Nanatsuki's guess was confirmed the moment he used Shadow Space to slip past a layer of spiritual particle shielding—revealing a very peculiar glasas tank.
"A Vasto Lorde-class Arrancar?" Nanatsuki murmured.
A Hollow with a human face was suspended in a tank filled with Reishi-rich nutrient fluid. Countless tubes pierced through the glass and into her body.
The upper half of her body resembled a human woman, but instead of legs, her lower body extended into octopus-like tentacles, all matching the silver-white hue of her hair.
"A cadaver... no wonder I couldn't sense any strong reiatsu outside." Nanatsuki frowned.
Upon closer inspection, he realized this human-faced Hollow was no longer alive.
The tubes connected to her were continuously injecting some kind of agent into her body, keeping the corpse biologically active.
Nanatsuki searched the room and quickly found documentation about the subject.
What he discovered was shocking: this was a naturally-born Arrancar!
More precisely, she was a hybrid—half-human, half-Hollow.
Miyamoto Tetsuya had conducted horrific experiments, using human spiritual bodies to give birth to children fused with Hollow essence.
This human-faced Hollow was the result of such an experiment.
She was born an Arrancar and possessed high-speed regeneration.
Miyamoto's spiritual growth agent had been made using her blood.
Possibly due to her hybrid nature, her Hollow-infused blood did not induce uncontrollable mutations in Shinigami bodies.
Nanatsuki flipped through a pile of experiment logs, which listed numerous failed attempts at creating similar hybrids.
Each log noted the source of the hosts and dates—most were residents of Rukongai with Shinigami potential.
It all began to click.
Back when he studied under Genryūsai Shigekuni Yamamoto, they had encountered suspicious shrines—most tied to the Lord of Hell.
The core members of these cults were actually artificial Reishi Shells created by Miyamoto Tetsuya.
By seeding these shells into Rukongai, Miyamoto spread the faith of the Lord of Hell and selected hosts from among the believers to conduct his experiments.
"His ambition is immense... he turned his eyes toward Hollows early on," Nanatsuki thought.
Failing to overcome the limitations of Shinigami shells, Miyamoto had shifted focus to other spiritual lifeforms.
After sacrificing countless spiritually gifted souls, this hybrid was born.
But even she was imperfect—possessing some unknown congenital defect.
She grew rapidly, maturing from infancy to adulthood in just seven days, but died shortly after.
Even so, she was a miracle, never replicated despite many attempts.
Other experiments degenerated into uncontrollable Hollow mutations at the embryonic stage, consuming the host and fully Hollowfying.
Still, Miyamoto found her blood useful for accelerating growth and regeneration in other spiritual bodies.
So he preserved her biological activity to harvest blood for spiritual growth agents.
"Looks like those Reishi Shells in the outer lab all contain trace amounts of Hollow essence," Nanatsuki concluded.
Since the growth agent was made from her blood, Hollow Reishi was inevitable.
Used short-term, like for healing, the low concentration didn't cause issues.
But for shells grown entirely on the agent, Hollow Reishi would accumulate.
Nanatsuki even suspected they could undergo Hollowfication.
He resolved to take some shells back for observation in his own lab.
This hybrid, of course, had to come with him. Her unique biology could significantly advance his research into Hollowfication and Shinigamification.
Most importantly, he wanted to try using the growth agent to cultivate a new body for Katori Batsu'unsai!
The early Reishi Shell tech of Seireitei couldn't grow adaptive bodies. Their Spiritual Pressure Cores were too fragile to handle Batsu'unsai's soul.
But Miyamoto's advancements allowed embryonic cultivation with growth potential and eliminated personality flaws.
Using the agent to rapidly mature the shell would push the Spiritual Pressure Core to its limit.
And as the body matured, the core would too—greatly reducing growth time.
He might just be able to create a core that could host Katori Batsu'unsai's spirit.
He needed to run tests and determine the limits of these enhanced shells.
Fortunately, there were plenty in the lab ready for observation.
Nanatsuki found the formula for the preservation agent used on the hybrid and committed the method to memory.
Miyamoto had prepared for a lab evacuation—the hybrid's life support tank was modular.
After some effort, Nanatsuki disassembled it.
He packed the tank and the agent into his Reishi Pocket.
Good thing he had it—otherwise transporting the tank would be impossible.
He also looted the chamber's research data.
After confirming nothing was left behind, he used Shadow Space to exit the sealed room.
Outside, Yoruichi was leaning against the wall.
"Anything else to pack up, Sensei?" she asked. "We've finished sorting our data."
"I need to bring some shells with me," he replied.
"Then let's take all the intact tanks. No point in letting Squad Twelve have them," she smiled.
Among the Gotei 13, only Squad Twelve had lab capabilities. The Onmitsukidō could only turn the shells in as mission evidence.
Such evidence would end up with Hikifune Kirio anyway.
"My Reishi Pocket's full," Nanatsuki said.
There were over a hundred intact tanks. His pocket already carried a bunch of odds and ends.
"You're out of space? Easy fix!" Yoruichi pulled out a scroll. "This one's big. I'll give it to you~"
Nanatsuki took the scroll and opened its space—and was immediately stunned.
"This Reishi space is massive... and stable too. This scroll's tech is impressive."
"It's a piece of Divine Armament. The sealed space inside is actually a portion of the Valley of Scream. It's not expanded with Kidō, so it's extremely stable," Yoruichi explained.
"This much space is more than enough to carry all the tanks," Nanatsuki marveled.
With help from Onmitsukidō members, they disassembled the usable tanks.
Nanatsuki packed them in. The others collected damaged ones as mission proof.
Three Onmitsukidō Shinigami approached Yoruichi. "Vice-Captain, the living area shells are all dealt with."
The strongest among them had power on par with regular Shinigami officers—no match for three elites.
Had it not been for fear of triggering alerts and letting Miyamoto escape, they wouldn't have bothered sneaking in.
Once the lab was cleared, they returned to Soul Society through the Senkaimon.
Since Nanatsuki had joined the mission, he accompanied Yoruichi to Squad One.
She had already informed them of their return, so all captains and acting captains were waiting.
Vice-Captain Chōjirō Sasakibe was already at the gate.
Nanatsuki didn't hide his injury, and Sasakibe immediately noticed it.
"Everyone's inside, but do you need treatment first?" Sasakibe asked.
"No need. It won't heal quickly, but Kaido is keeping it stable," Nanatsuki shook his head.
Kaido couldn't accelerate regrowth. Flesh wouldn't come back that fast.
"Tough mission, huh?" Sasakibe said with rare sentiment.
"I'm fine. Please lead us in," Nanatsuki smiled.
The three entered the meeting hall, where everyone's attention locked onto his hand.
"Oh no—what happened?!" Hikifune Kirio grabbed his arm, eyes wide with concern.
She treated Nanatsuki like a cherished junior. Seeing him like this clearly hurt her.
"No worries. I just used a technique beyond my limit. I'll recover," he said.
"Come to my lab later. I'll prepare something to help restore your body," she insisted.
"The enemy must've been strong to injure you like this," said Kyōraku Shunsui, brow raised. He knew Nanatsuki was already captain-tier.
"We underestimated him. The Mizutani family didn't have intel on his Bankai. Had we known, we would've sent more captains," Kirio said.
"He modified his Zanpakutō. It was hard to deal with. I had to go all out to end it quickly," Nanatsuki explained.
"Modified his Zanpakutō?!" Kirio gasped. "He had such technical talent... what a shame he went rogue."
A Zanpakutō was part of a Shinigami's soul. Tampering with it was dangerous.
That he succeeded only proved how deep his understanding of spirit research went.
"So you developed another high-powered sword technique? You should rest in Squad Four," said Unohana Retsu, eyes glinting despite her smile.
Even with her eyes half-closed, Nanatsuki felt her intensity.
"Thank you, Captain Unohana, but school term is starting soon. No time for rest," Nanatsuki replied righteously.
Resting in Squad Four... he feared Unohana might "train" him into the ground.
But the new technique was too dangerous for sparring, especially in Seireitei.
"You did well, Nanatsuki. This was technically Squad Thirteen's responsibility," said Ukitake Jūshirō, apologetic.
Mizutani intel never covered Miyamoto's Bankai, so they hadn't anticipated his full power.
"Though Miyamoto's body is gone, the Gate of Hell took his soul," Nanatsuki said. "And based on what we saw, we may have to face him again."
He explained Miyamoto's Zanpakutō: spores that replicated and infected any spirit they touched, causing grotesque mutations.
"The Gate of Hell..." Yamamoto's gaze turned distant, recalling the day he slew his former comrade.
Old age often brought back old memories.
Snapping out of it, he declared, "Nanatsuki did well! With Miyamoto gone, Hell's Will won't so easily open gates in Soul Society again."
"All squads, begin internal investigations! Search the Gotei, Rukongai, and even the Noble Quarters—every Reishi Shell must be found!"
"Yes, sir!"