When Giyu pushed open the door to the room in the Kyogoku House, the figure in a black kimono was already seated at the table, idly twirling a hairpin between slender fingers—Muzan hadn't left yet.
Those crimson eyes swept across Giyu's neck, gleaming coldly with mockery. "You couldn't even finish off a few wounded Hashira. Were you holding back, or have you really fallen so low that even humans are beyond you?"
Giyu lowered his head without replying. He knew that any explanation would only arouse suspicion. Silence was the best defense.
"Speak."
Muzan's voice dropped, sharp as a blade. Black mist gathered in her palm.
Before Giyu could utter a word, that force struck his chest like a hammer. The mist exploded violently, blowing half his body apart in an instant. Flesh and black blood splattered across the walls, the remaining half of his body hitting the floor hard. Agonizing pain consumed him.
"Useless."
Muzan stood over him, looking down like a god passing judgment. "Do you remember what I ordered you? Fail again, and you can stop calling yourself Upper Moon Two."
With that, her figure slowly faded, dissolving into the air.
Giyu lay on the ground, black mist pouring from the severed half of his body, knitting his flesh back together rapidly. He clenched his teeth and stayed silent—the attack had been brutal, but not lethal. It was a warning, not punishment.
Recruiting a new Upper Moon was both Muzan's command and his opportunity. He had to choose someone trustworthy—someone who could cooperate with him from the inside.
Once his body finished healing, Giyu sat at the table, quietly organizing his thoughts.
The first name that surfaced was Ubuyashiki Kagaya. If Oyakata-sama became a demon, would his curse disappear?
The thought alone made Giyu's pulse quicken—but he crushed it immediately.
No. Oyakata-sama was the heart of the Demon Slayer Corps. If he became a demon, the entire organization would collapse. That path could never be taken.
Next came the Hashira.
Himejima Gyomei? Impossible. The Stone Hashira was the Corps' strongest warrior—turning him into a demon would cripple their greatest strength.
Uzui Tengen? No. His flamboyant nature would never tolerate becoming the very thing he despised.
Shinazugawa Sanemi? Out of the question. His hatred for demons ran too deep—his rage would devour him before he could ever act the part.
Kocho Kanae? No. Shinobu would never forgive him for that.
Tokito Muichiro? His talent was exceptional, but he was still too young. He shouldn't bear such a cruel burden.
Rengoku Kyojuro? His flame symbolized justice itself. To make him a demon would shatter everything he believed in.
He went through each name in turn—and then, one suddenly stood out.
Iguro Obanai.
The quiet swordsman with bandaged lips and sharp eyes. His swordsmanship was precise, his loyalty unwavering, and his nature calm and calculating.
Most importantly, he had the patience to endure. If anyone could disguise themselves flawlessly under Muzan's gaze, it was Obanai.
Giyu wasn't certain if Obanai had already become a Hashira, but if he hadn't yet, that would make the risk smaller. A single swordsman could disappear more easily than a pillar.
Decision made, Giyu pulled a blank sheet of paper toward him and began writing—with his left hand.
His right-hand handwriting was too easily recognizable. Though his left-hand strokes were clumsy, they'd conceal his identity.
The letter said only this:
"A loyal, calm, and talented swordsman is needed. Three nights from now, at midnight, meet by chance at the Northern Mountain Shrine. You will 'encounter' Upper Moon Two and act as though captured. Do not resist—follow all further instructions thereafter."
At the end, he added a detailed location marker.
He folded the note and sealed it in an envelope, unsigned. On the front, he wrote only:
"To Rengoku Senjuro, private."
Senjuro—Kyojuro's younger brother. When Giyu had helped the Rengoku family with their correspondence in the past, Senjuro had seen his left-hand writing. He would recognize it as familiar and pass it immediately to his elder brother, who would then bring it to Oyakata-sama.
Once the letter was sent, Giyu waited.
He didn't know if Iguro Obanai was already a Hashira, nor whether Oyakata-sama would approve such a daring plan.
But it was the only path forward. Only by planting an ally within Muzan's ranks could they gather the intelligence needed—information that might one day bring about Muzan's destruction.
At the Demon Slayer Corps headquarters, the air was heavy with tension.
Inside the Butterfly Mansion's infirmary, Uzui Tengen's right arm was fixed in a splint, Shinazugawa Sanemi's left leg was wrapped tightly in layers of bandages, and Rengoku Kyojuro still lay unconscious, his chest wound only recently past the danger point.
Aside from Himejima Gyomei, Kocho Kanae, and Tokito Muichiro, who hadn't joined the battle, every other Hashira was severely injured—unable to return to combat anytime soon.
Rengoku Senjuro held a sealed letter in his hands as he approached Ubuyashiki Kagaya.
"Oyakata-sama, this letter was sent to me. The handwriting looks like Tomioka-san's left-hand writing. I didn't dare open it myself, so I brought it to you."
Kagaya took the letter, unfolded it, and read carefully. His brow furrowed deeper with each line—then slowly eased. A faint light of admiration flickered in his eyes.
Placing a swordsman inside the Upper Moons' ranks—such an audacious plan. No one had ever dared suggest something like it before. If discovered, the infiltrator would be slaughtered by Muzan without mercy, and Giyu himself would be doomed. But if it succeeded, the Corps could gain direct intelligence from inside Infinity Castle—no longer fighting blind as before.
"Senjuro, thank you. You may go and rest."
Kagaya's tone was soft yet deliberate. Once Senjuro left, he pondered for a moment, then called a member of the Kakushi. "Bring me Iguro Obanai immediately."
A short while later, Obanai entered the study. His uniform was dark, his face half-concealed beneath bandages, only his sharp eyes visible. "Oyakata-sama, you asked for me?"
"Sit, Obanai."
Kagaya gestured for him to take a seat and handed over the letter.
"This came from Giyu. He requests our cooperation in a plan—to pretend to be captured by him and become a demon, infiltrating Muzan's ranks."
Obanai read through the letter quickly. His expression didn't flicker with surprise, only resolve. "For you, Oyakata-sama, and for the Demon Slayer Corps, I am willing to become a demon."
Kagaya nodded slightly, his voice calm but heavy. "This mission is dangerous. Muzan may take control of you. Your comrades might misunderstand you. And it's possible… you'll never become human again. Have you truly decided?"
"I have."
Obanai rose and bowed slightly. "I will follow the instructions in the letter. I'll be at the Northern Mountain Shrine on time and won't expose my identity."
"Good."
Kagaya smiled faintly, his tone filled with quiet faith. "Remember—no matter what happens, the Corps will always be your support. Tamayo-san is developing a medicine to turn demons back into humans. If you can endure, we will bring you home."
Obanai said no more. He turned and left the room.
Back in his quarters, he packed lightly. Without telling anyone of his mission, he left only a farewell note with the Kakushi before departing alone toward the Northern Mountain Shrine.
Three nights later, at midnight, beneath the pale moonlight, the old shrine stood silent.
Moonbeams streamed through the broken lattice, falling across the dust-covered statue of the mountain god.
Obanai gripped his Nichirin Sword and stood in the center of the shrine, waiting for the "chance encounter."
Footsteps echoed outside. A dark figure stepped through the doorway—it was Tomioka Giyu, now in the form of the demon "Mushiki."
He gazed at Obanai, the dark blue slit pupils glinting faintly. He confirmed what he had suspected—Obanai had already become the Serpent Hashira.
"The new Hashira of the Corps, huh?"
Giyu's voice was calm and cold.
Obanai didn't answer. He raised his sword at once, the faint purple energy of Serpent Breathing coiling around the blade. "Serpent Breathing, First Form: Winding Serpent Slash!"
His sword struck like a venomous viper, twisting sharply toward Giyu's chest.
Giyu shifted aside, his own blade gleaming with a blue crescent. "Moon Breathing, First Form: Dark Moon, Evening Palace!"
The flash of his strike was faster than lightning. It grazed Obanai's arm, leaving a shallow line of blood.
Steel clashed again and again. Obanai's Serpent Breathing was agile and unpredictable, but no matter how he attacked, he couldn't pierce Giyu's defense.
Giyu kept perfect control over his strength. Each blow looked lethal but landed with precision—just enough to wound, never to maim. He had to make the act convincing without truly harming him.
After several exchanges, Giyu spoke quietly. "Was that your full strength?"
His voice stayed calm as he suddenly surged forward, his Nichirin Sword pressing against Obanai's throat.
Obanai glared back, fury rising for no reason he could explain—something about this man stirred an instinctive dislike deep in his blood.
He raised his blade again, eyes locked on Giyu.