Summoned from the seal on Kei's spine, the Zero-Tails erupted into the physical world. Instantly, the heavy, suffocating aura of dark chakra flooded the massive vault, mirroring the ancient, malevolent energy radiating from the Box of Ultimate Bliss.
Having absorbed enough ambient misery from Kei's patients to cross the threshold into a true, albeit minor, Tailed Beast, the Zero-Tails had developed a primitive, predatory consciousness. The grotesque, mask-wearing leech fixed its sightless gaze directly onto the colossal stone cube. It let out a low, vibrating roar, sensing the unfathomable ocean of despair trapped inside.
But it did not move. It waited for its master.
Seeing the beast's absolute obedience, Kei smiled beneath his porcelain mask. In terms of raw, apocalyptic firepower, the infant Zero-Tails was inferior to any of the nine true Tailed Beasts. However, its greatest tactical advantage was its complete lack of ego.
By mastering Shinnō's intricate jutsu formula, Kei had essentially bypassed the traditional, agonizing struggle of a Jinchuriki. He didn't need to negotiate with a demon for rent; he owned the house. Whenever he required power, the Zero-Tails provided it instantly and without question.
"Go," Kei commanded softly. "Devour it. Let us see how much you can grow."
The Zero-Tails shrieked in compliance. It surged forward, its long, smooth, slug-like body violently splitting into dozens of thick, muscular tentacles.
The dark tendrils slammed into the Box of Ultimate Bliss, wrapping tightly around the four demonic stone faces. The Zero-Tails unhinged its massive, leech-like maw and began to violently siphon the negative emotional energy directly through the stone.
Sensing the sudden, fatal drain on its lifeforce, the entity sealed within the Box panicked. The colossal stone cube began to shake violently. Monstrous, echoing howls erupted from deep within the artifact as Satori desperately attempted to break the seal and escape the parasite.
But it was entirely futile. Warden Mui had not yet sacrificed enough prisoners to fully charge the lock. The Box was dormant, unable to open its doors to defend itself.
"Damn you! Get away from it!"
Warden Mui sprinted into the vault, his face twisted in absolute terror and rage. The very first thing he saw was his village's legendary artifact being actively consumed by a dark, writhing abomination.
Refusing to lose his only hope of resurrecting his dead son, Mui screamed for his elite guards to open fire.
"Earth Style: Mud Wall!"
Before the volley of explosive kunai could reach Kei or the beast, Kakashi's hands blurred through a sequence of seals. A massive, reinforced wall of bedrock erupted from the floor, absorbing the barrage of explosions effortlessly.
Seeing the masked intruder repel a coordinated assault without breaking a sweat, Mui's pupils contracted. He instantly recognized the terrifying speed and precision of an elite Jonin.
But the fanatical desperation to save his son obliterated his tactical caution. Mui threw himself at the wall.
"Fire Style: Heavenly Prison Fire Sword!"
Mui channeled highly concentrated, white-hot fire chakra into his right hand, shaping the flames into a lethal, crackling blade. He vaulted over the earth wall, diving directly at Kakashi, desperate to break the defensive line and reach the Box.
Kakashi had absolutely no intention of engaging the Warden in a prolonged melee. As Mui descended, Kakashi exhaled a massive Great Fireball to blind him, simultaneously using his Sharingan beneath his mask to perfectly copy the Warden's unique jutsu sequence.
When Mui violently dispersed the fireball and swung his blazing blade, he froze in shock. The silver-haired intruder standing before him was holding an identical, perfectly formed sword of fire.
"How is that possible? Are you...?" Mui gasped, a horrifying suspicion regarding the intruder's identity flashing through his mind.
He didn't have time to finish the thought. Kakashi lunged forward, his copied blade crashing against Mui's in an explosion of sparks, locking the Warden in a deadly, high-speed duel.
On the opposite flank, Haru engaged the wave of Kusagakure guards pouring into the vault.
"Gentle Step: Twin Lion Fists!"
Brilliant, roaring lions of pure blue chakra ignited around her hands. Although Haru was only a Special Jonin, she was facing standard prison guards. She waded into their ranks without a fraction of hesitation. Furthermore, having been personally tutored by Kei in the mechanics of the Kaiten, she possessed an absolute defense. She was virtually untouchable.
With his two partners flawlessly holding the line, Kei focused entirely on his prize.
The progress was staggering. The Box of Ultimate Bliss bucked and screamed, but the Zero-Tails was a gluttonous black hole. As the minutes ticked by, an ocean of pure, concentrated terror and despair was violently ripped from the stone and converted directly into dark chakra.
Gorging on the feast, the Zero-Tails began to rapidly mutate.
Originally, the beast was roughly thirty feet long. As it drained the Box, its muscle mass exploded. The dark, leathery skin thickened into heavy armor. Within ten minutes, its size had doubled to over sixty feet, its massive body coiling heavily around the shrinking artifact.
And it wasn't stopping. The beast was still growing.
"Stop it! Who the hell are you people?!"
Desperately parrying Kakashi's relentless assault, Mui watched in pure agony as the dark aura surrounding his village's ultimate weapon visibly withered and died. He was on the verge of a total psychological collapse.
He simply could not comprehend what kind of monsters had invaded his prison. How did they know the Box was here? And more terrifyingly, what kind of abomination possessed the ability to eat the Sage of Six Paths' chakra?
Mui would never get his answers. He was a highly respected Jonin of the Grass Village, a master of complex fuinjutsu, but he was fundamentally outmatched in a physical duel against the legendary Copy Ninja.
Seeing that the outcome was already guaranteed, Kei decided to stretch his legs.
Because of his extended disguise, it had been a very long time since Kei had truly engaged in live combat. It was time to test his newly repaired eyes.
Kei flared his chakra. The veins bulged violently around his temples as the Byakugan activated, instantly painting the chaotic battlefield in flawless, 360-degree clarity.
Channeling lethal Gentle Fist chakra into his fingertips, Kei utilized the Body Flicker. He vanished, reappearing directly in the blind spot behind Mui, just as the Warden swung his sword at Kakashi.
Sensing the sudden, lethal drop in air pressure behind him, Mui abandoned Kakashi entirely. He spun around, swinging his fire blade in a desperate, sweeping arc to cut the blind-sided attacker in half.
But Kei was already moving.
With his perfect 360-degree vision, Kei read the microscopic twitch of Mui's shoulder muscles before the sword even swung. Kei stepped effortlessly inside the Warden's guard, casually swatting the blazing blade aside with a chakra-coated palm.
Mui gasped, feeling completely helpless. Every single counterattack he attempted was instantly read and neutralized before it even began. He was forced to expend every ounce of his stamina just to survive the barrage of lightning-fast, open-palm strikes.
Within three seconds, Mui's chest and arms were covered in bruising palm imprints. His chakra network sputtered and began to fail as Kei systematically shut down his tenketsu.
"The Gentle Fist... you are a Hyuga! Why?!" Mui screamed, his mind completely breaking as he realized the identity of his executioner.
The revelation made absolutely no sense. Kusagakure maintained a fragile, but officially allied diplomatic relationship with Konoha. Why would a Hyuga elite launch a black-ops strike against an allied prison?
The agonizing confusion caused Mui to hesitate for a fraction of a millisecond.
It was a fatal error. Kei's palm shot forward.
Every instinct in Mui's body screamed that he was about to die. Desperate, he channeled the last dregs of his failing chakra, forming a single hand seal to trigger the Body Flicker jutsu, hoping to teleport to the far side of the vault.
Mui vanished in a blur of motion, reappearing near a shattered stone pillar thirty feet away.
For a single heartbeat, Mui thought he had survived.
Then, he looked down.
Kei was standing directly in front of him. The doctor had matched the Warden's teleportation perfectly, appearing at the exact same destination a fraction of a second faster.
Kei's hand was buried deep in the center of Mui's chest.
"Why...?" Mui choked on a mouthful of blood, his eyes wide with disbelief as his heart was violently crushed. "Tell me... why?"
At that exact moment, as the darkness closed in, Mui knew he had lost everything. His grand dream of restoring his village's military supremacy was dead. His desperate prayer to resurrect his son was dead.
"My apologies, Warden," Kei said, his voice entirely devoid of emotion, his pristine Byakugan staring blankly into the dying man's eyes. "Your sacrifice will be remembered by the Avengers."
Kei felt absolutely zero remorse. He held no personal grudge against Mui. Under different circumstances, they might have been political allies. But Mui possessed the fuel Kei required to evolve the Zero-Tails. Therefore, Mui was an obstacle. And Kei eliminated obstacles.
Mui's pupils dilated. In his final, agonizing moments, he stared at the terrifying, mocking porcelain mask, desperately trying to burn the image into his fading memory. It was useless. His breath rattled in his chest, and he went limp.
Kei pulled his hand free, allowing the corpse to drop to the stone floor. He flicked the blood from his fingers, shaking off a fleeting, meaningless twinge of sentimentality.
He took no joy in slaughtering Mui. The Warden was a capable shinobi, but he possessed no Kekkei Genkai and no overwhelming power. Kakashi could have easily executed him alone. The monsters Kei would eventually have to face—Obito, Pain, Madara—were infinitely more terrifying. To survive them, he had to keep moving forward, ruthlessly crushing anyone who stood in his path.
Turning away from the corpse, Kei rejoined the fray, moving like a ghost through the remaining ranks of prison guards.
With his Byakugan active and his physical limits fully restored, Kei was an absolute force of nature. He moved through the chaotic brawl with terrifying grace, his palms striking with lethal precision. Every single time he moved his hands, a guard dropped dead, their internal organs shattered.
Watching their Warden fall and their ranks being decimated by a single, untouchable phantom, the morale of the Kusagakure forces completely broke. Panic and despair set in.
Kakashi moved to flank the remaining survivors, his blade flashing in the dark. As an ANBU veteran, he felt absolutely no sympathy for his enemies. But watching the cold, mechanical efficiency with which Kei was slaughtering the guards, a heavy, cold weight settled in Kakashi's chest.
Kei had sworn that every single atrocity they committed was necessary to secure the future of the Leaf Village. But watching the doctor effortlessly rip a man's heart out, Kakashi felt a deep, primal fear.
What happens if he loses control? Kakashi thought grimly, parrying a desperate strike. If he decides the village itself is an obstacle... could I even stop him?
Kakashi shook his head violently, forcing the treacherous thoughts away. He couldn't afford to doubt the mission now. The only way to ensure the brilliant, terrifying doctor didn't stray into total darkness was to remain firmly by his side, acting as his conscience.
The massacre ended quickly. Within minutes, the vault was entirely silent, save for the heavy breathing of the three Avengers standing amidst the sea of corpses.
The Zero-Tails finally released its grip on the Box of Ultimate Bliss.
The ancient artifact was now a dead, hollow husk of mundane stone. The unimaginable ocean of negative emotion, and the demonic entity Satori trapped within it, had been entirely devoured.
The Zero-Tails was massive. Its body had swollen to nearly a hundred feet in length, coiled thickly around the center of the vault like a grotesque, dark serpent. Having gorged itself on a feast of pure terror, the beast had fallen into a heavy, comatose slumber to digest the power.
"Did it succeed?" Haru asked, stepping carefully over the bodies, her Byakugan deactivated.
Kei placed a hand against the beast's leathery hide, sensing the violent, cascading mutation occurring deep within its chakra network. "The evolution has been successfully triggered. When it finally wakes, its power will have permanently crossed the threshold."
Kakashi sheathed his blade. He stared at the massive, sleeping abomination, and then turned to look at Kei. His visible eye was deadly serious.
"Kei," Kakashi said quietly. "May I ask you a question?"
"Of course, Captain. We are friends, are we not?" Kei replied smoothly, easily reading the profound, lingering dread in the older ninja's posture. "Ask whatever you wish."
"Everything we just did here tonight... the blood we just spilled... our crusade for 'revenge'..." Kakashi's voice trembled slightly. "It is truly for the ultimate protection of the Leaf Village. Is it not?"
Kei looked at the silver-haired assassin, seeing a man desperately begging for moral validation to quiet his own nightmares.
Kei stepped forward and placed a warm, reassuring hand on Kakashi's shoulder. "Kakashi. Have you not been standing by my side this entire time? Have you not seen the peace we are fighting to build?"
Kakashi let out a long, heavy sigh. "I know. It is just... looking at that monster... I am afraid."
Kei offered his trademark, gentle smile beneath the porcelain mask. "You have absolutely nothing to fear, my friend. The only people who should be terrified of that monster... are the men plotting to destroy our home."
Kakashi stared deeply into the dark eye-holes of Kei's mask for a long moment. Slowly, he nodded his head, choosing to believe the lie.