Chapter 10: Zero

Zero imagined himself as Kyojuro Rengoku and Kyojuro Rengoku’s elder brother, thinking from their perspectives.

But speculation is still speculation—without concrete evidence, it’s all just empty talk.

Still, he recorded his earlier conjectures in his notebook; if true, they might help his future self or the Demon Slayer Corps.

“What a pity… the lifespan of a Marked Swordsman is so short! I wonder if any current Swordsmen have awakened the Mark? But I doubt it—after all, the strongest Pillar of the Demon Slayer Corps is already twenty-four. If he had truly awakened the Mark, he’d probably be dead… maybe…”

He put the notebook back on the shelf, took out another notebook to open, then remembered the “Transparent World” recorded in the previous one.

“By the way, what exactly is this ‘Transparent World’?”

【Search: Transparent World】

【Searching… Search successful】

“Phew… good, it’s searchable. Let me see what this so-called ‘Transparent World’ really is.”

【Transparent World: A world visible when the mind becomes transparent—a “supreme realm” reached only after pushing oneself to the limit and enduring unbearable pain.】

【In this state, by focusing and shutting down excess senses, a living being’s body appears as if transparent.】

【One’s movement speed, prediction of attacks, and evasion all improve significantly; the opponent’s pulmonary blood flow becomes clearly visible, muscle contractions are perceived faster, and even qi can be freely shut off as needed—because perception accelerates, the enemy’s movements appear slower.】

“Hmm… how is this different from X-ray vision plus infrared sensing…? Still, it does seem powerful—for this world, anyway…”

“No! If it were only Breathing Technique, the Mark, and the Transparent World, Kyojuro Rengoku could never have cornered Muzan Kibutsuji! There must be something else—perhaps… his weapon?”

During his sparring with the Pillars, Zero noticed their weapons differed—and all were called Nichirin Blades.

Shinobu Kocho’s weapon was a Nichirin Blade with a white hilt, pale green blade, serrated hamon, and the characters “Evil Demon Extermination” engraved on it; the tsuba was black and octagonal.

Inosuke Hashibira’s Nichirin Blade was a curved, double-edged serpent blade, purple in color, with a circular tsuba engraved with serpent patterns.

Thus, Kyojuro Rengoku’s weapon must have had its own unique, distinctive qualities.

After organizing his thoughts, he returned the current notebook to its place and retrieved the previous one to continue reading.

He found no relevant information, but uncovered another detail.

According to the record, Kyojuro Rengoku once encountered Muzan Kibutsuji on a certain night and fought him; Muzan, unable to prevail, fled, accompanied by a female demon—but Kyojuro Rengoku did not kill her, instead letting her go.

When Demon Slayer Corps members arrived, they found only massive bloodstains and the fleeing female demon, and rebuked Kyojuro Rengoku for doing so…

Because Muzan Kibutsuji escaped and the female demon was released, Kyojuro Rengoku was scolded by the Demon Slayer Corps and subsequently expelled…

The record was incomplete, even fragmented! But Zero felt the female demon Kyojuro Rengoku let go must have been different in some way—or held some significance.

He drew another line beside Kyojuro Rengoku’s name in the notebook and wrote: “The Released Female Demon.”

“Who could this released female demon be? If she’s still alive, what identity does she hold now? Why did Muzan Kibutsuji keep her by his side? Or what could she do for him?”

Zero couldn’t fathom it, nor understand why the Demon Slayer Corps had acted so foolishly then—expelling Kyojuro Rengoku without even asking why?

And Zero had found records showing Kyojuro Rengoku had taught the Demon Slayer Corps his Breathing Technique, elevating their strength and making them less helpless against demons!

Though they couldn’t master Sun Breathing, Kyojuro Rengoku guided them to adapt it, giving rise to other Breathing Techniques.

He was, without doubt, indispensable.

“What cruel fate!”

He flipped through other notebooks but found nothing about Kyojuro Rengoku’s weapon.

“Looks like I won’t find it… no, not impossible.”

He recalled the two Lower Moon demons he’d fought before, and the information his Analysis Glasses had provided: the thermal effect of Dark Alloy Type IV infused with Energy Crystals produces the same effect as a Nichirin Blade.

“Thermal energy? When heat becomes extreme, it causes burning—like high-purity iron exposed to sustained flame turning red and emitting fire…”

“Nichirin Blade… if my Dark Alloy Type IV infused with Energy Crystals mimics the Nichirin Blade’s effect, then is the Nichirin Blade itself forged from some special material?”

【Search: Nichirin Blade】

【Searching… Search successful】

【Nichirin Blade: Forged from sunlight-absorbing Gyokuto steel, along with materials “Shinshinhi Iron” and “Shinshinhi Ore”—the only weapon capable of harming demons, its color varies according to the user’s attributes.】

“So that’s why the Pillars’ weapons all have different colors! And the Nichirin Blade’s material absorbs sunlight!

It must have been exposed to prolonged sunlight near the sun—but it’s not just that; if I’m right, the Gyokuto steel’s location must have been near a volcano or similar!”

“But unlike my Dark Alloy Type IV, which I crafted myself using highly precise instruments over multiple iterations, and which uses specially modified dark alloy!”

Zero then took out two Energy Crystals: one glowing brilliant red, the other translucent and snow-white.

They were formed by converting thermal energy from near the solar system and cryogenic energy from near Pluto.

Created by special high-precision instruments that absorbed and compressed the energy, then infused it into crystals forged from unique meteoric iron.

Crystals infused with solar energy turn red; their immense energy can continuously power armor and weapons for combat against otherworldly beasts.

Crystals infused with Pluto’s cryogenic energy produce freezing effects, primarily used to cool armor and immobilize otherworldly beasts.

After observing them briefly, he returned the Energy Crystals to his nanospace.

“From this, I infer Kyojuro Rengoku’s weapon was likely forged from the purest Gyokuto steel. If I’m correct, it should have been black, red… or both?”

He drew another line beside Kyojuro Rengoku’s name and wrote: “Nichirin Blade.”

“My Type I and Type II, when infused with Energy Crystals, generate thermal energy that suppresses demon regeneration—likely by destroying their cells, which, combined with decapitation, turns them to ash!”

“But is it really that simple? What if a demon, even after decapitation, cannot die? What then? Like Muzan Kibutsuji? He’s lived a thousand years—he must have many survival techniques; perhaps overcoming decapitation is one of them?”

He drew a line under Muzan Kibutsuji’s name and wrote: “Overcome Decapitation?” with a question mark.

Then he looked at the line for the released female demon and drew another line connecting it to Muzan Kibutsuji’s name.

“What’s the relationship between this female demon and Muzan? It can’t be just superior-subordinate—there’s only one possibility: enemies.”

“This female demon must possess something—otherwise why would Muzan keep her? What? No, no… it’s not what she possesses—it’s who she is that helps Muzan Kibutsuji!”

After much thought, Zero still couldn’t find the answer. Just as he was about to give up, a sudden flash of insight struck his mind.

He recalled a line from the earlier notebook: Kyojuro Rengoku encountered Muzan Kibutsuji on a certain night…

“Demons can only move at night. Muzan must have been searching for a way to resist sunlight—and this female demon must have been skilled in medicine, so Muzan turned her into a demon and kept her close! If so, her identity must have been a physician specializing in pharmacology—perhaps she just happened to cross paths with Kyojuro Rengoku…”

Realizing the reason, he drew a line beneath “The Released Female Demon” and wrote: “Physician.”

Though all this was Zero’s speculation, his gut told him it wasn’t that simple.

“The identity of the released female demon is only my guess, but something deep inside tells me I’m right… I should look into physicians—especially those who only practice at night—perhaps I can find her.”

He drew another line beside Muzan Kibutsuji’s name and wrote: “Sunlight-Immune Being?” with a question mark.

“A being immune to sunlight… I remember Xiao Yue once found some information about a plant called ‘Blue Higanbana’—Muzan Kibutsuji has been searching for it for ages…”

【Search: Blue Higanbana】

【Searching… Search successful】

【Blue Higanbana: A flower that blooms only during the day, blooming for only two or three days per year.】

“Pfft…” Seeing this, Zero suddenly found it hilarious—and pitied Muzan Kibutsuji’s idiotic intelligence.

Muzan Kibutsuji had searched for a thousand years for the Blue Higanbana, which blooms only during the day and survives for just two or three days annually.

Imagine his expression if he learned this—Zero couldn’t help laughing. And even worse, the exact days it blooms each year were unknown!

“Muzan’s an idiot—his IQ’s pathetic! No, no, no—I can’t laugh…!” But picturing Muzan’s furious, ridiculous reaction made him burst out laughing anyway.

Though he didn’t know what Muzan Kibutsuji looked like, that didn’t stop Zero from imagining.

Luckily he held back his laughter—if not, Yoriichi Tengen outside the door might have wondered what was wrong.

Calming himself, he drew a line under “Sunlight-Immune Being” and wrote: “Blue Higanbana,” but felt something was missing.

He added: “Idiot, IQ pathetic,” then drew an arrow pointing directly at Muzan Kibutsuji’s name.

Hidden in the Infinite Castle, Muzan suddenly sneezed, a chill running through his body.

“Achoo! What’s going on? Why do I feel someone’s plotting against me?!”

Zero looked at his notebook—he’d already gathered surprisingly important notes. Not many, but more and more complete than what he and Xiao Yue had collected earlier.

He began searching for information on Breathing Techniques, writing on another page: “Origin Breathing. Sun Breathing.”

“According to the earlier notebook, the Origin Breathing is Sun Breathing—but most couldn’t master or endure its solar power, so other Breathing Techniques emerged. Can I find related information?”

“Let’s try. Maybe I’ll find what I need.”

【Search: Sun Breathing】

【Searching… Search successful】

【Sun Breathing: The origin Breathing Technique, the ancestor and foundation of all Breathing Techniques, created and used by its originator, Kyojuro Rengoku.】

【Sun Breathing emits energy identical to the sun, devastating to demons. It can also directly coat the blade with energy, forming a blazing crimson blade—the Hekito.】

【When plunged into a demon’s body, it destroys internal organs, causing agonizing pain as if the demon’s insides were burning in flames, and prevents regeneration as the body gradually disintegrates.】

【Due to its overwhelming power, most cannot endure it, so five foundational Breathing Techniques derived from Sun Breathing emerged: Water, Thunder, Flame, Rock, Wind.】

【Demon Slayer Corps members later created numerous derivative Breathing Techniques from these five foundations.】

“So that’s how many types of Breathing Techniques exist—and these five foundational ones must have branches too. For example, Inosuke Hashibira’s Snake Breathing must be a derivative of one of these five.”

He drew five lines around “Origin Breathing. Sun Breathing” and wrote the names of the five foundational Breathing Techniques.

“But what exactly is this ‘Hekito’ mentioned here?”

“Hekito… the blazing crimson blade? The energy described… I think it’s sunlight absorbed by Gyokuto steel, combined with Shinshinhi Iron and Shinshinhi Ore—but what’s the trigger?”

He drew a line beside “Nichirin Blade” and wrote: “Hekito,” pondering the key.

“My Type I and Type II generate thermal energy by adding Energy Crystals—same effect as Hekito. So, does the Nichirin Blade need something added? No, no—it must be triggered by something else! What?”

“Breathing Technique, Mark, Transparent World, Hekito… the Nichirin Blade requires a wielder—so it’s grip strength? But can people in this world really have that much grip strength? No, no—Kyojuro Rengoku could. Or rather, with Breathing Technique and the Mark, strength increases—enough to do it… maybe…”

“Ah, forget it. I’ll ask Yoriichi Tengen later—borrow a Nichirin Blade and test it myself. If I can do it, I’ll teach them the method—give them an extra edge in battle…”

He drew another line beside “Hekito” and wrote: “Grip Strength.”

Just as he was about to continue searching the other notebooks on the shelf, a knock came at the door.

Knock knock knock!

“Zero, it’s nightfall. Do you need any assistance?” Yoriichi Tengen’s voice came from outside the door.

“Wait, I’ve been here for a whole day?!” Zero thought, then packed up his notes into the nanospace, shut off the projector of his analysis glasses, returned the notebook to the bookshelf, and left the study.

Opening the door, he saw Ubuyashiki Amane standing there, who said: “I’m sorry, Lady Amane, for keeping you so long!”

“Lord Ubuyashiki instructed me that if you need anything, you may tell me—we will do our utmost to assist you.”

“Actually, there is one thing I’d like to ask: do you have a portrait of Kyojuro Rengoku from the Sengoku period? The one who created the Original Breathing Technique?”

Zero’s reasoning was simple: if a portrait existed, he could use his analysis glasses to extract data from it, gain more information, and perhaps use that data to deduce Muzan Kibutsuji’s appearance and uncover further useful details.

“I’m sorry, Lord Zero, I don’t know about that. But I can ask Lord Ubuyashiki—he might know.”

“Alright, then I’ll trouble you. I’ll come back tomorrow. Also, I’ve found some information that might help the Demon Slayer Corps.”

“Understood. I will inform Lord Ubuyashiki. Thank you, Lord Zero!”

“Oh, by the way, could you arrange for me to have a Nichirin Blade? A standard one will do—I plan to run an experiment. If my theory is correct, it might make dealing with demons easier.”

“Understood. I will inform Lord Ubuyashiki as well.”

Finally, he bid farewell to Ubuyashiki Amane and left the mansion.

Ubuyashiki Amane relayed this matter to Ubuyashiki Kagaya.

Upon hearing this, Ubuyashiki Kagaya began searching for a portrait of Kyojuro Rengoku; if it could aid Zero, it would also benefit the Demon Slayer Corps.

The night was deep and still.

On his way to Chouji House, Zero heard Xiao Yue calling out.

“Master!” Xiao Yue sprinted over and landed on Zero’s shoulder, hugging his neck tightly.

“What’s wrong, Xiao Yue? Are you unwell?”

“Master, are you abandoning me?”

“Why would you say that?”

“I didn’t see you all day. I thought you’d left me behind! Waaah…”

Hearing this, Zero couldn’t help but smile wryly. He took Xiao Yue off his shoulder, wiped her tears with his finger, and held her in his palms as he comforted her: “There, there, don’t cry. I never said I’d abandon you. I went to the Ubuyashiki mansion today to look for information and lost track of time—came back late. I’m sorry, Xiao Yue, for worrying you.”

“Really?”

“Of course! You’re so adorable—how could I ever abandon you?”

“Waaah… That’s good! I thought you’d left me all alone!”

“You silly child! I can’t even scold you. How about tomorrow—you come with me? That way, you’ll be reassured, right?”

“Mm-hmm!”

“It’s getting late. Let’s head back to Chouji House and rest.”

“Okay!”

In the end, Xiao Yue settled inside Zero’s coat pocket, her head poking out, as they returned together to Chouji House.