The key lies in the method of increasing body temperature and heart rate.
Under normal circumstances, when humans engage in high-intensity exercise for an extended period, their body temperature can rise to 38-39 degrees Celsius, with 39-40 degrees being the human limit.
Similarly, heart rates can approach or even reach 200 during high-intensity, or even extreme, exercise.
A small number of elite athletes, after extensive professional physical training, can sustain such a state for a certain period.
However, activating the Demon Slayer Mark requires two additional crucial factors besides pushing the body's physical state to its human limit.
One is achieving an extremely high level of Breathing Style, which is almost like Constant Breathing reaching its peak.
Constant Breathing allows one to continuously use a Breathing Style for twenty-four hours a day, and beyond that, it involves fully unleashing the potential of that Breathing Style.
The body must reach its utmost physical state, and the Breathing Style must also reach its utmost potential.
The last remaining factor is the first person to activate the Demon Slayer Mark.
Four hundred years ago, it was Tsugikuni Yoriichi; now, it is Tanjiro.
For Qingshui, with his high level of control over his physical body, he could choose whether or not to activate the Demon Slayer Mark.
For ordinary people, activating the Demon Slayer Mark typically leads to a lifespan of no more than twenty-five years, with Tsugikuni Yoriichi being the sole exception.
For Qingshui, he wouldn't face that limitation either.
That's because a human body cannot constantly endure such extreme conditions. But for him, with his perfect Demon transformation, this burden was entirely manageable.
There was nothing left to hesitate about.
Sending another Fifth Form: Wind-Lightning Separation Blast towards Muzan, Qingshui gripped his Nichirin Blade tightly, his heart rate climbing steadily.
The surrounding air was continuously drawn into his lungs through his mouth and nose. Thanks to Qingshui's immense lung capacity, a thick white mist began to emanate from his body.
There were even sporadic sparks, similar to those seen with Sun Breathing, though entirely blue.
A faint, simple blue pattern subtly appeared at the junction of Qingshui's shoulder and neck.
The Demon Slayer Mark pattern was formed by several lines, resembling lightning composed of two slightly rigid flowing cloud patterns. It looked somewhat like two oracle bone script characters for "electricity," with one inverted and combined with the other.
Qingshui could feel a slight burning sensation from his Demon Slayer Mark. After the surrounding air was inhaled, it felt as if a rich energy filled every inch of his meridians, strengthening his physical power.
And so, Qingshui fought Muzan with even greater vigor.
The other Hashira also noticed changes in themselves, more or less, and their strength increased significantly.
Soon, Upper Rank Five was slain by the Wind and Water Hashira, who had both activated their Demon Slayer Marks.
Of the Twelve Kizuki, only the strongest, Kokushibo and Akaza, remained.
Kokushibo was still being besieged by the original three top-tier Hashira.
At this moment, all three of them had also activated their Demon Slayer Marks during the battle.
With their combined abilities and the power of the Red Blades, they fought Kokushibo to a four-to-six split.
The Hashira were at a slight disadvantage, with Kokushibo holding the superior position, but they were able to hold him off, preventing him from joining other battlefields.
The remaining Hashira, without exception, all joined the encirclement of Akaza.
As a result, Akaza was taking quite a beating.
Originally, Tanjiro and Rengoku Kyojuro alone were barely enough to hold him off. After activating their Demon Slayer Marks, they had a fighting chance. And then, four more Hashira—Wind, Water, Mist, and Love—arrived as reinforcements.
Every one of them had activated their Demon Slayer Marks. Not to mention, Tokito Muichiro, just like in the original timeline, had awakened the Transparent World through his life-and-death struggle.
However, Tokito Muichiro wasn't the only one who had activated the Transparent World.
Because Rengoku Kyojuro and Kamado Tanjiro had also awakened the Transparent World during this battle.
But there was also bad news.
Although Akaza was being severely beaten in this battle, under immense pressure and the threat of death, he had managed to reach the pinnacle of martial arts in this world, comprehending the Transparent World.
Even worse news was that when they finally managed to cut off Akaza's head after much hardship, they discovered that Akaza could continue fighting as a headless body, and his head was rapidly regrowing.
"He's actually overcome the weakness of dying from decapitation... Everyone, what do you think we should do next?"
Tokito Muichiro floated gracefully out of the clouds after decapitating Akaza, his expression grave.
"Tch, he's not like that Demon from earlier, is he? With a real body hiding somewhere?"
Shinazugawa Sanemi blocked a punch from Akaza, shouting.
"At least for now, I haven't detected any other Demons."
Rengoku Kyojuro disengaged from the battle, pulled out a new battery, inserted it into the flashlight strapped to his wrist, and then used the searing blade of his Red Blade to cauterize his still-bleeding eye, stopping the flow.
"Mr. Rengoku is right, I haven't found any other Demons either."
Tanjiro cut off Akaza's arm with an "Inferno Dance" technique, responding as he fought.
By now, all the remaining Demons had been annihilated, leaving only Muzan and the two strongest Upper Ranks—assuming Tamayo and Yushiro were excluded.
In the perception of the several swordsmen who had activated the Transparent World, indeed, only these few Demons remained.
"Who cares? I don't believe he can live forever! I'll keep slashing until dawn and the sun rises. We'll see if he dies or not!"
Shinazugawa Sanemi spat out a mouthful of blood, wiped the crimson from the corner of his mouth, and charged forward again.
Five Hashira, plus Tanjiro, who possessed Hashira-level combat power—every one of them had activated their Demon Slayer Mark and Red Blade, and half of them had mastered the Transparent World. Even though Akaza, as a Demon, had superior physical strength, he was inevitably suppressed until he was beheaded once more.
"Akaza! I can sense that you have an unresolved obsession keeping you from leaving. What is it that a Demon like you can't let go of?" Tanjiro shouted at Akaza, who was still throwing punches.
His sense of smell was unique; not only could he detect openings in combat, but he could also discern others' emotions.
"I will climb, step by step, to become the strongest! I will reach the pinnacle of martial arts!" Akaza roared with a mouth that had just regenerated.
"But you're already so strong. With such immense power, what will you use it for? Even if you become the strongest, how will you use it?" Tanjiro continued to try and talk him down.
He was still using his ability, trying to discern the true reason why Akaza's obsession hadn't dissipated.
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