Ubuyashiki looked into Hiru's eyes, which had at some point turned gray. The smile on his lips did not change. "If it would affect Muzan Kibutsuji's defeat, erasing my memories is no issue."
"But you'd just figure it out again sooner or later," Hiru sighed. "Just say what you want to say."
"Honestly, I was only guessing until just now." Ubuyashiki glanced at Hiru's speechless expression and chuckled. "No matter how one thinks about it, the idea that the original Swordsman didn't die, but instead became a demon who can withstand the sun… it's far too unbelievable."
"Yeah," Hiru propped his chin up, "I should have died centuries ago. But while painting that picture, I unexpectedly overcame sunlight. My brother couldn't bring himself to kill me, and he feared I'd lose control, so we ended up living on like this."
"Do the two of you need to eat humans?"
"No. I rely on animal blood for energy. As for my brother…" Hiru curled his lip. "He's basically a plant."
"I see…" Ubuyashiki Kagaya lowered his eyes slightly. When he looked back at Hiru, his expression was calm. "The next question may be presumptuous… but may I ask—how should we ultimately slay you?"
Hiru narrowed his eyes. "That kind of question… are you looking to die?"
"Not at all. I simply understand that demons and humans cannot coexist." Ubuyashiki Kagaya's expression did not waver. "I am only preparing for the possibility of a day that may come."
"…If my brother doesn't help you, the Demon Slayer Corps alone can't kill me. As for my brother…" Hiru sighed. "Honestly, sometimes I wonder why I didn't die properly back then."
"Wasn't Mr. Yoriichi transformed by you? Doesn't a Demon King control the life and death of the demons it creates?"
Hiru looked away. "…Three hundred years ago, yes."
Ubuyashiki: …?
"Three hundred years ago, I got lost while going down the mountain to buy supplies. My brother broke that restriction entirely in order to find me. He's now an unbound, perfect being." Hiru turned back to look at him. "If I became your enemy, you could still trade lives to bring me down. I have little combat ability besides demon strength. But my brother…
You must understand—back then, with a human body, he sliced Muzan into pieces in an instant and forced him into hiding for decades."
Ubuyashiki Kagaya: …
He suddenly felt a sharp pain in his stomach.
"But don't worry too much." Hiru extended a hand toward him. "I've never given up on returning to being human."
"That's good." Ubuyashiki Kagaya smiled, lowered his head, and closed his eyes, waiting for his memories to be wiped. Whether he would realize anything afterward didn't matter—he had obtained the answer he needed.
He felt something drawn out of his body, yet he hadn't forgotten anything. Instead, his body felt lighter.
Did it fail?
Confused, Ubuyashiki Kagaya opened his eyes—and saw Hiru trembling as he held a pale pink stone no larger than a fingernail.
"I only wanted to test it. I didn't expect you to be cherry-blossom-colored…" Hiru stared at it like it was unbelievable, then sighed with regret. "Why aren't you a demon…"
"Huh?" Ubuyashiki looked bewildered as Hiru carefully placed the stone into a small box. "Apologies, I don't quite understand."
"Simply put, my Blood Demon Art and Muzan Kibutsuji's Blood Demon Art just had a small 'contest' through your body." Hiru closed the lid with satisfaction and looked at him. "I don't know what this particular Blood Demon Art does, but without a doubt—you've been affected by one."
"…Blood Demon Art?" Ubuyashiki Kagaya froze. "You mean this isn't a curse, but a Blood Demon Art?"
"Mhm." Hiru nodded slightly. "Since my art worked, it's definitely a Blood Demon Art."
"But… but… doesn't Muzan Kibutsuji's Blood Demon Art turn humans into demons?"
Seeing his confusion, Hiru explained, "Turning humans into demons is an ability all Primordial Demons have. Even though my brother broke free from the restriction, he doesn't retain that ability—or rather, he can, but the success rate is less than one percent. Non-Primordial Demons converting others is far too inefficient.
Modern demons can convert others only because the blood inside them, belonging to Muzan Kibutsuji, was separated from their bodies. They do not inherently have that ability."
"Uh… but isn't Hiru a demon who was transformed and then broke free?"
"I'm a Primordial Demon." Hiru's face darkened. "I just said only Primordial Demons can transform humans."
"Hah—I've lost my composure…" Ubuyashiki noticed Hiru's expression darken further, let out a long breath, and relaxed slightly. He smiled apologetically. "Could you explain in more detail? This is a rare chance. I'd like to understand more about demons."
"Demons can be divided into four levels." Hiru raised four fingers. "First, the lowest—mindless demons who only crave flesh and haven't awakened a Blood Demon Art.
Second, those still fixated on eating humans, but who have regained their will, can communicate, and possess Blood Demon Arts.
Third, Primordial Demons—me and Muzan.
And lastly, the kind like my brother—demons who have broken free from the Demon King's control.
The first type is easy—they're nothing more than trash dangerous to ordinary people.
Second-tier demons vary wildly. Depending on how many humans they've eaten, their Blood Demon Arts differ. The weaker ones can be killed by skilled swordsmen or even strong civilians. The strong ones—like the Twelve Kizuki—can easily kill swordsmen.
Third-tier demons, like me and Muzan, can no longer grow stronger through simple feeding. We create many demons, let them feed, then retrieve their blood. That alone is enough to strengthen us…"
"Between you and Muzan, who is stronger?"
"…Muzan." Hiru lowered his eyes. "If we fought, I would be devoured without question."
Ubuyashiki Kagaya paused for a moment. "I see. Please continue."
"As Primordial Demons, we can convert humans. But Blood Demon Arts themselves come from a person's obsessions and human-life experiences…" Hiru extended his hand again. "Don't close your eyes this time."
Ubuyashiki once again felt something being drawn out of him. A small grain-sized stone appeared in Hiru's hand.
"This is…"
"One of my Blood Demon Arts: Banshoku Seki." Hiru placed the tiny stone in Ubuyashiki's hand. "Banshoku Seki only activates on things carrying a demon's aura. But you are without doubt human. That's why I deduced from the start that you were affected by Muzan Kibutsuji's Blood Demon Art.
But during my brief encounter with Muzan, he didn't use any Blood Demon Art. And when I later killed demons and read their memories, I found no trace of it. So I believe—when he first transformed into a demon, he unconsciously used his Blood Demon Art, and has been continuously draining something from the Ubuyashiki clan ever since."
Ubuyashiki stared at the pale pink stone in his palm. "I see…"
"My Blood Demon Art can dispel the curse, but only delay it," Hiru said. "Your body is too fragile to withstand a full confrontation between our Blood Demon Arts. You'd die before it concluded."
"No… this alone is enough." Ubuyashiki Kagaya handed the stone back. "I only wish for this failing body to witness Muzan Kibutsuji's death."
Hiru accepted the stone and stored it away. "…You will see it."
"Please continue with the fourth type of demon." Ubuyashiki smiled lightly. "Its representative must be Mr. Yoriichi, correct?"
"Yes. As I said, the fourth type is a demon who has broken free from the Demon King." Hiru nodded. "Any demon capable of that has power equal to or beyond the Demon King.
In other words—if a Primordial Demon fails to convert its victim the instant their blood mingles, then that victim, by steadily strengthening themselves, can absolutely surpass the Demon King who transformed them."
"Then… how long did it take Mr. Yoriichi to turn into a demon?"
"One—"
"What?"
"One month… a little more. And from the very beginning, I couldn't control my brother's actions or thoughts."
Ubuyashiki Kagaya: …
"And another thing," Hiru turned away from Ubuyashiki's stunned eyes, "since I had already overcome wisteria and sunlight when I gave him my blood, my brother inherited that. Wisteria poison and the Bright Red Nichirin Sword barely harm him.
But I can still be hurt by the Red Nichirin Sword—though very few can use it. Anyway, the Demon Slayer Corps has no chance of killing my brother now or ever. Unless something can instantly vaporize a person, it's only a dream."
Ubuyashiki Kagaya silently clutched his stomach.
"Are you alright? Why the sudden pain?" Hiru straightened. "Did my Blood Demon Art affect you?"
Ubuyashiki forced a smile. "…No, it's not that. Just… stomach pain."
"Stomach trouble too?" Hiru studied him, then stood to fetch brush and paper. "Then I need to adjust your prescription… hmm—what herbs were good for the stomach again?"
Ubuyashiki Kagaya: My digestion is poor, but that is definitely not why my stomach hurts…
"Right—any dietary restrictions?"
"…None."
"Good." Hiru lowered his head and began writing. "Take this medicine twice a day, morning and night. If you experience hallucinations, stop immediately and contact me."
"…Alright."
Ubuyashiki Kagaya watched Hiru and quietly sighed in his heart.
At this point, all I can do is take things one step at a time.