The Sekkiseki Mines Laboratory.
Nanatsuki and Aizen were conducting adaptation training with Shinigami who possessed Hollowfication abilities.
So far, there were already two Shinigami who, after using the Hollowfication Vaccine, had successfully regained their sanity and were suppressing their Hollow powers through sheer willpower.
The success of the vaccine wasn't a fluke—so long as one had strong enough willpower, it was possible to restrain the Hollow power within.
As such, Nanatsuki had left it up to Katori Batsu'unsai to decide whether to replace her Spiritual Pressure Core with a Hollowfied Artificial Body.
A bone-like mask formed over Miyanō's face, and in the next instant, she lost all reason.
Her mind was dragged into the Inner World, where she fought against the Hollow power.
"Hahahaha!!!"
The crazed Matsuri let out a twisted laugh, wildly slashing at empty air.
"Kyōka Suigetsu's complete hypnosis really is useful," Nanatsuki chuckled. Matsuri had already been hypnotized by Aizen Sōsuke and was now attacking a nonexistent enemy with all her might.
"Entering the 'Masked' state voluntarily causes a massive spike in Spiritual Pressure, but they still lose rationality."
Aizen recorded Matsuri's condition.
This power couldn't be sustained for long. The mask on her face would fall off soon, which was why they referred to a Shinigami's Hollowfication as "Masking."
"Let's hope she can tame the Hollow power this time. If she can maintain rationality, the Masked form could become a trump card comparable to releasing a Zanpakutō."
Nanatsuki nodded.
As the two conversed, clarity returned to Matsuri's eyes.
"Ah!!!"
Matsuri tore off the mask from her face with a firm yank.
"Huff… hah… huff…"
Traces of fear lingered on her face.
As a Shinigami, she was terrified of harboring Hollow powers.
"Matsuri, there's no need to be afraid. You've already overcome it, haven't you?" Aizen gave her a gentle smile. "You're strong. I believe in you."
"Vice-Captain Aizen!" Matsuri's gaze gradually steadied. "You're right… there's nothing to fear. I've already conquered it!"
"Matsuri, there's one more thing I need to tell you..."
Aizen lowered his voice. "Your Shinigami registration number has been revoked."
"What?"
Matsuri was stunned.
A revoked registration meant the Shinigami had either retired or died.
But she was clearly alive and hadn't filed for retirement—how could her number be revoked?
"I'm sorry… I mentioned before that our experiments couldn't be disclosed."
Aizen's expression was full of apology and reluctance. "Your injuries were examined by Squad 4, but neither they nor the Technology Bureau could create an Artificial Body and install a Spiritual Pressure Core for you in such a short time.
If you returned to duty, it would cause problems. So I reported that 'Miyanō Matsuri died from critical injuries in the Squad 5 infirmary.'"
"I see. It's okay. Just being alive is already a blessing." Matsuri was saddened by losing her identity, but she still nodded. "I don't want to cause trouble for Vice-Captain Aizen… or Captain Katori."
"I'm glad you understand." Aizen smiled gently again. "Don't worry. You've seen my Zanpakutō's abilities—I can help you return to the light under a new identity."
"Thank you! To have the help of Vice-Captain Aizen… and Captain Katori, I'm truly lucky!"
Matsuri was touched. Vice-Captain Aizen had thought so much about her.
No wonder he was so beloved in Squad 5—his kindness was famous.
"Go get some rest now. A battle of willpower can be incredibly draining." Aizen gave a small nod.
Nanatsuki and Aizen left the lab room.
"She trusts you a lot," Nanatsuki said with a smile.
"This lab contains too many things that can't be shown to the outside world," Aizen sighed lightly. "If the Central 46 found out what we were researching, we'd have no place left in the Soul Society."
There were many Soul King fragments in this lab—a secret that could never be leaked.
The Hollowfication Vaccine Aizen developed was actually a byproduct of his research into those fragments.
If this were revealed, it could shake the Soul Society to its core.
"Batsu'unsai has decided to replace her Spiritual Pressure Core. I'll need a Hollowfication Vaccine," said Nanatsuki.
"The Shishō truly lives up to being a First Generation Captain—completely fearless," Aizen said with admiration.
The two moved to another lab, where Aizen prepared a dose of Hollowfication Vaccine.
"The separation agent is inefficient, but fortunately, the amount of 'Power of the Soul King' needed per dose isn't much."
Nanatsuki accepted the vaccine Aizen handed over.
Its main ingredient was trace amounts of Power of the Soul King, extracted from the Soul King fragments.
It could slow the spread of Hollowfication and buy time for Shinigami to resist their inner Hollow.
"This is the sixth version we've iterated," Aizen said with a wry smile. "Separating the 'Power of the Soul King' is unbelievably difficult. Still, the hardest part of the 'Hōgyoku Project' has been solved."
The project name—Hōgyoku Project—had been coined by Nanatsuki. Its goal: to transcend the limits of Shinigami.
Their biggest obstacle had been separating the Power of the Soul King from the fragments.
The existence of Hollowfied Shinigami proved that this power did indeed allow Shinigami to surpass their limits.
"The separation agent needs further refinement. Otherwise, just processing the Soul King fragments we took from the Tsunayashiro Clan will take decades," said Nanatsuki.
"That'll be my main focus going forward." Aizen paused, then added, "Sensei, I plan to apply for the Captain qualification exam."
"Looks like Shinji's been giving you pressure—you're even moving aside voluntarily…" Nanatsuki nodded. "I'll recommend you to the Head-Captain."
"You don't seem surprised that I've mastered Bankai," Aizen raised a brow.
"Among all my students, your talent is the best. Mastering Bankai is as easy as drinking water for you," Nanatsuki smiled. "I know you don't like drawing too much attention. You probably mastered it long ago and waited for Kana and Kensei to catch up before showing your hand."
Over the years, many of their students had progressed. Both Azashiro Kana and Muguruma Kensei had already attained Bankai and were applying for their own Captain exams.
In truth, Hirako Shinji had achieved Zanpakutō materialization even earlier than Kensei. But his Zanpakutō was rebellious, refusing to submit or reveal its true name.
Since Shinji didn't have a Bankai Training Doll, he couldn't forcibly summon his Zanpakutō's spirit to fight.
"Sensei flatters me. I'm just a little diligent."
Aizen modestly raised a single finger to mimic a galaxy.
He had indeed mastered Bankai long ago, but hadn't revealed it so as not to attract attention.
Besides, in Squad 5, he already operated no differently from the Captain.
Still, with Shinji nearing completion of his own Bankai training, Aizen couldn't allow his Squad to slip from his grasp—he needed to pass the exam first.
A few days later, Nanatsuki brought Batsu'unsai to the Sekkiseki Mines Laboratory.
"I've heard you talk about this place often, but this is my first time here."
Batsu'unsai looked around curiously, completely unbothered by the challenge ahead.
"You never cared for these kinds of things," Nanatsuki commented.
"You guys do all the brainy work… not my style."
Batsu'unsai was self-aware—fighting was her thing, not thinking.
"Come with me. Rie's off today, but she'll be assisting me with the Spiritual Pressure Core replacement."
Nanatsuki led the way to the Artificial Body Lab.
"She's become your right hand these days. Hard to believe she almost flunked out of the Spiritual Arts Academy."
Batsu'unsai sounded impressed.
"Rie's worked hard these past years."
Nanatsuki gave a soft nod.
When they entered, Hoshino Rie had already prepped the surgical setup.
"Sensei, Shishō!"
"We'll leave it to you, Rie!"
"Don't worry, Shishō! I've done this surgery several times with Sensei—I'm well-practiced."
"Then let's begin."
Batsu'unsai lay down on the operating table and quickly fell unconscious under the effects of powerful anesthetics.
Soon after, Nanatsuki and Rie completed the procedure—implanting the Hollowfied Spiritual Pressure Core into Katori Batsu'unsai's Artificial Body.
As expected, her inner Hollow power reacted immediately. Nanatsuki injected the Hollowfication Vaccine directly into her artery.
"Ahhhh!!!"
Batsu'unsai let out a layered, beast-like roar as her Reiatsu surged violently.
The terrifying pressure swept through the room, slamming all equipment against the walls.
Nanatsuki shielded Rie from the chaotic torrent of Reiatsu.
The force of Batsu'unsai's release far surpassed that of Matsuri.
In that moment, Rie felt the terror of a First Generation Squad 8 Captain.
Even with Nanatsuki shielding her, the crushing aura was overwhelming.
Her spiritual perception screamed in alarm, throwing her mind into chaos.
Rie's body trembled uncontrollably.
"You should leave and lock the door."
Nanatsuki sensed the change in his student and gave instructions.
The walls and doors were made of Sekkiseki, isolating internal Reiatsu turbulence.
Since Rie wasn't a combatant, she had never undergone pressure resistance training.
She wouldn't be able to withstand a force that could incapacitate even most seated Shinigami.
"Understood. Please be careful, Sensei!"
Rie quickly exited the surgery room.
In the next moment, shadows spread from beneath Nanatsuki's feet, pulling him and Batsu'unsai into the Shadow Space.
A brutal battle was about to begin.
Until Batsu'unsai defeated her inner Hollow in her Inner World, she would remain in an irrational, berserk state.
Nanatsuki would have to endure her indiscriminate attacks for a while.
Inside Batsu'unsai's Inner World—
"So, you're the 'Hollow Power'."
Batsu'unsai stared at a figure identical to herself, save for the color difference.
"That's right."
The white Batsu'unsai's voice echoed with layered resonance.
"Then let's fight!"
A fiery fighting spirit surged on Batsu'unsai's face—she hadn't had a proper battle in a long time.
Clang—!!!
Their blades collided.
Crack
The Sekkiseki bindings on Batsu'unsai's hands and feet snapped from her brute strength.
She was now covered in a vicious bone-like armor, looking every bit like a humanoid Hollow.
Nanatsuki raised his hand, summoning several shadows to bind the Hollowfied Batsu'unsai.
A dull drumbeat sounded—she vanished.
The next instant, she appeared directly above Nanatsuki's head.
He sank into the shadows, narrowly dodging her attack.
"Sonído…"
Nanatsuki realized: she wasn't using Shunpo—she had used the Hollow's Sonído.
Sonído was a form of spatial teleportation that could break restraints instantly.
With her target lost, Batsu'unsai's bone mask glowed with blackish-red light.
A terrifying beam of energy—far beyond a standard Adjuchas-class Cero—struck the Shadow Space's dark wall.
BOOM—!!!
Cracks appeared in the Shadow Space.
"I'll have to stop her..."
Though the space could be repaired with Reiatsu, if a Cero escaped, it would destroy everything in the lab.
Even if the opponent was Batsu'unsai, Nanatsuki was no longer who he once was.
This version of her had only Hollow instinct—no Zanpakutō techniques or swordsmanship. Nanatsuki could handle it with ease.
A figure wrapped in golden lightning burst from the shadows.
BAM! Nanatsuki cleaved the Cero in half.
"ROAR!!!"
The Hollowfied Batsu'unsai sensed life and lunged.
Nanatsuki vanished in an instant; she struck nothing.
Their figures flickered rapidly through the air.
With multiple buffs stacked, Nanatsuki's speed had become the fastest in Seireitei—even quicker than Sonído.
Every attack from Batsu'unsai missed its mark.
Not wanting to hurt her, Nanatsuki merely evaded with speed.
Eventually, Batsu'unsai's eyes cleared, and the bone armor crumbled.
"You're awake?"
Nanatsuki used Shunpo to catch her mid-fall as her strength gave out.
"I won," Batsu'unsai panted. "Haven't practiced my swordplay in a while—it's gotten rusty."
Worried she'd hurt Nanatsuki in her berserk state, she rushed the fight, ironically giving the Hollow more time to resist.
But now, feeling her Reiatsu steadily stabilizing, she was exhilarated.
"Nanatsuki, thank you." She knew how much time and effort he'd spent solving the Spiritual Pressure Core problem for her.
At last, it had paid off.
"There's no need for thanks between us." Nanatsuki smiled.