*Thud!*
With a faint landing sound, Riezel touched down on the rooftop of a building.
It was a palace.
Riezel landed on the open rooftop of this palace and entered a place that resembled an altar.
In this altar, the surrounding walls were adorned with murals, particularly on two sides that were not only facing each other but depicted entirely opposite scenes.
On the left mural was a sun, radiating light in all directions, beneath which were many people extending their hands toward it in a gesture of worship, resembling a ritual to a god, or a hero.
In contrast, the right mural depicted a moon emitting light in all directions but with no people or animals below, only a desolate nighttime landscape that appeared somewhat barren.
Riezel stood directly before the sun mural, facing the moon mural on the opposite side.
"Hmm, the so-called throne doesn't seem to be here..."
"Upper One."
As Riezel said this, a figure emerged slowly from the shadow of the moon mural across from him.
He held a sword in his hand.
He was dressed in a kimono.
With long black hair tied up in a high ponytail, adorned with red markings on the top left side of his forehead and the bottom right of his chin, his face bore six eyes.
A six-eyed samurai from the Sengoku period had appeared beneath the moon mural.
Indeed, he was none other than the Upper One, Kokushibou.
"...It's you?"
Kokushibou gazed at Riezel with his six eyes, his vision cold and filled with murderous intent.
"...You call yourself Tsugikuni Yoriichi?"
Kokushibou, the strongest Upper Rank, posed such a question.
"What? Have a problem?" Riezel looked back calmly. "Or do you think I'm not up to par?"
Kokushibou did not answer his question but slowly drew the sword from his waist—a terrifying sword adorned with eyes on its blade and handle.
After drawing such a sword, Kokushibou spoke again.
"Whether you're up to par... let me test that."
*Whoosh!*
With those words, Kokushibou suddenly vanished on the spot.
"Moon Breathing..."
*Swish—*
A figure resembling a bright moon breaking through dark clouds suddenly appeared before Riezel.
"First Form - Dark Moon, Evening Palace."
It was an iai slash executed with such speed that the sword was barely visible, leaving behind a trail of crescent-shaped slashes. It was like a moon blade competing with the sun's brilliance, carrying a chilling coldness as it sliced toward Riezel's neck.
*Whoosh!*
In the next moment, the blade that conjured countless crescent moons was intercepted by two fingers, held between them.
"Hou?" Kokushibou's six eyes narrowed slightly. "You stopped it so easily?"
"Easily? Is it difficult for you?"
Kokushibou's words earned only a calm question from Riezel, clearly unmoved.
"Even some Hashira nowadays could easily block such a probing attack. If you want to test my limits, show me something more powerful."
With that, Riezel released the blade from his fingers and surged forward, delivering a fierce kick to Kokushibou's chest.
*BANG!*
Amidst the muffled sound, Riezel's sudden kick landed heavily on the timely sword raised by Kokushibou, unleashing astonishing force.
Kokushibou was sent flying backward as if struck by a train, his feet skidding violently across the ground, retreating a considerable distance.
Riezel's kick had directly knocked Kokushibou back beneath the moon mural, returning him to his original position.
Kokushibou stared at his hands gripping his sword, trembling slightly.
It was clear how much power Riezel had unleashed with that kick alone.
"Incredible strength..." Kokushibou muttered darkly. "A mere human... possessing such strength... No wonder the former Upper Five and Upper Six fell to you..."
Speaking like this, his six eyes revealed not resignation but a more complex emotion as if standing before him wasn't Riezel, but some other existence entirely.
"Your monstrous, inhuman strength... does bear some resemblance to him..." Kokushibou said coldly. "But I must say... you have no right to use that name... even if you've somehow learned Sun Breathing..."
"Heh..."
At his words, Riezel chuckled.
"Coming from you, that's quite the irony..."
Riezel retorted casually, shaking his head.
"At least someone is using that name, unlike you, who has forsaken even your own name, reduced to this grotesque appearance."
Upon hearing this, Kokushibou narrowed his six eyes, exuding an even stronger murderous aura.
"...You know about me?" Kokushibou asked in a low voice.
Naturally, the answer was yes.
"Many people who know about the history of the Demon Slayer Corps during the Sengoku era have heard of Tsugikuni Yoriichi, but they don't know that he actually had an older twin brother."
Riezel's voice slowly filled the air.
"He was once highly esteemed, eager to become a powerful swordsman."
"He was favored more than his younger brother by their father, seen as the sole heir of the family's legacy."
"Due to the strange mark on his younger brother's forehead from birth, his younger brother faced rejection and neglect from their family. Because of this, he gained all the love and growth environment that should have belonged to his younger brother."
"However, there was one thing he could never surpass his younger brother in... talent in swordsmanship."
Riezel's words caused Kokushibou's hand gripping the sword to tighten slowly, yet Riezel seemed oblivious, continuing unabated.
"Just like his younger brother, he had also joined the Demon Slayer Corps and became a Demon Slayer."
"In his goal to surpass his younger brother, he tirelessly honed his swordsmanship."
"Eventually, he not only awakened the Demon Slayer Mark and the Transparent World but also stepped into the Domain of Supremacy that martial artists dream of, becoming one of the strongest in the Corps."
"Even so, he still couldn't surpass his younger brother."
"Faced with his younger brother's dazzling achievements and unparalleled talent, he could only ever gaze at his back."
Such was Tsugikuni Yoriichi's elder brother, a man consumed by an obsession with strength, constantly striving to improve himself while harboring intense jealousy toward his supremely talented younger brother.
"Tsugikuni Michikatsu, that's his name..."
Riezel's eyes gazed calmly into Kokushibou's.
"How does it feel to hear your abandoned name after so long?"
Riezel's words caused Kokushibou to close his six eyes.
After a long while, he opened them again.
"Since it's a name I've long abandoned... what is there to cling to?" Kokushibou said lightly. "Tsugikuni Michikatsu has long been dead and the only one here is Kokushibou, the Upper One... that's all."
"You seem pretty relaxed..." Riezel remarked emotionlessly. "Especially... since you're the one who caused your younger brother's downfall."
Initially, Yoriichi was banished from the Demon Slayer Corps and even asked to perform seppuku for three reasons.
First, for allowing Muzan to escape.
Second, for sparing Tamayo voluntarily.
Third, for having a close relative within the Demon Slayer Corps who assisted Muzan in killing the then Master.
Truthfully, any one of these acts would be unforgivable for anyone.
Just like Zenitsu's master, the former Thunder Hashira, who took his own life simply because his disciple became a demon, and Tanjirou, who nearly committed seppuku for sheltering his demon younger sister.
When these three incidents converged on one person, even someone nearly legendary like Yoriichi, he was ultimately banished from the Demon Slayer Corps.
Michikatsu's betrayal was the final straw that led to this decision.
If the Master of the Demon Slayer Corps hadn't been killed, perhaps Yoriichi's contributions and abilities would have prevented his banishment from the Corps.
It was Yoriichi's elder brother who killed the beloved Master of the Demon Slayer Corps, plunging its members into irreparable sadness and anger, with no possibility of reconciliation.
Given this, Riezel's accusation that Kokushibou was responsible for Yoriichi's downfall was not unfounded.
However, Kokushibou clearly didn't see it that way.
"You... What do you understand...?"
Kokushibou spoke softly, his head lowered.
"Do you understand the feeling of never being able to catch up to someone by your side, no matter what...?"
"Do you understand the feeling of pushing yourself to the limit in training, only to fall short of someone else's casual attempt...?"
"Can you understand... that feeling when everything becomes futile... when even the right to strive is taken away...?"
As Kokushibou's chest began to rise and fall, he lifted his head, revealing bloodshot eyes fixed on Riezel's.
"You don't understand... because you, like him, are both naturally gifted..."
Kokushibou truly envied Yoriichi, envied his own younger brother.
Because of this envy, he despised Yoriichi to the point of anger whenever he thought of him.
His dream was to become a powerful swordsman, but that dream was effortlessly fulfilled by his younger brother, who had been ignored and looked down upon since childhood.
How could anyone else understand such a disparity?
If not for this, Kokushibou wouldn't have become a demon, betrayed the Demon Slayer Corps, and killed the then Master.
However, in truth, the main reason Kokushibou became a demon was actually the Demon Slayer Mark.
He was a Marked Demon Slayer, evidenced by the marks on his forehead and chin.
When he learned that Marked Demon Slayers would die before the age of twenty-five, his first emotion was resentment.
Resentment that he no longer had time to train.
Resentment that he no longer had the opportunity to surpass his younger brother.
Because of this, in order to gain eternal time to perfect his swordsmanship, Kokushibou accepted Muzan's invitation, became a demon, and killed the Master of the Demon Slayer Corps at the time.
In addition, Kokushibou also participated in the ruthless eradication of those who knew about Sun Breathing initiated by Muzan.
"Now that you've used his name... used his breathing style... be prepared..."
As Kokushibou spoke, a terrifying aura gradually emitted from his body, growing stronger by the second.
"I will definitely kill you right here... right now..."