Sora confirmed his guess. After passing through the reishi barrier with his shadow space, he saw a unique glass tank.
"A Vasto Lorde Arrancar?" Sora was slightly taken aback.
A Hollow with a human face was submerged in the tank's reishi nutrient fluid. A dense network of tubes passed through the glass and was inserted into her body.
The upper torso of the human-faced Hollow was that of a human female. Her lower half had no legs; instead, there were limbs resembling octopus tentacles, the same color as her long, silver-white hair.
"It looks like a corpse... No wonder I didn't detect any strong spiritual pressure from outside the room," Sora frowned.
Upon closer observation, he realized this human-faced Hollow was not a living being.
The tubes inserted into her were injecting some kind of chemical to maintain the activity of her corpse.
Sora searched the room and quickly found the research file on this Hollow.
He soon made a startling discovery—
This human-faced Hollow was actually a natural-born Arrancar!
More accurately, she was half-human, half-Hollow.
Tetsuya Miyamoto had conducted inhumane experiments.
He used human spiritual bodies as subjects, forcing them to bear the offspring of a human and a Hollow.
The human-faced Hollow Sora was looking at was the result of this experiment.
She was born an Arrancar and possessed the ability of high-speed regeneration.
Miyamoto's Spiritual Body Growth Agent was created by extracting this Hollow's blood.
Perhaps because she was half-human, her blood, which contained Hollow power, did not cause uncontrolled mutations in Shinigami.
Sora flipped through a stack of experimental records and saw numerous failed attempts at creating half-human, half-Hollow spiritual bodies.
The records noted the origin of the "vessels" and the experiment dates. They were mostly residents from the Rukongai who had Shinigami potential.
At this point, Sora quickly put some things together.
Most of the mysterious shrines he had seen during his study tour with Genryūsai Shigekuni Yamamoto were connected to the Lord of Hell.
The core members of these shrines were mostly Reigai created by Miyamoto.
Through these Reigai, Miyamoto spread the faith connected to the Lord of Hell in the Rukongai, and then selected suitable vessels from among the believers for his human experiments.
"Miyamoto's ambition is considerable... to have set his sights on Hollows so early on," Sora thought with some emotion.
Miyamoto must have discovered the limitations of Shinigami Reigai after many attempts and decided to work on other types of spiritual bodies instead.
After sacrificing countless souls with Shinigami potential, this one half-Hollow was born.
And even she was not perfect; she seemed to have some unknown congenital defect.
Her growth was incredibly fast, developing from an infant to an adult form in just seven days, but she died suddenly after living for only a short period.
Even so, this human-faced Hollow was a miracle of chance. Miyamoto's many subsequent attempts to replicate her were all failures.
The other test subjects underwent uncontrollable Hollowfication and mutation during the embryonic phase, devouring their mothers before completely turning into Hollows.
Miyamoto also discovered that the human-faced Hollow's blood was special, capable of promoting growth and high-speed regeneration in other spiritual bodies.
He used technology to maintain the activity of her corpse, extracting her blood to create the growth agent.
"It seems the Reigai being developed in the labs outside all contain trace amounts of Hollow power," Sora thought.
Since the growth agent was made from the human-faced Hollow's blood, it would inevitably contain Hollow reishi.
Short-term use, like how Miyamoto used it to heal his injuries, was fine. The trace amounts of Hollow reishi wouldn't put a significant strain on the spiritual body.
But for the Reigai that were force-matured with long-term use of the agent, the accumulation of Hollow reishi would become greater and greater.
Sora even suspected they might have the potential to undergo Hollowfication.
He decided he had to take some of the Reigai back to his own lab to observe their growth.
Of course, he couldn't pass up the human-faced Hollow in this secret room. This natural-born half-Hollow was extremely special and would be of great help to his research on Shinigami Hollowfication and Arrancar creation.
Most importantly, he wanted to try using the growth agent to develop another Reigai of Katori Batsu'unsai!
The Seireitei's original Reigai technology couldn't produce a Reigai capable of growth, resulting in a spiritual core that was too fragile to handle Batsu'unsai's spiritual body.
But Miyamoto had iterated on the technology, allowing him to develop Reigai from an embryonic phase.
Such a Reigai had room for growth and also eliminated the problem of personality defects.
By using the growth agent, he could quickly mature the Reigai's body to the limit that its spiritual core could handle.
The body's development would, in turn, promote the development of the spiritual core.
This greatly shortened the body's growth time.
It might be possible to use this accelerated development technology to produce a spiritual core that could handle Batsu'unsai's spiritual body.
He would have to take them back and test the limits of a Reigai created this way.
The laboratories outside had plenty of ready-made subjects for observation. The Reigai being developed there were mostly grown using this method.
Sora found the data on the drugs that maintained the human-faced Hollow's body activity. After a quick review, he knew what to do.
Miyamoto was prepared to move his lab at any time, so the life-support tank was designed to be detachable. After some effort, Sora managed to dismantle it.
Sora packed the tank and the life-support drugs into his reishi space pouch.
It was a good thing he had the pouch; otherwise, there would have been no way to take this large tank with him.
He didn't spare the data in the secret room either, packing it all away.
After confirming nothing was left behind, Sora once again opened his shadow space and passed through the room's reishi barrier.
The moment he came out, he saw Yoruichi leaning against the wall.
"Is there anything else you need to pack, Professor?" Yoruichi asked. "We've finished organizing the data on our end."
"I need to take some of the Reigai as well," Sora said.
"Then let's take all the unbroken tanks. Leaving them here would just be a gift for the 12th Division," Yoruichi said with a smile.
Currently, only the 12th Division had a research lab. The Reigai the Onmitsukidō brought back would only serve as mission evidence.
Such evidence would ultimately be sent to the 12th Division for Kirio Hikifune to handle.
"My reishi space pouch doesn't have enough room," Sora said.
There were still one or two hundred intact tanks. Sora's pouch was already filled with various items and simply couldn't hold that many.
"Your pouch is full?"
"That's easy to solve!" Yoruichi took out a plain-looking scroll. "I have a rather large one here. It's a gift for you, Professor~"
Sora took the scroll. When he checked the space inside, he was stunned by its size.
"This space is huge. For such a large and stable reishi space, the technology of this scroll must be quite advanced," Sora said, surprised. It was the largest storage-type reishi space he had ever seen.
"This 'Tenzen Heisō' has a sizable 'Valley of Screams' space sealed within it. It's not a reishi space expanded by Kidō, so its structure is very stable," Yoruichi explained.
"With this much space, there's more than enough room for all these tanks," Sora remarked.
With the help of the Onmitsukidō members, they spent some time dismantling the intact tanks in the laboratory.
Sora packed up the intact tanks, while the Onmitsukidō took the damaged ones and the dead Reigai as mission evidence.
Three Onmitsukidō members approached Yoruichi and said, "Vice-Captain, the Reigai on the upper living area have been dealt with."
The strongest of those Reigai were only equivalent to an ordinary squad member and stood no chance against the three elite Shinigami.
If not for the fear of them sounding an alarm and allowing Miyamoto to escape, they wouldn't have had to sneak in so carefully.
After clearing out the laboratory, the group returned to the Soul Society through a Senkaimon.
Since Sora had participated in the mission, he went with Yoruichi to the 1st Division.
Yoruichi had already informed them of their return via Denreishinki. The captains and acting captains of the various divisions were now waiting in the 1st Division barracks.
The 1st Division's lieutenant, Chōjirō Sasakibe, was already waiting at the entrance.
Sora hadn't deliberately hidden his injury, and Sasakibe noticed it.
"The captains are waiting inside, but do you need to treat your injury first?" Sasakibe asked.
"No need. This injury can't be healed quickly. As long as I maintain it with Kaido, it won't worsen," Sora shook his head gently.
His healing could not achieve high-speed regeneration. The lost flesh wouldn't grow back in a short time.
"You've worked hard. It seems this mission was not easy," Sasakibe said, becoming slightly more talkative when faced with an injured person.
"I'm fine. Please lead the way, Vice-Captain Sasakibe," Sora said with a smile.
Sasakibe nodded lightly and led them into the 1st Division barracks.
When the three arrived at the assembly hall, all eyes were drawn to the injury on Katori Sora's hand.
"Oh my—how did you get hurt so badly!" Kirio Hikifune took Sora's left hand. Seeing the exposed bones of his index and middle fingers, her face filled with distress.
She had come to see Sora as a close junior. Seeing him so severely injured pained her.
"Don't worry. I just used a sword technique that exceeded my body's limits. It will be fine after some treatment back home," Sora said.
"Come to my lab later. I'll prepare some medicine to help your body recover," Kirio said.
"Was the enemy this time so strong that he could injure you this badly, Katori-kun?" Shunsui Kyōraku raised an eyebrow. He was well aware that Sora's current strength was definitely at a captain's level, yet he was still so injured.
"We didn't expect Miyamoto to have become so much stronger. The Mizutani family didn't have specific intelligence on his abilities, otherwise we should have sent two more captains," Kirio said.
"He modified his Zanpakutō. Its ability was quite troublesome. I didn't want to get into a drawn-out fight, so I had to use a technique that was beyond my body's limits to end it quickly," Sora explained.
"He actually modified his own Zanpakutō?" Kirio was surprised, and a little regretful. "Miyamoto was a talented technician. It's a shame he went down the wrong path and turned against the Seireitei."
A Zanpakutō is part of a Shinigami's soul and not something that can be easily modified.
The slightest mistake could have very serious consequences.
The fact that Miyamoto could modify his own Zanpakutō was proof of his extraordinary skill in spiritual body research.
"Have you developed an even more powerful sword technique, Katori-kun? Since you're so badly injured this time, why don't you rest and recover properly in the 4th Division?" Retsu Unohana said with a pleasant smile.
Even though her eyes were narrowed, Sora could still feel the intensity in her gaze.
"Thank you for the kind offer, Captain Unohana, but the Academy is about to start. Now is not the time for rest," Sora said righteously.
If he went to the 4th Division to recover, Unohana would probably drain him dry before letting him go.
But his new technique was too powerful and couldn't be used in the Seireitei. It was also hard to control and not suitable for sparring.
"You've worked hard, Katori-kun. Miyamoto's base was in the World of the Living, so it should have been the 13th Division's responsibility," Jūshirō Ukitake said, apologizing for not participating in the mission.
Miyamoto had never released his Bankai in front of the Mizutani family, so they had no intelligence on it.
"Although Miyamoto is dead, the 'Gate of Hell' that appeared afterward took his reishi away," Sora continued. "Based on what we saw with Captain Takezuna, it's highly likely we will have to deal with Miyamoto again in the future."
Sora then described Miyamoto's Zanpakutō ability: "He modified his Zanpakutō to allow spore-like parts of it to proliferate endlessly. His Bankai also enhances this ability. When these spores touch another's spiritual body, they cause an infection that leads to mutation..."
"'Gate of Hell'..." Genryūsai Shigekuni Yamamoto's eyes were deep, as if he were once again reliving the scene of striking down his former comrade.
As one grew older, one tended to reminisce about the past.
Yamamoto came back to his senses and said slowly, "Katori Sora has done well! He has eliminated the cancer that was Tetsuya Miyamoto. Without him creating Reigai, it will not be so easy for that 'Will of Hell' to open gates in the Soul Society."
"Next, all squads will conduct another internal search. The Seireitei, the Rukongai, and the noble district will all be thoroughly swept. We must find all remaining Reigai!"
"Yes, sir!"
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