Not to be outdone, Kokusibo, with his six sharp eyes full of determination, met El's every attack with precision movements.
His sword, which was made of flesh and blood, might be able to be effectively used to flow his Demon Blood Art that had been modified with the Moon Breathing Style, but it would not last long and was destroyed eventually.
That was why, Kokusibo preferred to use his old sword instead of using the sword made of flesh and blood, this was also to be prepared in case this battle happened
As they exchanged strikes and dodges with El, Kokusibo's body movements were like the shadow of the moon on the water. Each time their blades clashed, the sound of the boom grew louder and louder, shaking the empty space around them.
Sparkling sparks exploded in the air, along with visuals of burning embers, clashing with blue and golden yellow visuals like moonlight, surrounding them.
Each swing of El and Kokusibo's swords clashed with tremendous force, creating an impact that shook the buildings and corridors around them, decorating the space of Infinity Castle with sparks that radiated like exploding stars.
If he felt that his position was not favorable, then El utilized the air flowing around her to increase his speed, adjusting his posture to remain stable, even as he continued to fall in the endless abyss-like darkness of Infinity Castle.
As he glided swiftly through the air, El would occasionally spin, creating a succession of attacks in an impressive and sacred dance-like form. The red glow of her attacks illuminated the face of Kokusibo who remained focused and alert.
Although seemingly pressured by El's incredible swordsmanship, Kokusibo did not stay still. Calmly, he read El's attack patterns and activated the World of Invisibility, watching every movement of El's muscles to find an opening.
Seeing an opening in El's attack, Kokusibo immediately drew blood onto his old Nichrin sword, leaving it covered in blood. Calmly, Kokusibo said, "Tsuki no kokyū."
"Ni no kata: Shuka no Rōgetsu." In an instant, Kokusibo swung his sword with incredible power, creating three crescent-shaped slashes that glowed bright yellow.
Shing! Shing! Shing!
The blood on his sword splashed into the air, creating hundreds of tiny bright yellow crescent-shaped blades, simultaneously tearing into El from various directions.
The blades split the air with lightning speed, spinning and spreading around El, attacking him from all directions with terrifying sharpness.
"This is bad!" muttered El softly, realizing that his position in the air without a foothold precluded the use of Gen'nichi Kō to dodge.
With no other choice, El focused the flow of oxygen in the body's blood, hardening the skin to withstand the impact of this attack.
Kokusibo's crescent slash struck El's body brutally. Several sharp blades tore into his skin and muscles, leaving reddened wounds.
While the overwhelming impact of the striking attack left his right arm fractured with radiating pain.
El's body continued to tumble, rolling uncontrollably through the ruined corridors, shattering the wooden walls along its path.
Finally, El's body came to a brutal halt when it slammed into a giant pillar, which stood tall in one of the rooms inside Infinity Castle, creating a huge crack around it.
El recognized this place; the vast room with tall pillars rising up in Infinity Castle, the place where Gyomei, Sanemi, Tokitou, and Genya had once fought desperately against Kokusibo.
The place consisted of tall surrounding pillars rising from the floor to the ceiling, as if they were supporting an infinite world above.
Each of the massive pillars was made of jet-black wood, with strokes that looked alive, sinuous and as if they were moving under the dim light that filled the room.
The light emanated from mysterious cracks in the walls and floor, giving the impression as if the room had a terrifying and intimidating aura.
For the size of this place seemed to have no end, it was vast as far as the eye could see.
In his weakened state, El tried to maintain his consciousness. The stinging from the wounds from Kokusibo's last attack began to become more and more painful, and fresh blood flowed along her body, soaking her clothes.
El's breathing was labored, a sign that her stamina was running out.
Even Rinnegan's indigo glow in his eyes began to fade, the six tomoe orbiting like a solar system disappeared one by one, gradually El's body began to lose strength as it reached its limit.
El realized the Demon Slayer Mark's enhancement effect would soon disappear, making his extremely vulnerable. However, El still wore a determined expression, his weakened eyes still radiating enthusiasm.
Also, El is not Yoriichi Tsugikuni who has almost unlimited stamina. Because of this, El was still prone to fatigue even though he had activated Mark in the previous intense battle.
"Ugh... Apparently, my body has almost reached its limit due to the intense battle in the air," El muttered while trying to catch her breath, entering a brief pause to calm down and regain focus.
Though only momentary, this pause was significant, helping to restore stamina, while also easing muscle tension and reducing the risk of the extreme fatigue El was experiencing.
With more stable breathing and recovered stamina, El felt his body regain energy, enough to maintain the Demon Slayer Mark and minimize severe side effects.
In the world of Kimetsu no Yaiba, the active duration of the Demon Slayer Mark is not specified. But in general, its use is limited by physical condition and how long they can withstand the increased pressure on the body.
In other words, activation of the Mark can indeed significantly increase STR, VIT, and AGI, or represent increased strength, speed, and endurance.
However, these improvements come with severe consequences, accelerating physical fatigue as well as causing serious side effects.
The side effect of activating Mark for such a long period of time now, made El start to run out of stamina faster.
This was due to El activating Mark simultaneously with Rinnegan, plus the constant use of the Sun Breathing Style, which also drained his stamina and mental concentration.
The previous side effect even made El vomit blood, due to damage to some of her cells and organs, even though Mark was only active for a short time.
Logically, Mark's active duration ranged from 5 to 10 minutes in intense combat. However, El had already used Mark for more than 8 minutes in the fight against Kokusibo, even when they were down and fighting in the air.
Although Mark gave El enough time to utilize his increased strength and abilities without inflicting too much body damage, this time the situation was different.
Her opponent was Kokusibo, a Rank 1 Upper Moon Demon, with strength equivalent to Muzan. El guessed that Kokusibo was at the peak of Authority level 5, and if added with his combat skills, his attribute value could reach 130 points.
While El, even with the addition of passive skills, active skills and personal attribute values, only had a total of 127 points.
It's a slim margin, but El is undaunted. With the Sun Breathing Style, the pinnacle of human power, El was confident of being able to face Kokusibo even if she had to exceed her own limits.
"So far, I've only activated Mark twice, so I don't fully understand the limit of its duration," El muttered seriously.
El realized that if she kept the Mark for much longer, the risk of serious physical damage would increase dramatically, including internal bleeding, muscle damage, and even the potential for sudden loss of consciousness.
"If only Akaza and Doma had joined the fight, I could have killed one of them right away," El muttered, sighing.
El already had a plan to kill one of them, to complete the Side Mission and Main Mission and then immediately return to the Rencarnation Room for the Mission Evaluation.
However, on the positive side, this time El had enough footing to maximize the Sun Breathing Style, including the use of the Ju San no Kata Move when fighting against Kokusibo.
In the previous wooden platforms and corridors or when fighting in the air, the lack of footing made it difficult for El to use Ju San no Kata.
That is why, El did not use Ju San no Kata from the start when fighting against Kokusibo, Doma, and Akaza, before finally giving full resistance to Muzan, unless El was really forced to use it.