Cheng Mo stood still, not immediately approaching the ice crystal that emitted faint energy fluctuations.
He listened to the soul named "Hotaru" (firefly), who, with a tearful, tender voice, intermittently continued to tell their story.
Later, during a mission to find the "Blue Spider Lily," perhaps because they were far enough from Muzan, or for some other reason, the older sister, "Yuki" (Snow), actually recovered her memories from her human days.
"My sister... she recovered her memories..." Hotaru's voice was filled with pain. "She remembered everything, she remembered... she remembered me... and she also remembered that she... ate 'me'..."
Even through the ice, Cheng Mo seemed to feel the sorrow that almost tore the soul apart.
"My sister couldn't bear it... She didn't want to continue like this, she wanted to run into the sunlight..."
Hotaru's voice trembled.
"I desperately called out to her, telling her I was still here... That's when my sister realized that a part of me... was still inside her body..."
"My sister was very happy... truly very happy... But... she was even more in pain..."
"We killed too many people, ate too many people... My sister said we were deeply sinful and didn't deserve forgiveness... But she couldn't resist that terrifying man..."
Cheng Mo listened quietly, but his brain was rapidly processing the information.
Hotaru continued to narrate, her voice tinged with a hint of weariness: "...Later, a guy wearing a hat with a strange smile tried to take my sister's position... My sister then thought of a plan."
"She used almost all of her power and demon blood to create a fake 'her' to deal with him... Then, my sister took me, used her last bit of strength to return here, the place our family has guarded for generations..."
"My sister said she wanted to use the power of the spiritual pulse, and her final will, to seal ourselves away..."
Hotaru's voice gradually lowered.
"That way, that man wouldn't be able to find us, and we... wouldn't hurt anyone anymore... My sister was very tired and in pain, and she fell asleep after doing all this... I stayed awake, watching over my sister..."
"We finally found peace with great difficulty..." Hotaru's voice carried a plea and fear, "...I don't want anyone to disturb her again... and I don't want to hurt anyone anymore..."
As she finished her narration, the legend of the demon "Yuki-onna" (Snow Woman) abruptly ceased.
No one knew that in this deep mountain ice cave, such a pair of struggling, fate-defying sister souls were sealed.
Silence fell all around, with only the faint chill emanating from the ice crystal deep within the cave.
"Soul..." Cheng Mo murmured to himself.
Coming from a new era, once a staunch materialist... after experiencing transmigration, a system, cultivation, Chakra, and even a superpower like the "Eye of Truth," his acceptance of the existence of "souls" had greatly increased.
Moreover, he knew that the concept of souls already existed in the original Demon Slayer World, and there were even clear settings for heaven and hell.
Although he couldn't see souls, Hotaru's narration perfectly explained why the energy here was so abnormal.
The superposition of two souls, residual spiritual power, the self-sealing technique's struggle against Kibutsuji Muzan's blood, and the interweaving of the leyline's spiritual energy.
All of this formed this unique energy singularity.
Thinking of this, Cheng Mo's breathing almost imperceptibly quickened for a moment.
As a researcher, an explorer eager to unravel the mysteries of things.
This case before him held an unimaginably huge attraction for him!
This was practically a living sample touching upon the fundamental rules of the world!
His gaze once again swept over the massive, ethereal blue ice crystal, his eyes becoming incredibly focused.
As if scrutinizing a rare treasure.
He organized his words and said in as steady a tone as possible: "Hotaru, your situation is very special, and extremely... rare."
"But with all due respect, your current state is not a long-term solution. Self-sealing may seem safe, but it is actually consuming the very foundation of your existence."
"Especially you, Hotaru, your soul state is not stable. And who can guarantee that Muzan will never find this place? Or that the seal will weaken on its own one day?"
The chill around the ice crystal seemed to fluctuate slightly.
Hotaru did not respond immediately, as if her thoughts had been spoken aloud.
Cheng Mo continued: "I have already achieved some preliminary research results on Kibutsuji Muzan's blood, and how to reverse or counteract its effects."
He did not exaggerate; his collaboration with Tamayo and his research on Himura Senichi had indeed provided him with much advanced information and technological directions.
"I can try to help you."
He dangled the bait.
"At the very least, to make your current state more stable, reduce pain and risks. And perhaps, in the future, it might even be possible to find a way to separate you, or seek true liberation."
"Really?"
Hotaru's voice carried a trace of bewilderment and faint hope.
"Yes."
Cheng Mo responded earnestly.
"But this requires research, your most detailed cooperation, providing me with the information I need, and, when necessary, allowing me to conduct some... tests and observations."
He particularly emphasized: "This is not gratuitous aid, Hotaru; this is a transaction. I possess the knowledge and technological possibilities that you urgently need."
"And you possess the unique samples and information that I desperately need. We each take what we require."
The girl in the ice crystal remained silent for a long time.
Hotaru seemed to be processing Cheng Mo's words, weighing the pros and cons and... the risks.
Finally, the tender voice spoke again, with a hint of hesitation: "...Is it really... possible... to make my sister no longer so much in pain?... Will it really... not hurt anyone else?"
"I cannot give you a hundred percent guarantee."
Cheng Mo's answer was as factual as ever, but this honesty actually added a touch of credibility.
"Scientific research itself involves variables and uncertainties. But compared to passively waiting for the seal to fail or for self-destruction, my plan at least offers an active direction with possibilities."
After all, these sisters did not appear in the original work.
They were either permanently sealed away, or they died together after Muzan's death, affected by the demon blood curse.