Some might wonder, if that's the case, couldn't Mitsuba just find a time to examine all the causal threads on every geisha in this pleasure house at once?
Why go through all this trouble, wasting time every night coming here with her brother's face pretending to be a customer?
Eh~ you just don't understand~
As the saying goes, in work, there must be balance between labor and rest, pushed forward bit by bit.
For a "worker," the most important thing to avoid is finishing all the work at once.
That way, when you later have nothing to do and your "boss" sees it, he won't think you're very capable—instead, he'll think—
"Oh, you seem quite free, come here and help me handle this too!"
In the end, your salary doesn't increase, but your workload actually grows.
So, rather than solving everything at once, Mitsuba feels it's better to push forward little by little.
After all, she has plenty of time, and Lord Muzan isn't rushing her to resolve this matter immediately...
Life as an oni is all about enjoying the process~
Some might also be curious, why does she have to use her brother's face of all things—wasn't that deliberate?
To this, Mitsuba must seriously state—
Yes, she did it on purpose.
She just wanted to see her brother's serious, rigid face appearing in the entertainment district while frolicking with geishas.
Not only that, later she'll also find a way to drag her brother—who has nearly devolved into a caveman—personally to the entertainment district.
Then find ten talented and beautiful geishas to surround him at once, properly changing his habit of always having a sour face, letting him have some fun too.
Hehe~
"Lord Yori-tsu~ what song would you like to hear tonight~"
"Whatever, you decide."
"Ugh~ Lord Yori-tsu always says that~ don't tell me you hate us sisters~"
"Nonsense, how could I hate such beautiful girls."
"Lord Yori-tsu~ here~ let me feed you fruit~"
"That won't be necessary, I don't eat such things at night."
Inside the room, with Yori-tsu's face, one hand supporting her head, Mitsuba relaxed half-lying on the bed.
While skillfully responding to the geishas around her who were so enthusiastic they wanted to pounce and kiss her, she repeatedly swept her gaze across them.
[Hmm, seems like tonight's batch of geishas doesn't have her either...]
Like the previous days, among tonight's geishas, Mitsuba also didn't see any causal threads intersecting with that mysterious wealthy merchant.
[Seems like tonight I can enjoy myself leisurely too... hmm?]
Just as Mitsuba sat up, preparing to join the geishas' musical performance and have some fun.
Her peripheral vision accidentally noticed a geisha sitting alone in the corner of the room, from beginning to end bowing her head in silence, quietly following her companion playing the biwa.
This was a somewhat special geisha.
Her kimono wasn't as bright and magnificent as the other geishas, even somewhat worn, her long bangs hanging down covering her face, making it impossible to see her features clearly.
Since entering the room until now, this geisha seemed to always sit in that corner, neither speaking with others nor showing any intention of pleasing Mitsuba.
With her extraordinary memory, Mitsuba recalled this geisha's name.
Her name was Akari. Earlier downstairs, the madam seemed to have ordered her to bring Mitsuba and the other geishas to this room.
Of course, what truly caught Mitsuba's attention wasn't the strange behavior shown by this geisha Akari.
But rather the causal thread coiling around her body, a thread so thin it was almost invisible.
Seven days ago, she had served the mysterious wealthy merchant Mitsuba was looking for.
Seven days...
This matched exactly with the time Mitsuba had traced earlier, when this mysterious wealthy merchant intersected with the caravan leader.
Good, she finally found her.
"..."
"Miss Tamayo, are you certain we really must do this?"
Midnight, at a residence where the leader of the Demon Slayer Corps was hiding, with no outsiders present.
Ubuyashiki, whose face had begun showing traces of the curse, looked at the girl with dark purple pupils before him, asking doubtfully.
The girl was undoubtedly Tamayo, who had disappeared for nearly a hundred years.
Since parting with Yoriichi more than eighty years ago, Tamayo had been continuously striving, attempting to complete on her own the medicine she envisioned, which could turn demons back into humans.
However, not long after beginning her research, Tamayo realized that to develop this medicine, demon blood was essential.
Without available research samples, nothing could begin.
But the key problem was, with Tamayo's own combat strength, she could at most obtain blood from extremely weak demons.
For truly powerful demons, even if she encountered them, it would only result in a meaningless death.
And the Muzan cells in these weak demons' blood were extremely scarce, completely insufficient as her research samples...
The research had reached a dead end here.
After several hesitations, Tamayo finally decided to seek help from the Demon Slayer Corps.
She still needed to rely on the Demon Slayer Corps' power to obtain blood from those powerful demons.
Of course, when asking for help, one certainly couldn't come empty-handed.
So, over the past several decades, Tamayo temporarily set aside her research on the "human-reversion medicine," instead diligently developing "poison" that could effectively damage powerful Upper Rank demons.
Turning demons back into humans was difficult, but if one simply considered how to kill demon cells, the difficulty was much simpler.
After all, there did exist a plant that could naturally suppress demons—wisteria flowers.
Wisteria flowers to demons were like arsenic to humans, both were deadly poisons.
Even powerful demons were no exception; it was merely a matter of quantity.
As long as enough was accumulated, even the mightiest demon could be poisoned to death.
The only difficulty Tamayo faced was how to make demons unaware of the wisteria poison's presence in their bodies before the poison took effect.
To overcome this, she spent decades, finally successfully developing an inhibitor that could temporarily suppress wisteria poison.
This substance was non-toxic and harmless, able to suppress wisteria poison for a certain period, thereby temporarily preventing the demon cells in the body from showing rejection reactions to wisteria.
When the inhibitor completely lost effect and the wisteria poison began to explode in concentration, these toxins had already successfully spread throughout the demon's body.
In this way, even if it couldn't instantly kill those powerful demons, it could greatly weaken their strength in a short time, creating favorable opportunities for demon slayers to behead them.
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