Yoriichi once again drew his long sword. As the blade turned a deep, fiery red, he brushed his fingers along it. To his surprise, there was no searing pain—only a pleasant, gentle warmth. The confusion in his eyes deepened.
[It seems... to have become something extraordinary.]
Beside him, Hiru recalled the sensation of being branded by molten iron and quietly shifted one step away.
Watching the once-fearsome Demon King now looking so obedient and cautious before Yoriichi, Tamayo couldn't help but find the sight amusing. She actually let out a soft laugh.
Both brothers turned to her, puzzled. Tamayo smiled faintly.
"It's just... truly remarkable."
"What is?" Hiru looked at her with confusion. "What's so remarkable?"
Yoriichi silently slid his sword back into its sheath, though his expression also held curiosity.
"I just find it amazing how close the two of you are," Tamayo said warmly. "Before Yoriichi's return, Mr. Hiru was so anxious and restless he seemed ready to destroy everything around him. But the moment he saw you, he calmed down completely. Even his demonic aura has faded. It seems Mr. Hiru truly relies on you, Mr. Yoriichi."
Hiru turned his head away. "Well, we've been together for decades. He's always looked after me, so of course I was uneasy when he suddenly disappeared without saying a word."
"My apologies for worrying you," Yoriichi said gently, reaching out to ruffle Hiru's hair. "I didn't expect the transformation into a demon to take so long."
"It's a blessing you retained your sanity, Mr. Yoriichi," Tamayo said, visibly relieved. "When I first saw you return as a demon, I truly despaired for this world.
If it's alright, I'd like to run a few tests—to record your physical data."
"That's only right," Yoriichi replied with a small nod. "If we ever lose control, your records could help future generations discover our weaknesses."
"...Do you even have weaknesses, Brother?" Hiru gave him a skeptical look. "You're a demon who isn't harmed by sunlight and isn't affected by the Demon King."
"Any being that can think and feel will have weaknesses," Yoriichi said calmly. "I'm no exception. How do you plan to test us, Miss Tamayo?"
"Well..." Tamayo began listing various known weaknesses of demons in a composed tone—aside from sunlight, which both brothers had already conquered. She spoke of poisons, recovery rates, and wounds inflicted by a Crimson Blade.
To ensure proper comparison, Yoriichi even went out after several days of adaptation and captured two demons for Tamayo to use as control subjects.
The two demons nearly fainted from the agony of Tamayo's experimental poisons, while Yoriichi and Hiru merely commented on the "smoothness" of their texture.
After over a dozen different tests, Hiru extracted the two demons' memories and turned them into pigment stones. Finally, Tamayo compiled her findings.
First, Hiru's results:
As a Primordial Demon, his blood possesses an extremely high infection rate, easily transforming lesser demons into subordinates. His Blood Demon Art primarily targets the mind.
Completely immune to all known anti-demon toxins, including those derived from wisteria. No signs of rejection.
Reacts to humans with rare blood. Requires regular consumption of animal blood for survival. Cannot digest human food and perceives no flavor from it.
Unaffected by sunlight. Wounds from Nichirin Blades heal normally, but he remains vulnerable to Crimson Blades—though they leave no permanent marks. A wound one centimeter deep and long took seventeen days to fully heal despite his regeneration.
Emotional stability remains high except when concerning Yoriichi or painting. Holds prejudice and hostility toward humans.
Comparatively weak in close combat.
Retains around 80% of human appearance when demonized—develops horns, markings, claws, and distinct pupil changes.
Next, Yoriichi's results:
A demon created by Hiru's blood, yet free from his control. Possesses complete willpower and no Blood Demon Art. His blood is highly corrosive—any demon it touches dies instantly, making assimilation impossible.
Immune to all known demon toxins, including wisteria. However, he unconsciously reacts with discomfort to medicine or incense containing wisteria wood or flowers.
No response to human or animal blood. Since his transformation, he has never consumed blood. Can digest human food, though finds it tasteless and unsatisfying.
Unaffected by sunlight. Wounds from Nichirin Blades heal normally; wounds from Crimson Blades heal slower, though still quickly overall.
Displays little emotion unless spoken to by Hiru.
Exceptionally powerful.
No visible demonic traits.
Holding the paper Tamayo had written on, Hiru glanced over it, then looked at his brother with mild disbelief.
"You really are strange, Brother. You're so powerful, yet you don't have a Blood Demon Art? I thought the stronger a demon, the faster and more potent their Art would awaken!
And I've restrained myself for decades—now I need to drink at least one chicken or rabbit a week just to stay sane. How are you surviving without eating anything? Do you feed on air?"
"Hmm... Just standing under the sunlight and practicing my Breathing and sword forms is enough," Yoriichi said, his pale hair now threaded with faint streaks of black. He held his own copy of the report, glancing down at it. "Is this what demons are like in detail? Truly... what terrifying beings."
Hiru: ...
Setting aside the fact that we're immune to sunlight and wisteria, if we're talking about something terrifying, isn't my brother—the man who, as a human, nearly slaughtered the Demon King—far more frightening?
Hiru fell silent in thought.
Tamayo examined the results, her violet eyes clouded with worry. But just as she had once been powerless against Muzan, she could do nothing to these brothers either.
She sighed softly. "Mr. Yoriichi, Mr. Hiru... in many ways, you've both surpassed what could even be called 'demons.' What will you do from here?"
"Until Muzan shows himself again, we'll remain in hiding," Hiru said, setting the report aside. "We don't fear him, but if he flees and starts creating more demons, that'll be troublesome."
"When we face him again, I won't let him escape," Yoriichi said calmly, his gaze still on the paper. "A human might fear losing strength when wounded—but demons have no such weakness. So we'll simply kill him before he has the chance to run."
"But considering he's been hiding for decades now..." Hiru folded his arms. "The moment he sees you, he'll probably split apart and flee, right? If that happens, even you might not catch him."
"As for appearance..." Yoriichi fell silent, then the mark on his forehead slowly faded. His hair, darkened by demonic energy, paled once more. "Would this make a difference?"
"Huh? Your Slayer Mark can disappear? I thought it was natural!"
"When I was human, I could suppress the Transparent World sometimes, but I couldn't hide the mark," Yoriichi said simply, as though it were nothing remarkable. "But with this body, it's easy. Honestly, I should thank it—I can finally see the world as normal people do."
Tamayo thought for a moment. "In that case, for safety's sake, while I continue researching the human transformation drug, I'll also develop one that prevents demons from splitting."
After that, Hiru reapplied his contact mark on Tamayo and told her that besides him, members of the Demon Slayer Corps might reach out.
If they came with goodwill, she could share the demon-slaying drug formula. Otherwise, she should use the mark to call for him—he would help her escape. But no matter what, she must never reveal information about him or Yoriichi, to prevent Muzan from learning of their existence.
Tamayo agreed.
A few days later, Yoriichi and Hiru slipped away amid the chaos of the town's spring festival, their destination unknown.
To be safe, Tamayo soon relocated as well, not even bothering to sell or manage the now-valuable house she left behind.
Three days after their departure, a four-member Demon Slayer Corps team arrived at the building at dusk.
"There are traces of demons here, but they're long gone."
"There are also signs of human life—food scraps, utensils... Damn it, did someone get killed?"
"Kakushi's report said the town doctor mentioned a patient hadn't come out in weeks. It's possible the doctor turned into a demon and killed them."
"Or the patient became a demon and killed the doctor. The doctor was well respected, after all."
"Guess we came for nothing. Let's report to the Master and move on."
"Right."
With that, the four quietly left, just as swiftly as they had arrived.
Though frustration lingered, there was no time for grief. Intelligence wasn't always reliable, and when they arrived too late, there was often nothing left to do but mourn.
But it didn't matter. As long as they continued to slay demons, as long as the will was carried on, humanity would one day eradicate them completely.
When the Demon Slayers departed, the small house grew silent once more. Bathed in the sacred moonlight, it stood still and silent—like a mute giant, quietly witnessing everything.