At the same time, in Infinity Castle.
Muzan ended his brief rest and opened his eyes to look at a nearby corner.
"Nakime, report the situation."
"Lord Muzan, it has been confirmed that the Demon Slayer Corps headquarters is to the west of the Rengoku Residence. As for the exact location, further investigation is required."
"How long will it take?"
"...I am not sure."
"Not sure?"
Muzan narrowed his eyes, a terrifying pressure emerging within his crimson pupils.
"Nakime, you should understand that is not the answer I want."
"Lord Muzan, please calm your anger."
Nakime quickly replied: "Although I cannot confirm the location of the Ubuyashiki Residence, during my search, I discovered that the Flame Hashira of the Demon Slayer Corps did not return with the others, but remained at that estate."
So what?
Muzan instinctively wanted to question her.
However, at that moment, he learned Nakime's true thoughts through mind reading.
"You want to find the location of the Ubuyashiki Residence through that Flame Hashira?"
"Yes."
Nakime nodded.
As Muzan's logistics manager, her Blood Demon Art allowed her not only to control the Infinity Castle but also to create clones to remotely monitor the surrounding scenery.
Immediately after Doma's death, Nakime had created a large number of clones and sent them to the Rengoku Residence.
Perhaps due to good luck, Lynn and the others did not return directly to the Demon Slayer Corps after killing Doma.
By the time they officially set off, Nakime's clones had already occupied the entire area around the Rengoku Residence and followed them immediately.
Although they were lost midway due to insufficient speed, Nakime still collected a large amount of information during the process, confirming the direction of the Ubuyashiki Residence.
If she set up a surveillance network in advance based on this and found someone to tail again, finding the Ubuyashiki Residence would not be impossible.
"According to my investigation, that Flame Hashira named Rengoku Shinjuro stayed behind because he lost his fighting spirit and didn't return to the Demon Slayer Corps with the others."
"As long as we find a way to reactivate his fighting spirit, he will definitely head to the Ubuyashiki Residence to regroup with the others before the final battle arrives."
"However, doing so might add another enemy for you, Lord Muzan."
"He is but a single Hashira. Ten or a hundred more would make no difference to me."
Muzan didn't care about that.
Compared to the enemy's combat power, Muzan cared more about whether he could find the Ubuyashiki Residence in advance and launch an attack before the appointed time.
"Find a way to reactivate his fighting spirit. I want to see the Ubuyashiki Residence within a week."
"Yes!"
...
The next day, at the Ubuyashiki Residence.
Kocho Shinobu slowly woke up from her deep sleep.
Opening her eyes, Lynn's handsome face came into view.
"You're awake. The surgery was a success."
"You are now a girl."
"I already was one."
Kocho Shinobu rolled her eyes and looked at the others standing nearby.
"Am I the last one to wake up?"
"That's right, you're the weakest one," Lynn nodded bluntly.
"Get up and adapt to your new power. You should have awakened a Blood Demon Art after turning into a demon, right?"
"A Blood Demon Art... I did awaken one, but it seems to be related to my Breathing Technique."
Although she had only just woken up, Kocho Shinobu already had a faint inkling of what her Blood Demon Art was.
Rising from the bed, she picked up her nichirin blade and took a deep breath.
Dance of the Butterflies: Caprice!
The sword skill she had practiced day after day was unleashed instantly. A pair of butterfly wings appeared behind Kocho Shinobu, and she vanished from the spot like a real butterfly.
Cold light cut through the air, and the sharp blade pierced the ground, leaving several gashes.
Lynn walked over to take a closer look and found that some butterfly powder remained around the gashes, gradually corroding the ground and widening the holes.
"This is my Blood Demon Art."
Kocho Shinobu reappeared to the side: "After activating the Blood Demon Art, my Breathing Technique is enhanced to a certain extent, making its effects even stronger."
"Wow! Shinobu, your Blood Demon Art is so pretty!"
Kocho Kanae walked over from nearby.
Hearing Kocho Shinobu's words, she gave her nichirin blade a casual swing, emitting a faint floral fragrance.
"See, my Blood Demon Art isn't as flashy as yours; it can only create hallucinations through floral scents."
"Floral scents? Sister, is your Blood Demon Art also an enhancement of your Breathing Technique?"
Kocho Shinobu seemed to realize something and looked again at the others nearby who were adapting to their new powers.
Uzui Tengen was swinging his dual blades, creating massive noise.
Tomioka Giyu stood silently with his eyes closed, water splashes constantly appearing around him.
Rengoku Kyojuro was moving nimbly, emitting scorching flames.
Clearly, after completing the demon transformation, the Blood Demon Arts awakened by the members of the Demon Slayer Corps were all enhancements of their Breathing Techniques.
"Strange, why is it like this?"
A look of confusion appeared on Kocho Shinobu's face.
While it was a good thing that the awakened Blood Demon Arts could effectively increase their strength, the fact that everyone's Blood Demon Art was related to their Breathing Technique was clearly no coincidence.
There must be a reason for this situation.
"This might be related to Tsukiguni Yoriichi."
As if guessing what Kocho Shinobu was thinking, Lynn's voice suddenly rang out.
"Tsukiguni Yoriichi?" Kocho Shinobu had some impression of the name. "Are you referring to the creator of that Sun Breathing Technique?"
Lynn nodded. "Do you know how Sun Breathing was created?"
"It should be like existing Breathing Techniques, modified using some other Breathing Technique as a template."
Just like how Kocho Kanae created Flower Breathing by referencing Water Breathing.
And how she created Insect Breathing by referencing Kocho Kanae's Flower Breathing.
In Kocho Shinobu's view, Tsukiguni Yoriichi must have also come into contact with some special Breathing Technique and used it as a prototype to create Sun Breathing.
However, to her surprise, Lynn shook his head after hearing this response.
"In fact, there was no creation process. Sun Breathing is simply Tsukiguni Yoriichi's normal breathing frequency."
"Eh? Really?"
A look of surprise appeared on Kocho Shinobu's face. "You mean, Tsukiguni Yoriichi was born having attained the Total Concentration: Constant state of Sun Breathing?"
"Actually, it might be even more exaggerated than that."
"Even more exaggerated?"
"Yeah, the Sun Breathing passed down by Tsukiguni Yoriichi is likely a simplified version, more suitable for ordinary people to practice."
"Why do I feel like the Tsukiguni Yoriichi you're talking about is some kind of monster in human skin?"
Kocho Shinobu couldn't help but complain.
As far as she knew, many of the original members of the Demon Slayer Corps created the Five Main Breathing Techniques based on it because they couldn't learn Sun Breathing.
If everything was as Lynn said, and the Sun Breathing Technique passed down by Tsukiguni Yoriichi was a simplified version...
Then could Tsukiguni Yoriichi, who naturally mastered the original version of Sun Breathing, really still be called human... Hmm?
Kocho Shinobu suddenly turned around and stared at Kocho Kanae, looking her up and down.
"Sister, has my appearance changed?"
"No~ Shinobu, you're still as cute as ever."
"Don't use the word 'cute' to describe me."
After habitually retorting, Kocho Shinobu fell into deep thought.
Perhaps because Tamayo's skills were good, Kocho Kanae's appearance hadn't changed after her Demon Transformation Surgery.
Similarly, the other members of the Demon Slayer Corps, including herself, hadn't changed in appearance either.
In other words, although their true identity was now demons, they still looked like humans on the outside.
Then... what about Tsukiguni Yoriichi?
Was it possible that Tsukiguni Yoriichi, who naturally mastered the original Sun Breathing, was truly an alternative existence in human skin, just as she had joked?
"It seems you've understood what I mean."
Lynn's voice rang out again, and Kocho Shinobu snapped out of her thoughts.
Looking at the black-haired youth talking eloquently beside her, Kocho Shinobu asked curiously.
"Lynn, what do you think is the deal with Tsukiguni Yoriichi?"
"He's like Kibutsuji Muzan after obtaining the blue spider lily and completing his final evolution."
"Kibutsuji Muzan?"
"You should still remember the 'New Human' analogy I told you before, right?"
Kocho Shinobu nodded. "Of course I remember. You said Muzan is an incompletely evolved New Human who needs the blue spider lily to be completed."
"Exactly. If Kibutsuji Muzan is a New Human who evolved later in life, then in my view, Tsukiguni Yoriichi is a natural-born New Human."
"Influenced by this, the Sun Breathing created by Tsukiguni Yoriichi and its derivative Breathing Techniques can be seen as incomplete means of evolution."
At this point, Lynn paused slightly, then switched to another question: "From an evolutionary perspective, what do you think the Demon Transformation Surgery is?"
"...An incomplete means of evolution?"
"Exactly. In my view, the Demon Transformation Surgery is also an incomplete means of evolution."
Lynn raised both hands, holding up a finger on each and slowly bringing them together: "When two incomplete evolutionary methods meet, they influence each other."
Lynn's fingers crossed: "The final result is that the Blood Demon Arts you awakened after turning into demons are all enhancements of your Breathing Techniques."
"I see."
Kocho Shinobu nodded thoughtfully.
Although Lynn's perspective was quite unique, it sounded quite reasonable.
"Alright, enough small talk."
Having answered Kocho Shinobu's doubts, Lynn turned around and called over Uzui Tengen and the others, who were still adapting to their new power nearby.
"Everyone, I have something to tell you. The time for the final battle might have to be moved up."
"Moved up?" Uzui Tengen frowned. "Did something unexpected happen?"
Lynn nodded. "We were followed when we came back yesterday. The tracker is most likely one of Muzan's subordinates, a demon named Nakime."
Lynn briefly introduced Nakime's Blood Demon Art. "Although being followed wasn't in my plan, I intend to take this opportunity to beat them at their own game and set a trap."
"I see."
Uzui Tengen nodded thoughtfully.
When a person thinks everything is under control, that's actually when they are most relaxed.
Using this mentality to set a trap for Muzan, there's a high probability the other party will walk right into it.
"What's your plan?"
"I plan to have the Flame Hashira, Rengoku Shinjuro, act as bait to expose the location of the Ubuyashiki Residence."
Lynn looked at Rengoku Kyojuro. "For the plan to go smoothly, I need your help."
...