"Head Instructor Tsugikuni Yoriichi is fighting the Upper One!"
When this news spread from the mouth of Kasugai Crows and echoed across the Infinity Castle, everyone momentarily paused their battles.
'Lord Head Instructor is fighting!'
Every member of the Demon Slayer Corps had this thought flash through their minds.
'Upper One is fighting!'
On the other hand, all the demons were equally shaken by this news.
No choice.
After all, the famous head instructor of the Demon Slayer Corps, who could easily slay the Upper Ranks, causing its members to be absent for the first time in a century, was fighting the strongest Upper Rank.
Undoubtedly, there was no reason for this fight not to move everyone deeply.
"Moon Breathing - Second Form - Pearl Flower Moongazing."
On the top floor of the altar-like palace, the strongest demon under Muzan exploded from in front of the moon mural, rushing forward while spinning his body and delivering three slashes.
His three slashes, carrying many crescent moon blades, cut through the ground as they swept toward Riezel with such force it seemed they could shred the entire area to pieces.
Facing such a fierce and sharp slashing storm, Riezel didn't retreat but advanced instead.
*BOOM!*
He stomped his foot, igniting a blaze beneath him, and dashed forward with his back to the sun mural.
In the next moment, Riezel grabbed his scabbard and swung it like a blunt weapon toward the incoming crescent moon blades.
*Clang!*
With a metallic clash, Riezel deflected one crescent moon blade, sparking a burst of fire.
*Clang!* *Cling!* *Clang!* *Cling!*
A series of intense clashes followed, as Riezel's scabbard, now an afterimage, deflected all the crescent moon blades flawlessly.
After Riezel easily neutralized Kokushibou's attack, he flashed and darted straight toward Kokushibou.
"Moon Breathing - Third Form - Loathsome Moon, Chains."
Kokushibou immediately twisted his sword and performed two extremely broad slashes which released a storm of crescent moon blades.
As Kokushibou's slashes came too fast, his sword couldn't even be seen, only the slashes and numerous crescent moon blades of varying sizes could be seen, cutting through the air and slicing toward Riezel.
However, in this instant, Riezel's figure suddenly vanished, leaving the crescent moon blades to slice through the empty ground, shredding the floor and sending countless fragments of debris flying.
"?!"
Kokushibou's heart screamed with warning signs as he suddenly ducked and squatted to the ground.
*Swish!*
Almost at the same moment, a still-sheathed sword, whistled past Kokushibou's head, grazing his scalp and nearly blowing his head apart.
"Not bad."
Riezel's calm voice reached Kokushibou's ears.
"Dodging that strike shows your strength is way beyond the other Upper Ranks. As expected from a swordsman who awakened the Mark and the Transparent World before becoming a demon. You must've sensed my next move through the Transparent World, yes?"
Unfortunately, Riezel's words of praise didn't make Kokushibou feel honored at all.
"Moon Breathing - Sixth Form - Perpetual Night, Lonely Moon - Incessant!"
Kokushibou turned around and slashed toward Riezel, who stood behind him.
It was a high-speed combo slash far faster than before.
It was a multitude of curved slashes several meters, which released a wild barrage of crescent moon blades, enveloping Riezel in a storm of slashes, leaving no room to retreat.
In an instant, the spot where Riezel stood was instantly reduced to fragments, while the floor and the nearby pillars were all neatly cut apart.
*Rumble—*
Suddenly, the palace echoed with a heavy rumble.
As the sound of pillars collapsing and the floor breaking mixed together, the entire palace began to shake, causing the open rooftop to raise a cloud of dust that resembled a natural disaster.
Kokushibou's fierce attacks and strong slashes were evident from this.
In fact, the Moon Breathing he used was a breathing style derived directly from Sun Breathing, similar to the five fundamental breathing styles. Yet, it wasn't entirely accurate to call Moon Breathing a derivative of Sun Breathing, as it was more of a degraded version.
Due to the harsh conditions of Sun Breathing, Michikatsu created his own degraded breathing style when he couldn't withstand the sun's power while learning Sun Breathing from his younger brother, Yoriichi.
Ironically, Moon Breathing's existence was a testament to Michikatsu's inferiority to Yoriichi, for no matter how bright the moon was, it could never outshine the sun.
Moreover, this degraded breathing style constantly reminded Michikatsu that he was always inferior to his chosen brother, even after awakening the Demon Slayer Mark and the Transparent World.
After all, Yoriichi was born with the Demon Slayer Mark and already understood and awakened the Transparent World from childhood, an achievement no one could match.
Nevertheless, this didn't mean Moon Breathing wasn't powerful.
On the contrary, because it corresponded with the 'Sun', in terms of sheer power, Moon Breathing surpassed all derivative and branch breathing styles, including the five fundamental breathing styles.
Its effect was to release a wide range of sword auras, with slashes always accompanied by numerous crescent moon-shaped blades. All of its sword forms were high-speed, wide-range killing moves with extremely shocking lethality.
In Michikatsu's hands, this breathing style's power was almost infinite. Coupled with him mastering the Demon Slayer Mark and the Transparent World, its power was even more terrifying.
Consequently, even before becoming a demon, Michikatsu was undoubtedly a Hashira-rank swordsman. Although he couldn't match the broken existence of Yoriichi, he wouldn't lose to any current Hashira, including Gyoumei, praised as the strongest and the pillar of the current Demon Slayer Corps.
In Riezel's view, Michikatsu, before becoming a demon, had power equivalent to the limit of Level 4. Compared to the current Gyoumei, who had also awakened the Demon Slayer Mark and Transparent World, he was definitely on par.
In such circumstances, after Michikatsu abandoned his past, transformed into a demon, and became the Upper One, Kokushibou, his power had undoubtedly reached a higher level.
Now, hundreds of years had passed.
In these centuries, since Kokushibou had devoured countless humans and increased his strength, his power had reached the true supreme realm of this world.
Just as Riezel had previously speculated, Kokushibou was at Level 5, completely above the other Upper Ranks.
In this era without Yoriichi, in terms of individual strength, besides the monstrous Muzan who had lived for a thousand years, no one could match Kokushibou.
In the original work, Kokushibou not only single-handedly fought against Sanemi, who had used the Demon Slayer Mark, but also against Gyoumei and Muichirou, who had both used the Demon Slayer Mark and the Transparent World, ultimately managing to kill one of them.
If it had been the Riezel before arriving in this world, with his status not yet reaching SSS-rank and without mastering Sun Breathing, it would have taken considerable effort to deal with such a first-class adventurer-level demon.
But now?
*BANG!*
In the slash storm that could shred anything, Riezel, wrapped in flames, emerged unscathed and kicked Kokushibou's chest, sending him flying.
*Cough!*
*BOOOM!*
Kokushibou immediately spat out a mouthful of blood, crashing into a pillar and shattering it into pieces.
As the fragments of the stone pillar fell, they instantly buried Kokushibou, making him disappear from Riezel's field of vision.
Riezel, however, did not intend to stop his attack. With his sword still sheathed, he slashed toward the pile of rubble mercilessly.
*BOOOM!*
As his slash hit the rubble, a scorching explosion blasted it, sending debris flying.
Kokushibou emerged from the rubble, rolling on the ground along with various debris, tumbling a considerable distance.
*Cough!* *Cough!*
Kokushibou lay on the ground, coughing up blood, his body completely charred by flames.
"Damn it…"
Feeling the unbearable pain, Kokushibou gritted his teeth, shakily stood up from the ground, and looked at Riezel with both seriousness and anger in his eyes.
The seriousness stemmed from Riezel's great strength, which had far surpassed his imagination.
He knew Riezel was strong and could easily kill an Upper Rank, but to behead an Upper Rank in the blink of an eye was something he could also do, so he didn't think he would be utterly powerless against Riezel.
In his view, Riezel was indeed strong, but that didn't mean he would lose to him. They might be evenly matched, with their strengths at the same level.
However, he never expected that even when facing him, Riezel would still seem so relaxed, without even breaking a sweat.
Such overwhelming power made Kokushibou's heart tremble, and for a moment, he even thought it was his younger brother standing before him.
Remembering that monstrous existence, as powerful as a god—could there really be a second one in this world?
Was that even possible?
Kokushibou's anger gradually arose.
In fact, before Yoriichi died, he had met him one last time.
It was during that meeting that Yoriichi died, right in front of his eyes.
For this reason, he had sworn long ago that he would never lose to anyone again in this life.
Yet now, in front of Riezel, he was not only in a miserable state and completely outmatched, but Riezel hadn't even drawn his sword and was merely using its scabbard.
"Are you looking down on me?!"
Yelling angrily, Kokushibou glared at Riezel with his six eyes, which were almost popping out of their sockets.
Riezel gazed at the enraged Kokushibou, his expression calm, even had a hint of indifference.
"Don't get me wrong. You're still alive because I have a question for you."
Riezel's tone sounded like he wasn't in a battle but merely completing a tedious task.
"Where is the throne?"
Yes, the so-called 'throne' was all Riezel cared about right now.
At this moment, he had completely lost interest in Kokushibou.
Previously, he had had some expectations for this Upper One, but now it seemed Kokushibou was also no match for him.
Given this, Riezel's goal was left with only one thing—Kibutsuji Muzan.