Kokusibo looked up, staring at El who stood in the corridor, above him like a bat, with sharp, analyzing eyes. "There's no mistaking it... That move... Must be the Sun Breathing Style," he muttered, emphasizing, his eyes narrowing.
The frown on Kokusibo's forehead deepened, seemingly in deep thought, "Where did that human learn that?" he muttered again. But soon Kokushibo shook his head softly. "No, that's not important right now. I have to be careful."
On the other hand, El with a flat expression and a cold look said quietly, "If I had been hit earlier, my body would have been full of cuts."
'This is strange... I'm already inside Infinite Castle, but why a side mission: Infinite Castle Invasion hasn't been declared complete yet?' El thought in her mind, his expression remaining calm despite the confusion in his eyes.
'Perhaps I should kill one of the Upper Moons here?' El wondered, while thinking about which Upper Moon to kill.
El paused for a moment, observing Kokusibo with an intense cold gaze. Then, with no hesitation in his voice, he said, "I won't hesitate anymore."
Within himself, El tried to communicate with Demon Slayer Mark, a very mysterious power. This caused Rinnegan's eyes to sparkle brightly, emitting an aura that made the space around subtly tremble.
Suddenly, a mark that looked like a maroon-colored scar glowing dimly in the dark, resembling a fierce flame, appeared and spread from El's left chest, even to his cheek.
The appearance of the mark seemed to trigger an explosion of power in his body. El's heartbeat quickened, his blood rushed, his body temperature increased, making his muscles and bones feel more alive, as if they were writhing with incredible strength.
Gripping Metsu tightly, the originally black blade of the sword turned bright red, emitting a heat that seemed to make the surrounding air sizzle.
However, amongst the familiar feeling of increasing power, El felt something different within his Rinnegan.
His vision had now changed, allowing it to penetrate to deeper layers, seeing parts of Kokusibo's body. "What is this... the world of invisibility?" he muttered in surprise, though his facial expression remained cool and controlled.
It didn't take long for El to understand the change in Rinnegan. "Is that so? The activation of Rinnegan together with Demon Slayer Mark triggered an extraordinary focus," he muttered, as he stared intently at the silent Kokusibo, his eyes fixed on El.
Kokusibo, who was able to access the World of Invisibility, noticed every blood flow and muscle contraction in El's body, noticing a significant change. "How could that human suddenly become so much stronger?" he muttered in astonishment.
Finally, Kokusibo's gaze fell on the blade of El's sword which had been black in color, now turning bright red due to the tremendous pressure.
Kokusibo was certain that if his body was slashed by that blade, his regeneration ability would have difficulty recovering to its original state.
Kokusibo frowned with a focused gaze. It didn't take long for him to realize the secret of El's sudden increase in strength.
"That mark... So that's what it is!" muttered Kokusibo in an increasingly serious tone, realizing the power of the Demon Slayer Mark emanating from El.
Before he could digest further, El's figure suddenly vanished, as if absorbed by spacetime itself, leaving a trail of destruction on the wooden corridor where he stood.
Kokusibo was startled, his eyes narrowed, trying to catch El's whereabouts which now seemed to merge with the shadows.
At first glance, El's figure appeared in Kokusibo's eyes, about to swing his sword that had turned bright red.
Realizing the imminent danger, Kokusibo deftly took out his old sword from inside his body. This was a sword that Kokusibo used before becoming a demon.
Kokusibo's sword made from his flesh and blood is powerful and has regenerative properties, but the attack from El's bright red Nichirin sword will still be very difficult to withstand.
The bright red Nichirin, which can be generated through El's techniques and her abilities as a Breath of the Sun user, has a special effect that weakens and inhibits demonic regeneration.
The bright red color of the blade is able to burn and suppress demonic power, including that of Kokusibo, even though he is a top-level demon with extremely high endurance.
Kokusibo may be able to withstand one or two initial attacks from the bright red Nichirin sword, however, gradually, the sword's flesh and blood will erode, and the regenerative ability will weaken.
Over a long period of time or in an intense battle, El's attack with the bright red Nichirin could completely destroy Kokusibo's sword and inflict a serious wound that would be difficult to heal, even for a demon as powerful as himself.
Dang!
The boom of two metals echoed loudly throughout Infinity Castle, creating powerful waves that cracked the wooden construction around them.
As Metsu's blade rubbed against Kokusibo's, red-orange sparks filled the air, adding to the tense atmosphere.
Kokusibo looked at El with a sharp gaze and a voice full of emphasis. "Where did you... learn the Sun Breathing Style?" he asked, tone flat yet full of deep curiosity.
El snorted, looking at Kokusibo with a slight mocking smile. "If I told you from Yoriichi," he said curtly, "would you believe me?"
Hearing Yoriichi's name come out of El's mouth, Kokusibo's anger instantly flared. He thought it was just nonsense. However, Kokusibo held back his anger, trying to think realistically.
How could a human live for hundreds of years? Besides, it was impossible for Yoriichi to have offspring, since his unborn child had been killed.
It was that tragedy that motivated Yoriichi to become a Demon Slayer, determined to finish off Muzan.
In an instant, from Kokusibo's body protruded a 50 cm sword, threatening anyone nearby. However, El, with Rinnegan's Eye of Insight combined with Demon Slayer Mark, immediately read the move.
El jumped back in a split second, keeping a safe distance from the sword that now adorned Kokusibo's body, like a cactus plant.
"You don't believe me do you?" said El coldly and flatly, "But this is the truth, I learned the Sun Breathing Style from Yoriichi, whether you believe it or not."
Kokusibo didn't say a word, just examined El's body with a sharp and serious gaze.
Slowly, Kokusibo retracted the swords that had emerged from his body, then sent a telepathic message, reporting to Muzan, 'Muzan-sama, this human is a Sun Breathing user.'
Muzan replied in a haughty tone, 'Kokusibo, for such a trivial thing, you don't need permission. Kill him, and reveal the secret of his body.'
"Yes, Muzan-sama," Kokusibo replied with confidence. The look in his eyes now implied a determination to finish El off, realizing that these were the orders he had to carry out.
El darted forward in an instant, quick as a blink of an eye, followed by Kokusibo who rushed to intercept his.
Clank!
A loud bang sounded as their blades met in a powerful collision. In the next second, El and Kokusibo came back into view, their blades interlocking, emitting sparks and creating an unstoppable wave.
Suddenly, the cracks in the corridor they were standing on widened, the supporting timbers cracked and brittle under the pressure of their strength, until they finally shattered, knocking down the two who still continued to fight in a fierce sword fight.
Clank! Clank! Clank!
As the two fell from the collapsed corridor, El and Kokusibo were still clashing swords in the air. It was like attacking, parrying and dodging but on the ground, falling further and further into the darkness of Infinity Castle.
Calmly catching his breath, El continued to increase the flow of oxygen and blood in his body to the extreme, using the Sun Breathing Style through her bright red Metsu. Each of his slashes created flashes of fire that ignited in the air, adding to the intensity of the fight.