
After breakfast came the obstacle course: an upgraded version of the "exciting mountain path," with longer routes, denser traps, and a requirement for higher speed. Tatsuya had to maintain extreme focus while his physical strength drained. Kuwajima's cane would strike his mistakes without mercy: "Right here! You were 0.1 seconds slow! If that were a demon's claw, your guts would be on the floor!"
Before lunch was explosive power training: short-distance sprints, frog jumps, and sudden bursts of speed from a crouched position. It required squeezing every ounce of power from his body in an instant. Tatsuya often fell and face-planted into the mud from sheer exertion.
Kuwajima would watch coldly. "Explosion isn't brute force! it's coordination! It's focus! Twist your power into a single rope!"
Afternoons were for strength training: push-ups, pull-ups, and swinging heavy wooden dumbbells. These dumbbells, handmade by Kuwajima, were much heavier than they looked, making Tatsuya suspect they were filled with solid iron. Every tear and regrowth of his muscle brought throbbing pain and hidden growth.
After dinner came the Suburi (swinging practice): This was the most tedious part of his swordsmanship foundation. Holding a massive wooden sword, he repeated basic strikes, slashes, and thrusts thousands of times. The goal was to turn correct form into muscle memory—to make the arm, the wrist, and the entire body one with the blade.
At first, his arms were so swollen he couldn't even hold a rice bowl steady. Kuwajima corrected every tiny angle: "Drop your wrists! Use your core! The sword is not an extension of your arm—it is an extension of your WILL! And your breathing? Have you forgotten to sync your breath with your movement?!"
Throughout all this, Kuwajima emphasized one core principle: a powerful physique is the vessel for everything.
"Breathing Styles are techniques to squeeze out human potential and surpass limits," Kuwajima explained during a rare break. "Without a strong enough vessel, forced breathing leads to only one end..." He paused, his gaze cold. "Bursting blood vessels and heart failure. You'd pop like an overfilled balloon. Is that what you want, boy?"
Tatsuya imagined the scene and shook his head like a bobblehead. He finally understood the purpose of these torturous repetitions. They were building a fortress—a body capable of containing the violent power known as Thunder Breathing.
As the days passed, Kuwajima watched Tatsuya practicing his swings under the setting sun, gritting his teeth with unwavering discipline. A rare, almost imperceptible trace of pride flashed in the old man's eyes.
This boy... Kuwajima thought. I thought I had estimated his natural talent and physical ceiling, but his progress over this month has shattered my expectations.
A normal trainee would take at least half a year to reach this level of fitness and basic form. He did it in one month. This bone structure... this resilience... I have never seen anything like it.
However, his face remained as still as a deep well. I cannot tell him. A raw gem needs careful polishing; geniuses break easily. A single word of praise might breed arrogance, and that would be his downfall.
So, he opened his mouth and yelled: "Tatsuya! Your wrist is loose again! Give me another five hundred swings!"
He was satisfied to hear his disciple's agonized groan.
Once Tatsuya's physical foundation and form reached the standard, Kuwajima began teaching him systematic swordsmanship—not just slashes, but parrying, deflecting, thrusting, and rotating strikes.
Tatsuya learned fast, but a question nagged at him. "Master, you said Thunder Breathing is incredible. Why don't you just teach me the Breathing Style directly instead of all these technical drills? Wouldn't I kill demons faster with it?"
Kuwajima snorted. "Stupid! The Breathing Style is the source of power; swordsmanship is the method of applying it! If the Demon Slayer Corps relied solely on a few breathing forms, we would have gone extinct long ago!"
He spoke sternly: "Demons are not mindless beasts. They communicate. Over centuries, our forms have been studied by powerful demons. If you only know how to use fixed 'Forms,' you are a dead man the moment you meet a demon who has figured you out!"
Kuwajima's expression darkened slightly. "And among the demons, there are those who are familiar with Breathing Styles... some can even use them." The old man instinctively touched his missing leg. "A demon born from a fallen Slayer is a thousand times more terrifying than a normal one."
He whispered those last words so quietly Tatsuya couldn't hear. Then, he pointed to Tatsuya's wooden sword. "True combat is ever-changing. You need a rock-solid foundation, the ability to adapt, and the skill to drive your blade into a demon's neck under any circumstance!"
"Only by perfectly combining the power of Breathing with flexible, unpredictable swordsmanship can you survive the slaughter. Do you understand?"
Tatsuya felt a chill and answered solemnly, "Yes, Master! I understand!"
But he hadn't missed his master's hand touching his stump. Was Master's leg lost to a demon who knew our techniques?
Three months passed in a flash. Tatsuya's body was lean and hard, and his swordsmanship was fluid. The heavy wooden sword felt light as a feather in his hand.
One morning, Kuwajima called Tatsuya to their usual training clearing. The old man's expression was more serious than ever before.
"Tatsuya, I have accepted your foundation," Kuwajima began slowly. "Today, I shall teach you the Breathing of Thunder."
Tatsuya's heart skipped a beat. After three months of anticipation, he was finally going to touch that mysterious power!
However, Kuwajima didn't start with explanations. He simply said, "Watch closely. This is one application of Thunder Breathing—"
In an instant, the old man's slightly hunched frame straightened. An invisible, heart-stopping aura exploded from him! It felt as if countless golden sparks were dancing in the air, creating a faint, soul-shaking crackle.