What Ren Kuroda had thrown to Kagaya Ubuyashiki was, in fact, an Inferior Demon Power Crystal—an item he had exchanged for in the system's reputation shop.
The function of this crystal was simple: it increased demonic power.
Of course, for a being at Ren's level, the boost was almost negligible. Most of the time, he treated it as a recovery item rather than a power amplifier.
However, for ordinary humans—or low-level demons—the increase provided by an Inferior Demon Power Crystal was extremely significant.
Ren's reasoning was straightforward.
He could clearly sense that an abnormal force already existed within Kagaya Ubuyashiki's body—one that was continuously damaging his flesh and draining his life force.
When an ordinary person consumed an Inferior Demon Power Crystal, their body would also be eroded by demonic energy. In other words, if Kagaya consumed the crystal, a foreign demonic power would forcibly enter his body.
Ren believed that since Kagaya's body was already being corrupted by a curse, introducing this intrusive demonic energy might create a counteracting effect.
After all, both forces sought to erode Kagaya's body.
And if they collided, they would inevitably clash—and partially neutralize one another.
Of course, when Ren tossed the demonic crystal toward Kagaya Ubuyashiki, he had also mentioned another possibility.
That the two forces might collide too violently.
And that Kagaya might end up dying even faster.
"This is my return gift—an exchange for the Nichirin Blade," Ren said calmly."Of course, whether you eat it or not is entirely your choice."
In truth, Ren hadn't needed to offer the Inferior Demon Power Crystal at all. Kagaya had already agreed to his request before this.
However, the magnanimity and open-mindedness Kagaya Ubuyashiki had shown earlier had genuinely earned Ren's respect.
And so, Ren offered what he called a return gift.
"Master, you can't just eat something like that!""We can look for other solutions to your condition!""If something goes wrong, the entire Demon Slayer Corps will be finished!"
The nine Hashira stared at the purple crystal in Kagaya's hand, their expressions tense as they urgently tried to dissuade him.
Holding the crystal, Kagaya Ubuyashiki's eyes shone with an unprecedented light.
Faced with the Hashira's pleas, he could only give a bitter smile.
Other solutions?
If there truly were another way to solve his clan's problem, would he have struggled for so long?
The Ubuyashiki clan was a cursed lineage.
Kagaya's frail health from childhood was not an individual problem—it was a curse borne by the entire family.
The Ubuyashiki clan was bound by blood to Muzan Kibutsuji.
When Muzan became the first demon a thousand years ago, the Ubuyashiki bloodline was cursed as well.
Every child born into the clan was weak and sickly. Most died shortly after birth.
The reason Kagaya Ubuyashiki had confidence in Ren's proposed offset method was because the clan's own method of survival was remarkably similar.
To prevent their bloodline from going extinct, each generation of the Ubuyashiki clan married women from priestess families, using spiritual light to suppress the curse.
This method did extend their lifespan—slightly.
Even so, almost no one in the clan lived beyond thirty.
Kagaya Ubuyashiki was already twenty-three years old.
And he could feel his body weakening with every passing day.
He had long grown tired of a life where he couldn't even wield a sword.
So when he learned there might be a way to escape his current fate, how could he not be moved?
At the same time, Kagaya never believed that Ren intended to harm him.
As Ren himself had said—Kagaya already knew his days were numbered.
Ren had no reason to go to such lengths. Doing so would only create unnecessary trouble for the Demon Slayer Corps' Pillars.
Clutching the purple crystal, Kagaya Ubuyashiki finally hardened his resolve.
Looking at the nine Hashira before him, he spoke calmly:
"If I die, my position will be inherited by my son, Ubuyashiki Teruya.And none of you are permitted to make things difficult for Ren Kuroda."
"This is my decision."
After speaking, Kagaya Ubuyashiki decisively swallowed the purple crystal.
Just like the resolve he had shown earlier, this seemingly fragile man did not hesitate for even a moment when faced with a choice.
The instant the crystal entered his mouth, a scorching heat surged through his body.
The sudden, intense pain caused him to clench his fists tightly.
His face—already disfigured for unknown reasons—twisted into something even more grotesque.
Yet not a single Hashira showed disgust.
They all watched anxiously, their attention fixed on their master.
"What are the chances,"Giyu Tomioka asked, unable to maintain his usual calm,"that the crystal you gave our master will successfully resolve his condition?"
Ren glanced at him and answered honestly.
"About fifty percent."
"If it were me, I'd take the gamble too. With his physical condition, he only had one or two years left anyway."
"I just didn't expect him to swallow it so decisively. Seems he really hates being powerless."
"And one more correction—the demonic energy crystal can only temporarily offset the curse. It can't erase it completely."
"The energy inside the crystal is limited. Like a plant without roots, it will eventually wither. When that happens, the curse will resurface."
As Ren finished explaining, Kagaya Ubuyashiki finally steadied himself.
He felt better than he ever had before.
When the Hashira moved to support him, Kagaya actually stood up on his own.
Then, without warning, he clenched his fist—
—and punched the wooden door beside him.
The blow pierced straight through the door, which was several centimeters thick.
The nine Hashira froze in shock.
Then, almost immediately, their faces lit up with pure elation.
Breaking through a wooden door was nothing for them. Even a trained ordinary person could manage it.
But Kagaya Ubuyashiki accomplishing such a feat—
—that was astonishing.
Even at his healthiest, Kagaya had barely been able to swing a sword a few times.
Now, his physical strength clearly surpassed that of an average adult.
"It seems I made the right bet," Kagaya said, his voice filled with vitality."That crystal didn't just neutralize the curse—it practically transformed me."
For the first time in his life, Kagaya Ubuyashiki felt a surge of unrestrained excitement.
He even felt an overwhelming urge—
—to pick up a sword and wield it freely.