By the time Tanjiro and his companions arrived at Ōsumi Village, the once prosperous and peaceful settlement had become a scene straight out of hell—utter devastation and misery.
The bodies scattered across the ground and the wreckage of buildings all bore marks of wounds caused by Thunder Breathing. Seeing them, Zenitsu felt his head spin, his spirit drained as though his very life force were being sucked away. His breathing grew ragged, his eyes bloodshot with fury.
Through gritted teeth, Zenitsu kept muttering the same name, over and over—
Inadama Kaigaku.
On the far side of the ruined village, Kaigaku, who had been gleefully wreaking havoc, suddenly stopped. As if sensing Zenitsu's presence, he crushed the neck of the villager he was holding and hurled the body aside before racing toward Zenitsu's direction.
When Kaigaku finally saw Tanjiro, Zenitsu, and Inosuke, his face twisted into a cruel, feral grin.
"So, you finally came…! Zenitsu! Today is the day you and your friends die!"
His sharpened fangs gleamed; his ears were pointed, his skin deathly pale, and black tiger-like stripes coiled across his face. The kanji for Upper Rank Five was etched into his eyes.
Overcome by rage, Zenitsu could no longer contain the storm inside him. He stepped forward, lightning crackling around his body, and launched the first attack.
"Thunder Breathing, First Form: Thunderclap and Flash!"
Zenitsu's figure vanished from sight. The sound of thunder ripped through the village as a blinding golden streak of lightning cut toward Kaigaku.
But Kaigaku only sneered.
"I've abandoned my humanity and gained power beyond compare! Compared to me now, your speed is pathetic!"
Without even invoking a Breathing Form, Kaigaku blocked Zenitsu's strike with sheer physical power and speed alone—then countered with a vicious kick that sent Zenitsu flying.
Inosuke leapt forward, catching Zenitsu before he crashed to the ground, while Tanjiro rushed in to cover them.
"Water Breathing, Third Form: Flowing Dance!"
Tanjiro's movements became fluid and swift like water, his body gliding through the air with such speed that multiple afterimages trailed behind him, confusing the eye.
Kaigaku smirked mockingly, as if toying with him. He gathered strength into his legs and burst forward with explosive force.
His speed dwarfed Tanjiro's by several times—his afterimages multiplied, sharper and clearer than Tanjiro's own.
Tanjiro swung his Nichirin Blade, but every strike met only Kaigaku's illusions.
Amid those afterimages, the real Kaigaku emerged—his sword flashing upward in a deadly arc aimed straight for Tanjiro's neck.
Feeling the chill of death at his throat, Tanjiro twisted his body at the last instant to deflect the blow.
"Water Breathing, Sixth Form: Whirlpool!"
Whirlpool—a form best used in unstable or uneven terrain. By spinning his body with fierce momentum, Tanjiro generated a powerful vortex that sharpened into a massive blade of water, intercepting Kaigaku's strike.
The two forces clashed violently. Tanjiro leapt back, creating distance between them.
His expression grew grim. The Kaigaku before him was nothing like the one they once knew.
Before, even when Kaigaku used Thunder Breathing, his movements could still be followed by the naked eye. But now—now, as a demon—he could achieve that same overwhelming speed without even using any Breathing Form at all.
For Tanjiro, Zenitsu, and Inosuke, this was their first time facing an Upper Rank demon without the aid of the Hashira.
Even though the three had grown strong—each now possessing the skill level of a Hashira—they could feel it.
The crushing, suffocating pressure of Kaigaku's strength.
"Zenitsu! Inosuke! Let's go!"
With just that single shout from Tanjiro, Zenitsu and Inosuke instantly understood his intent — the bond between them was that seamless.
"Hinokami Kagura — Setting Sun Transformation!"
Tanjiro took a deep breath, shifted his breathing style, and in the next instant, his body blurred upward into the air. With an inverted slash, he brought his blade down — a horizontal arc wrapped in flames blazing like the sun itself.
"Thunder Breathing —Fifth Form: Heat Lightning !"
Kaigaku unleashed an eerie surge of black lightning, striking directly toward Tanjiro, who was midair and unable to adjust his posture.
"Beast Breathing — Tenth Fang: Circular Fang Rotation!"
Before the black lightning could hit, Inosuke had already dashed ahead of Tanjiro. Wielding both Nichirin blades in a high-speed spin, he formed a whirling shield of steel that blocked the demonic thunder.
With Inosuke's help, Tanjiro's Setting Sun Transformation sliced forward unhindered — straight toward Kaigaku's head.
"Thunder Breathing — Sixth Form: Rumble and Slash!"
From the center of his body, Kaigaku erupted with countless lightning-shaped slashes radiating in all directions.
Faced with the storm of electric blades, Tanjiro couldn't break through Kaigaku's attack range — his Sun Breathing strike fell short.
On the ground below, Zenitsu's body crackled with lightning as he whispered:
"Thunder Breathing — First Form: Thunderclap and Flash — God Speed!"
In an instant, Zenitsu vanished, piercing straight toward Kaigaku with blinding velocity.
Seeing the technique that had once overwhelmed him — the Thunderclap and Flash: God Speed — Kaigaku's lips curled into a wild grin.
"So you've finally used it again, Zenitsu Agatsuma!" he howled with manic delight. "Ever since you defeated me with that move, I've thought of nothing but how to shatter it — to crush your spirit completely!"
Obsessed with surpassing Zenitsu's God Speed, Kaigaku had endlessly refined his Thunder Breathing forms — yet no matter how he tried, he could never reach that same transcendent speed.
Then, on one stormy, thunder-filled night, staring up at the sky, inspiration struck him.
Lightning first flashes within the clouds — the light comes before the bolt, and the bolt before the thunder.
From that, he realized:
The speed of lightning's flash exceeds that of the bolt, and the bolt itself is faster than the thunderclap.
A standard Thunderclap and Flash reaches the speed of thunder.
God Speed reaches the speed of lightning itself.
So, to surpass Zenitsu — to be faster than God Speed — one must attain the speed of lightning's flash.
But achieving that was nearly impossible. Even as a lower Moon demon, Kaigaku's demonic body could barely withstand the pressure of breaking past the speed of the bolt. Each attempt tore his flesh apart before he could ever reach the flash.
Only after Muzan Kibutsuji granted him more blood — elevating him to the rank of Upper Moon Five — did his strengthened demon body finally endure the strain and transcend the limits of lightning.
Kaigaku exultantly raised his Nichirin blade, aiming it at the onrushing Zenitsu.
"Thunder Breathing — Seventh Form: Flaming Thunder God!"