Three days after the successful capture and treatment initiation of Nakime, Su Chen convened a war council in the Ubuyashiki estate's main hall. Every available Hashira had assembled, along with Tamayo, Yushiro, and Su Chen's core team. The atmosphere was tense with anticipation—everyone understood that they were planning the operation that could finally end Muzan Kibutsuji's thousand-year reign of terror.
Kagaya Ubuyashiki sat at the head of the gathering, his health visibly improved each day as his body recovered from the curse's removal. His presence lent authority to the proceedings, though everyone understood that Su Chen would be directing the actual combat operations.
"Three days of interrogation and observation have provided critical intelligence," Su Chen began, activating a holographic display—technology that made several Hashira flinch in surprise before they adjusted to yet another demonstration of impossible capabilities. The display showed a three-dimensional map of Japan with multiple locations marked in red. "Nakime's memories, combined with Tamayo's centuries of research and the Corps' accumulated intelligence, have allowed us to narrow Muzan's possible locations to these seventeen sites."
The map zoomed in, showing detailed layouts of each location—abandoned estates, hidden underground facilities, even several locations within populated cities where Muzan had apparently been hiding in plain sight by disguising himself as a wealthy businessman or noble.
"Seventeen locations is still too many to search efficiently," Gyomei observed, his blind gaze somehow focused on the display through his spiritual perception. "Muzan has survived this long by being extraordinarily cautious. If we investigate even one site too obviously, he'll detect our activity and relocate."
"Which is why we won't investigate," Su Chen replied. "We'll eliminate the possibilities through process of elimination using methods Muzan won't detect. Tamayo, you've been tracking demonic energy signatures for centuries. Can you identify which of these locations show the kind of concentrated demonic presence that would suggest Muzan's actual hiding place?"
Tamayo studied the display carefully, her centuries of experience evident in how she analyzed each location. "These three," she finally indicated, highlighting sites in Tokyo, Osaka, and a mountain region. "The others show either no demonic presence or signatures consistent with lesser demons. But these three show spatial distortions and energy concentrations that could mask Muzan's presence."
"Nakime's spatial manipulation," Sanemi growled, understanding immediately. "He's using her technique—or rather, another demon with similar abilities—to hide his actual location behind warped space."
"Correct," Su Chen confirmed. "Muzan is paranoid enough to employ multiple layers of concealment. But he made one critical error: he didn't account for his most trusted servants being captured and interrogated by someone with abilities exceeding this dimension's normal parameters."
He manipulated the display, causing the mountain location to pulse. "This is his actual hiding place—the Infinity Castle. It's not a physical structure in normal space. It's a pocket dimension created and maintained by spatial manipulation demons, existing slightly out of phase with reality. That's why the Corps has never been able to locate it despite centuries of searching. You were looking in the wrong dimension entirely."
"A pocket dimension," Tengen said slowly, his shinobi training helping him grasp the concept faster than some others. "That's... that's actually brilliant from a defensive standpoint. If it doesn't exist in normal space, normal attacks can't reach it. How do we even begin to assault something that isn't technically 'there'?"
"By forcibly dragging it into normal space," Su Chen replied with cold confidence. "I've already analyzed Nakime's spatial manipulation principles and the structural framework maintaining the Infinity Castle. I can create a formation that will anchor the pocket dimension to our reality, forcing it to manifest physically. Once it's manifested, it becomes vulnerable to conventional assault."
He paused, then added, "Additionally, forcing manifestation will trap everyone inside. Muzan won't be able to escape by simply phasing back out of normal space. He'll be locked into direct confrontation, which is exactly what we want."
"When?" Kagaya asked quietly, his voice carrying the weight of generations of Ubuyashiki family members who had dedicated their lives to this goal. "When do we strike?"
"Tonight," Su Chen stated flatly. "Every hour of delay risks Muzan detecting our preparations. He's survived this long through paranoid caution—he monitors his servants, tracks unusual activities, and relocates at the first sign of organized opposition. We've been fortunate that Nakime's capture hasn't alerted him yet, but that luck won't last indefinitely."
"Tonight," Rengoku repeated, his perpetually enthusiastic expression becoming even more intense. "We end a thousand years of suffering tonight. The flames of justice will burn bright indeed!"
"Assault team composition," Su Chen continued, bringing up personnel assignments on the display. "Primary strike team: myself, Saeko, Esdeath, and Xiao Yi Xian. We possess the highest individual combat capabilities and can operate effectively in unstable dimensional environments. Secondary team: all available Hashira who wish to participate. Your role will be eliminating the other Upper Moons and any lesser demons defending the castle while I engage Muzan directly."
"You're fighting Muzan alone?" Shinobu asked, her analytical mind immediately recognizing the tactical implications. "That's either extreme confidence or—"
"Accurate assessment of relative power levels," Su Chen interrupted. "Muzan is approximately Spirit Transformation Realm equivalent—peak power for this dimension. I'm Formation Arrangement Realm with divine energy backing and techniques from multiple universes. Additionally, I've specifically prepared counters to his known abilities: his shapeshifting can't fool my dual pupils, his cellular regeneration is inferior to regeneration methods I've encountered and overcome, his blood manipulation won't work on my hybrid physiology, and his thousand years of experience become irrelevant when facing combat styles he's never encountered."
He looked around the assembled group, his expression serious. "I'm not being arrogant. I'm being realistic about power disparity. Muzan is a significant threat to this dimension. He's not a significant threat to me. The question isn't whether I can defeat him—it's whether I can do so before he attempts something desperate like triggering the castle's collapse to achieve mutual destruction."
"Then we prevent that scenario by eliminating his support structure first," Gyomei concluded. "The Hashira engage the Upper Moons and hold them away from Muzan. You defeat Muzan before he can resort to desperate measures. The castle remains stable because its maintaining demons are either dead or contained."
"Precisely," Su Chen confirmed. "However, I want to make one thing absolutely clear: this operation is optional for all Hashira. You've fought demons for years, risked your lives repeatedly, and earned the right to choose whether to participate in this final battle. No one will be judged if they decide the risk is too great."
Silence fell across the gathering as the Hashira exchanged glances, silently communicating through looks and subtle gestures. Finally, Rengoku spoke for the group, his voice carrying absolute conviction.
"We've dedicated our lives to eliminating Muzan. Every Hashira in this room has lost family, friends, or comrades to demons. The opportunity to participate in Muzan's final defeat isn't something we would refuse regardless of the risk. We're all in."
Nods and verbal confirmations came from around the room. Even Obanai, who had been skeptical about Su Chen's presence initially, showed determination to see this through to conclusion.
"Then we deploy at sunset," Su Chen stated. "Six hours from now. Use that time to prepare equipment, make peace with your affairs, and reach optimal mental state for combat. Tamayo, I'll need you to provide any anti-demonic compounds that might weaken Upper Moons. Shinobu, coordinate with Xiao Yi Xian on toxin deployment strategies. Saeko, Esdeath, conduct final equipment checks and coordinate with the Hashira on team tactics."
As the group dispersed to make preparations, Mitsuri approached Su Chen with unusual seriousness tempering her normally cheerful demeanor. "Su Chen-sama, I know I've been acting really enthusiastic about joining your organization, but... but this is real, isn't it? People might die tonight. I might die tonight. And I'm scared but I'm also ready and I don't know if that makes sense—"
"It makes perfect sense," Su Chen interrupted gently. "Fear and determination aren't mutually exclusive. The best warriors are those who feel fear but act anyway because the cause matters more than personal safety. Your fear proves you understand the stakes. Your determination proves you're worthy of the strength you possess."
"Thank you," Mitsuri said quietly, then seemed to gather herself. "Okay. I'll be ready. I'll fight my hardest. And afterward, when Muzan is dead, I'll officially join your organization and help people across dimensions just like I've been helping people here!"
After she departed, Tanjiro approached, Nezuko at his side. The demon girl's expression showed unusual seriousness—she understood that tonight's battle would determine the fate of all demons in this world, including herself.
"Su Chen-san," Tanjiro began, "Nezuko wants to come with us. She says she needs to see this through—to witness the end of the one who created her condition and killed our family. I think... I think she's right. She deserves to be there."
Su Chen studied Nezuko carefully, his dual pupils reading the determination in her demonic aura. She had grown significantly during her time with his organization—not just in power but in control and awareness. She was no longer the mindless demon Tanjiro had protected at the series beginning. She was becoming something new, something unprecedented.
"Nezuko can come," Su Chen decided. "But she stays with the secondary team and focuses on supporting the Hashira rather than engaging Upper Moons directly. Her blood demon art will be useful for enhancing their attacks. Agreed?"
Nezuko nodded enthusiastically, making pleased humming sounds. Tanjiro looked relieved—he had clearly been worried about her insisting on more dangerous participation.
The next six hours passed in a blur of preparation. Weapons were sharpened and blessed, toxins were synthesized and loaded into delivery systems, formations were inscribed and tested, contingency plans were developed for various scenarios. The Ubuyashiki estate transformed into a military operation center, with Kagaya coordinating logistics while his wife Amane ensured that detailed records were being made—if this operation succeeded, it would become the most important event in Demon Slayer Corps history.
As sunset approached, the assault team assembled in the courtyard. The Hashira wore their distinct uniforms and carried their personalized Nichirin blades. Su Chen's team wore their standard combat gear—Saeko in her modified fighting clothes, Esdeath in her signature white and blue, Xiao Yi Xian in flowing silver robes that seemed to shimmer with barely-contained toxins.
Su Chen himself had equipped simply: the artifacts he'd acquired from the Heavenly Tomb, emergency formations, and the Origin Mirror ready to copy Muzan's abilities during combat. He didn't need heavy equipment—his personal power was his primary weapon.
"Final briefing," Su Chen announced, causing conversation to cease. "Once I activate the dimensional anchor formation, the Infinity Castle will manifest physically in the mountain region. We'll have approximately five minutes before Muzan realizes what's happening and begins organizing defenses. Use that window to penetrate deep into the castle before organized resistance forms."
He manipulated a portable display showing the castle's probable internal layout based on Nakime's memories. "The castle is maze-like, designed to confuse and separate intruders. Ignore the maze—Saeko's sword intent can cut through walls, and Esdeath can freeze spatial distortions to create straight-line paths. We're not playing by the castle's rules."
"Upper Moons will likely be distributed throughout the castle," Su Chen continued. "Based on intelligence, we can expect to encounter Upper Moons One, Two, Three, and Six still at large. Upper Four is captured, Upper Five is dead. Each one is Hashira-level threat minimum, with Upper One potentially exceeding that. Hashira, work in coordinated teams against each Upper Moon—don't attempt solo engagements unless absolutely necessary."
"My team will proceed directly to Muzan's throne room," Su Chen concluded. "We'll force confrontation immediately rather than allowing him time to prepare or escape. This might seem like we're splitting our forces disadvantageously, but trust me: you're better off fighting Upper Moons without me present to draw Muzan's attention. Once I engage him, the entire castle will become a battlefield where being near me is more dangerous than facing demons."
"Any questions?" Su Chen asked. Silence answered him—everyone understood their roles and had made their peace with the risks.
"Then let's end this," Su Chen stated. He activated a spatial portal, its destination locked to the mountain coordinates where the Infinity Castle existed slightly out of phase with reality. "Through the portal. Stay in formation. And remember: tonight, we make history."
The assault team filed through the portal in disciplined order. When the last member had crossed, Su Chen stepped through himself, and the portal collapsed behind them.
They emerged onto a mountain peak shrouded in mist, the air thick with demonic energy that would have been oppressive to normal humans. Su Chen immediately activated the dimensional anchor formation he had spent the past three days preparing—a complex array combining Perfect World formation theory, Nakime's spatial manipulation principles, and his own understanding of dimensional mechanics.
Golden light erupted from his hands, spreading across the mountain in geometric patterns that burned away mist and cut through spatial distortions. The formation locked onto the Infinity Castle's dimensional signature, establishing anchor points that would drag the pocket dimension into normal space whether it wanted to manifest or not.
Reality screamed in protest as two dimensions were forcibly merged. The air split open, revealing glimpses of the castle's interior—endless corridors and rooms shifting and rearranging like a living maze. Then, with a sound like reality breaking, the entire structure materialized fully.
The Infinity Castle was massive—easily the size of a large town, its architecture impossibly complex with rooms and corridors that defied euclidean geometry even now that it existed in normal space. It towered above the mountain peak, multiple stories extending both upward and downward through space that should have been occupied by solid stone.
"It's beautiful," Mitsuri breathed, her aesthetic appreciation temporarily overriding tactical awareness. "Terrible and beautiful."
"It's a prison designed to trap and kill intruders," Sanemi corrected harshly. "Stay focused."
Alarms began sounding throughout the castle—Muzan had detected the forced manifestation. Demons would be mobilizing, traps activating, Upper Moons moving to defensive positions. The five-minute window of surprise Su Chen had mentioned was beginning.
"All teams, advance!" Su Chen commanded. "Hashira, engage any resistance you encounter! My team, on me!"
They charged into the castle, and chaos erupted.
The first wave of demons—lesser servants and guards—met them in the entrance corridor. Saeko's blade sang as she cut through them with contemptuous ease, each swing killing multiple opponents. Esdeath froze entire groups solid, then shattered them like ice sculptures. The Hashira deployed their breathing techniques, elemental effects manifesting as water, flames, thunder, and stone tore through demonic flesh.
They penetrated deeper, and the resistance intensified. Mid-tier demons with Blood Demon Arts attempted coordinated ambushes, but Su Chen's dual pupils detected them instantly and Xiao Yi Xian's toxins incapacitated entire groups before they could attack.
"First Upper Moon contact!" Gyomei bellowed, his spiritual perception detecting a massive demonic presence approaching from a side corridor. "Upper Moon Three—Akaza! He's moving to intercept!"
"Rengoku, Gyomei, Tengen—you three take Akaza!" Su Chen commanded. "Use coordination and overwhelming power! Don't give him time to analyze your techniques!"
The three Hashira peeled off to engage, their combined spiritual pressure creating a shockwave that cracked walls as they launched into combat against the Upper Moon who had killed countless Demon Slayers over the centuries.
"Upper Moon Six—Daki and Gyutaro!" Shinobu reported, her medical expertise identifying the siblings' synchronized spiritual signatures. "They're setting up ambush positions in the eastern wing!"
"Shinobu, Mitsuri, Obanai—handle the siblings!" Su Chen ordered. "Shinobu, your poisons are specifically effective against their regeneration! Mitsuri, your speed counters Gyutaro's!Obanai, your techniques can separate them to prevent synchronized attacks!"
Three more Hashira split off, racing toward the eastern wing where the demonic siblings awaited.
"Upper Moon Two—Douma!" Muichiro's detached voice suddenly sharpened with urgency. "He's in the northern chambers! I can sense his ice-based techniques activating!"
"Muichiro, Sanemi, Giyu—you're on Douma!" Su Chen commanded. "His emotional manipulation won't work if you maintain combat focus! Sanemi's wind techniques can counter his ice! Muichiro's mist can obscure his vision! Giyu's water techniques provide defensive support!"
The last three available Hashira departed, leaving only Su Chen's core team and Tanjiro with Nezuko. The final Upper Moon—Upper Moon One, Kokushibo—hadn't appeared yet, which was concerning. He was the strongest of Muzan's servants, and his absence suggested he was positioned directly with Muzan as final defense.
"We're approaching the throne room," Xiao Yi Xian reported, her poison-based senses detecting the massive concentration of demonic energy ahead. "Multiple presences—Muzan definitely, plus at least one other extremely powerful demon. Upper Moon One is likely with him."
"Expected," Su Chen replied. "Saeko, when we breach the throne room, you engage Kokushibo. He's a sword master—you'll enjoy the challenge. Esdeath, provide support and prevent escape attempts. Xiao Yi Xian, establish toxin barriers to prevent reinforcements from interfering. Tanjiro, Nezuko—you both stay near the entrance and handle any lesser demons that try to intervene. This is where I end this."
They reached massive double doors carved with demonic imagery. Su Chen didn't bother trying to open them—he simply released his spiritual pressure, and the doors exploded inward, the ancient wood unable to withstand Formation Arrangement Realm power.
Beyond lay a vast throne room, its ceiling lost in darkness above, its walls decorated with grotesque murals depicting Muzan's thousand years of atrocities. At the room's center sat a throne carved from what appeared to be bone, and upon that throne lounged a figure of terrible beauty—a pale man with blood-red eyes, dressed in Western aristocratic clothing, radiating power that made the air itself feel heavy.
Muzan Kibutsuji, the first demon, the Demon King, had finally been found.
Standing beside the throne was another figure—tall, with six eyes and a sword that looked eerily similar to those carried by Demon Slayers. Kokushibo, Upper Moon One, the strongest of Muzan's servants and the brother of the original Sun Breathing user.
"So," Muzan said with perfect composure despite his castle being invaded, his Upper Moons engaged, and his sanctuary violated. "The dimensional traveler finally shows himself. Nakime's betrayal suddenly makes sense—you're not from this world, so my control techniques wouldn't work normally on you. Clever. Inconvenient, but clever."
He stood smoothly, his movements carrying inhuman grace. "However, you've made a critical error. You've trapped yourself in here with me, thinking your foreign power makes you invincible. Let me educate you about the mistake of underestimating someone who has survived for over a thousand years."
Muzan's form suddenly exploded outward, his body transforming into a writhing mass of flesh and tentacles, each one capable of destroying buildings. His power erupted, shaking the entire castle, demonstrating why he had reigned as the unchallenged Demon King for so long.
And Su Chen smiled coldly, his dual pupils blazing with golden and silver light as he released his own spiritual pressure—Formation Arrangement Realm power backed by divine energy, refined through the Heavenly Tomb's trials, enhanced by techniques from a dozen different dimensions.
The collision of their auras cracked the throne room floor, shattered walls, and sent shockwaves through the entire Infinity Castle. Demons throughout the structure felt the overwhelming pressure and knew that something unprecedented was occurring in the throne room.
"Now then, Demon King," Su Chen said, his voice carrying absolute confidence, "let me show you what real power looks like. You've terrorized this dimension for a millennium. Tonight, that ends. Tonight, you learn that there are beings in the multiverse who make you look like a child playing at immortality."
He activated the Origin Mirror, ready to copy whatever abilities Muzan demonstrated before destroying him utterly. "Begin your final struggle. Make it entertaining enough that I remember your death among all the others I've witnessed."
The battle for the fate of the Demon Slayer world had begun in earnest. And Muzan Kibutsuji was about to learn that being the strongest being in one dimension meant nothing when facing someone who had transcended dimensional limitations entirely.
The end had come for the Demon King. He just didn't know it yet.