Nanatsuki and Saitō Seiko confirmed that the vortex within the Eye of Hell could not be used to reach Muken.
Whether it was a spirit body or a Reishi-formed object, anything approaching the sealing spell inside the vortex would be broken down into Reishi.
Saitō Seiko had no choice but to abandon her plan to travel to the Soul Society via the Eye of Hell for now.
Ever since reconnecting with Saitō Furaofushi, Seiko had been eager to return to the Soul Society—not for her own sake, but because she wanted to eliminate the Tsunayashiro Clan.
As long as the Tsunayashiro Clan remained, Furaofushi would never be released from Muken.
But since the vortex route was blocked, Seiko would need to find another method.
In gratitude for the information Seiko had provided—information that had led to his obtaining the Hell Pearl—Nanatsuki crafted another Sound of Hell for Furaofushi to pass the time.
He had previously gathered a good number of suitable conch shells during a trip to Hueco Mundo, and with the abundant Hell Reishi here, the materials and conditions to make another Sound of Hell were readily available.
Once finished, Nanatsuki and Furaofushi left the small sealed chamber in the depths of Muken.
"From everything we've seen so far, it's highly likely that whatever's sealed here has something to do with the Lord of Hell," Nanatsuki said. "I need to head to the Great Spirit Book Gallery to dig up more information."
Though he had no intention of breaking the seal, he still wanted to find relevant records if possible.
The Great Spirit Book Gallery contained the historical archives of the Soul Society—including many significant events.
There was a chance it held references to this particular seal.
"You could try checking the Saitō Family's archive first," Furaofushi suggested. "The Saitō line produced several Shinigami with Zanpakutō linked to Hell. For example, my grandmother's Zanpakutō could release a mist called Breath of Yomi—formed from the waters of the underworld. That same mist also exists in Hell."
"So our family knows more about Hell than most."
"Your grandmother's Zanpakutō can release Breath of Yomi, too?" Nanatsuki raised a brow. "What a coincidence. One of my newest disciples also has a Zanpakutō with the same ability. In fact, our recent contact with your grandmother was triggered because her Breath of Yomi activated the Sound of Hell."
"Another junior with similar powers to Grandma, huh..." Furaofushi idly twirled the end of her right ponytail. "If that girl ever reaches captain-class strength and dies—landing in Hell—she'd be terrifying. With high-density Hell Reishi enhancing her, her Breath of Yomi would turn into genuine underworld waters. The corrosion of the spirit and mind would multiply several times over."
"That's probably a long way off..." Nanatsuki scratched his face. "Kakyō is still young, still studying at the Shin'o Spiritual Arts Academy."
After chatting for a bit longer, Nanatsuki left Muken.
A few weeks later—
When Nanatsuki returned to the lab, he found Aizen Sōsuke immersed in his work.
The lab was full of Reishi Shells, all of which required growth serum to maintain their spiritual development. He and Hoshino Rie took turns monitoring them.
In addition to the Shinigami shells salvaged from Miyamoto Tetsuya's lab, the experimental Hollow shells they had been cultivating were also progressing steadily.
"Sōsuke, how are the shells developing?" Nanatsuki asked, observing their growth from the corridor.
"Miyamoto Tetsuya's spiritual growth serum is highly effective. Three of the shells have already matured to adolescent form—their Reiatsu is strong enough to qualify for academy enrollment." Aizen paused, then added, "But if Sensei plans to attempt Hollowfication experiments, I'd advise waiting until they reach the strength of Shinigami squad members."
"They're still too weak right now and won't survive Hollow particle invasion."
"I agree," Nanatsuki nodded. "Let's keep nurturing them for now. By the way, remember when we discussed the matter of the Soul King last time? Did you manage to look into it at the Shiba Clan's archives?"
Nanatsuki had intentionally brought up the Soul King in front of Aizen to provoke him into digging deeper, to uncover more truth.
Because soon, Nanatsuki planned to start research on Soul King fragments.
In his view, without the Hōgyoku, ordinary Shinigami bodies and Reishi shells couldn't bear the burden of Hollow power.
So he wanted to introduce the variable of Soul King fragments into the process of Shinigami-Hollow hybridization and vice versa.
He had no doubt in Aizen's talent—his contributions could vastly accelerate the research.
Now was the right time to let Aizen learn more about the Soul King.
Of course, the Shiba Clan wouldn't willingly grant Aizen access to their secrets. But Aizen's Kyōka Suigetsu allowed him to come and go undetected.
"I've been there," Aizen said, wearing a complex expression. "I never expected the Soul King—who supposedly reigns above all—to actually be the so-called linchpin of the Three Realms."
What he had learned at the Shiba archives had shaken Aizen to his core—enough to challenge his very worldview.
Both the Academy and the Gotei 13 taught that the Soul King ruled over the realms.
That he had created them, and kept them running.
But Aizen had never imagined that the Soul King's way of maintaining order was by becoming the literal Keystone of the Three Realms.
"Sensei, do you think the Soul King—reduced to an object like that—still retains consciousness?" Aizen asked.
"He does," Nanatsuki answered with certainty. "In fact, it's possible that even this conversation is already known to him. The Royal Guard can still receive the Soul King's orders."
"The Royal Guard? You mean the Division that serves the King directly?" Aizen's eyes widened slightly, concern rising. "Then... are we committing treason just by talking like this?"
"You don't need to worry," Nanatsuki said with a chuckle, amused by Aizen's wariness. "Even if you criticize the Soul King now, nothing will happen. Becoming the Linchpin of the Three Realms was the Soul King's own will."
"That was his choice...?" Aizen murmured, deep in thought. "Well... it does make sense. How could the Soul King have possibly been defeated by the Five Noble Clans?"