As Riezel breathing in and out, the abnormality around him quickly caught everyone's attention.
'His fighting spirit is... changing?' Akaza's eyes narrowed slightly.
In his perception, Riezel's fighting spirit suddenly changed. It wasn't a drastic change, still as massive and strong, but there was a sense of dangerous substance flowing within it, at least for him.
Unlike Akaza, the others each sensed the abnormality differently.
"His heartbeat suddenly became so fast!" Zenitsu, who had been crouching with his hands on his head, blurted out nervously.
"There's a tingling sensation all over my skin, what is he doing?" Inosuke rubbed his exposed arms.
"The rhythm and precision of his breathing have changed..." Kyoujurou sounded surprised and puzzled.
As for Tanjirou, he stared blankly at Riezel at this moment, feeling strangely familiar with something about him.
At this moment, the aura emanating from Riezel's body gave Tanjirou a sense of deja vu as if he was seeing his deceased father. In a daze, he even felt as if he was looking at a snowy landscape, seeing his father wearing priest attire dancing the Kagura dance.
In a mountain infested with spiders, a demon stood silently.
One of his eyes bore Japanese characters.
Unlike Akaza, who had both eyes engraved with characters, this demon only had his left eye bore two characters.
One was 'Lower Rank' and the other was 'Five'.
It was unmistakably Rui, the Lower Five who was recently defeated.
Several months ago, Tanjirou encountered this Lower Five and battled with him.
In that battle, Tanjirou was ravaged, subjected to unreasonable violence, repeatedly injured, and knocked down, until his sword broke like a butterfly caught in a spider's web, becoming prey to Rui.
Rui shot threads capable of shredding a human body into pieces toward him, weaving them into a net, literally leaving him with nowhere to escape.
However, in the very end, Tanjirou unleashed 'that' technique.
'Hinokami Kagura - Dance'.
It was the Kagura dance his father once performed in the snow.
It was the dance his father had taught and used.
Just as Tanjirou was forced into a corner facing the arrogantly impossible-to-defeat spider demon, he unleashed the Kagura dance passed down through generations of his family, even resorting to using unknown breathing, unleashing astonishing power, momentarily overpowering Rui.
In truth, the reason Tanjirou came to the Mugen Train to find Kyoujurou, aside from the mission from the Demon Slayer Corps, was for another reason—he wanted to ask Kyoujurou if he knew why Hinokami Kagura had turned into a breathing style usable for combat.
All of this was the idea of the current Insect Hashira, Kochou Shinobu.
As one of the few Hashira who harbored goodwill toward Tanjirou, upon learning about Hinokami Kagura from his mouth, Shinobu thought that Kyoujurou, who mastered Flame Breathing, might know something.
Perhaps because Hinokami Kagura and Flame Breathing were quite similar in power and characteristics, both employing swordsmanship that conjured flames.
For this reason, Tanjirou came here, to Kyoujurou's side, to uncover the secret of Hinokami Kagura passed down through generations of his family.
Unfortunately, before Tanjirou could find the opportunity to inquire, the incident on the Mugen Train occurred.
And now, Tanjirou was surprised to find himself facing off against the Upper Three, with Riezel holding Kyoujurou's Nichirin Sword in his hand.
Upon entering the state of Total Concentration Breathing, Riezel's appearance was identical to that of himself and his father when using Hinokami Kagura.
'What's going on?'
Unfortunately, Tanjirou couldn't utter the question in his mind, because he smelled danger.
"Watch out!"
Sensing the scent coming from Akaza, Tanjirou couldn't help but shout frantically at Riezel.
However, before his voice could fully resonate, Akaza's impending killing move was already unleashed.
"Destructive Death - Annihilation Type!"
A terrifying power suddenly erupted around Akaza's body, forming a dreadful shockwave, like an explosion, accompanying his two fists as it surged toward Riezel.
This was the most powerful technique among Akaza's martial arts techniques, named 'Annihilation Type'.
It concentrated all his fighting spirit and strength into the palm of his hands, which he used to strike his target with tremendous force, creating two powerful circular shockwaves.
Feeling the increasingly formidable threat from Riezel's fighting spirit, Akaza no longer held back, directly unleashing this technique.
If Akaza stood in Riezel's place, facing the charging Mugen Train on the train tracks, this might not have directly destroyed the entire train, but the locomotive that collided directly with this technique would certainly have been crushed.
Such a tremendously powerful attack, like a tsunami, swept out like an exploding shockwave, tearing through the earth, tearing through the air, and thundering toward Riezel.
"?!"
Seeing this, the faces of Kyoujurou and the other three changed drastically, all shouting something to Riezel, but their voices were drowned out by the explosion-like shockwave as they were blown away by the forceful wind and thrown from their position.
Riezel, holding Kyoujurou's Nichirin Sword, stood directly against this attack, giving the impression of a tiny human about to be engulfed by a tornado, powerless and fragile.
Yet, faced with this utterly destructive attack, Riezel unexpectedly closed his eyes.
*BOOOM!*
A shockwave, like an explosion, instantly engulfed Riezel's body, shattering the ground beneath him.
But within this astonishing shockwave, flames emerged.
Like the sun rising at dawn.
Like light appearing in the darkness.
As the flames ignited, it suddenly expanded.
*WHOOSH!*
Amidst the roar, the shockwave was sliced in half by the erupting flames.
At this moment, a fierce wind howled, sand filled the air, and the severed shockwave swept in all directions, stirring up an unstoppable gust, whistling as it went.
Meanwhile, the flames that cut through the shockwave leaped into the air, transforming into a dazzling fiery wheel, burning through the atmosphere, pushing back the impact.
Like a fleeting ray of sunlight, it suddenly tore through the void, slashing toward Akaza!
In everyone's eyes, this stunning display was akin to the embodiment of the sun gracefully dancing into existence.
Akaza only had time to shrink his pupils to the size of a pinhead and couldn't do anything else, not even change his stance, and remained in the posture of unleashing 'Destructive Death - Annihilation Type'.
At this moment, the flying flames cut through his field of vision, slicing through his fighting spirit, and ultimately slicing through his neck, disappearing behind him.
*Swish!*
"Sun Breathing - First Form..."
Only then did Riezel appear in the spot where the flames vanished, standing behind Akaza, back turned to him, slowly sheathing his sword.
"Dance..."
*Click*
With the clicking sound of the sword sheathing, everything returned to calm in that single moment.
The shockwave vanished.
The flames vanished.
The destructive power vanished.
It was as if nothing had ever existed, as if everything that had just happened was merely an illusion, making the entire scene fall into silence.
"..."
Whether it was Kyoujurou or the three youths, they all lay on the ground, eyes wide open, watching everything unfold, completely devoid of sound.
Even Akaza fell silent.
Riezel also remained silent and standing calmly, his expression unchanged.
Strangely, this silence persisted for who knew how long.
"Hey... what's your name?"
At a certain moment, Akaza asked in a hoarse voice, a question that had previously gone unanswered.
Riezel didn't turn around but still heard Akaza's question.
Yet, upon hearing Akaza's question, Riezel's initial reaction wasn't to give his name but to recall that dream, recalling that swordsman who had meticulously taught him every move of 'Sun Breathing'.
*Ding—ling—*
In this silence, the hanafuda earrings on Tanjirou's ears swayed just right, emitting a wind-chime-like sound.
"Tsugikuni Yoriichi..." Riezel uttered 'his' name.
"Tsugikuni... Yoriichi…" Akaza's voice began to slow down, lowering. "I will... remember that name..."
As his words fell, his head gradually tilted from his neck and fell from his body.
*Thud!*
His body collapsed as well, devoid of any remaining aura.
Needless to say, Upper Three Akaza had been defeated.
Facing an enemy whom even the Hashira found terrifying, a young man who had not yet joined the Demon Slayer Corps, was not a Demon Slayer, and didn't even possess a Nichirin Sword, slew an Upper Rank with just one slash.
This was an astonishing feat.
This was a terrifying achievement.
Kyoujurou and the other three stared dumbfounded at the scene, unable to snap out of it for a long time.
'…Dance?'
Tanjirou, staring blankly at Riezel, recalled the radiant and beautiful sword technique Riezel just displayed and felt his mind in chaos.
Indeed, Tanjiou's feeling wasn't wrong. When he saw the movement Riezel had just displayed, it was indeed very familiar to him.
Whether it was the Hinokami Kagura Tanjirou used against the Lower Five on Mt. Natagumo several months ago or the sword technique Riezel just used against the Upper Three, both were named 'Dance'.
'Sun Breathing?' Tanjirou's mind was in disarray, completely losing his ability to think.
Unaware of all this, Riezel had already walked over to Kyoujurou.
"Nice sword but it's too heavy for me." Riezel smiled faintly as he returned Kyoujurou's sword.
At his words, Kyoujurou smiled wryly.
After all the struggle, the events of the Mugen Train were truly coming to an end.
About ten minutes later, several swordsmen wearing the uniforms of the Demon Slayer Corps, along with a group of masked individuals, swiftly arrived from the other side of the train tracks to clean up the scene, tidying up the aftermath.