(T/N: Third Chapter!)
At night, a portion of the Thirteenth Division's archive room was shrouded in shadow.
Inside the Shadow Space, Nanatsuki was reviewing the Thirteenth Division's personnel transfer records.
During the Hyakuten¹ period, Nanatsuki had learned from Kensei that the three Shinigami previously in charge of securing Saitō Furuofushi'sSound of Hell had all been removed from the Penal Detention Squad.
The record scroll containing Sound of Hell had been tampered with—most likely by those same Shinigami who had handled the item.
After investigating their whereabouts, Muguruma found that all three had been transferred to the Thirteenth Division in the same year.
And so, Nanatsuki came to the Thirteenth Division to see if he could track them down.
Nanatsuki found the names listed in the archive:
"Kitano Asuka… Eighteenth Seat of the Thirteenth Division… Killed during World of the Living field mission."
"Sawada Taki… Thirteenth Division officer… Killed during World of the Living field mission."
"Kitayama Yūsei… Thirteenth Division officer… Killed during World of the Living field mission."
"They're all dead..."
"This happened more than a decade ago, before Ukitake Jūshirō even became the Thirteenth Division Captain."
Nanatsuki continued flipping through the records.
It was Miyamoto Tetsuya, the vice-captain at the time, who had transferred these three to the division.
They had been moved from the Ninth Division to the Thirteenth, assigned to World of the Living guard duty, and shortly afterward, were attacked by a Menos Grande and killed.
Nanatsuki frowned. Something about this didn't sit right.
Soon after, he located the personnel file of Miyamoto Tetsuya.
"Former Third Seat of the Twelfth Division. Promoted to Vice-Captain upon transfer to the Thirteenth. Disappeared during an investigation into Menos Grande attacks on Shinigami in the World of the Living. Whereabouts unknown to this day."
"'Hell consciousness'… 'Reishi Shell'… Third Seat of the Twelfth Division..."
Although Nanatsuki lacked concrete evidence, the web of connections gave him a nagging feeling—this Miyamoto Tetsuya was no ordinary man.
"Accessing the archives of the Twelfth Division won't be easy… they're bound to have multiple security layers against outsiders." Nanatsuki considered whether to ask Hikifune Kirio directly.
The three Shinigami removed from the Penal Detention Squad were all dead. The vice-captain who transferred them had vanished.
The Sound of Hell once held by Saitō Furuofushi was missing as well.
Since borrowing the Sound of Hell was no longer an option, Nanatsuki figured he might as well speak to Hikifune directly.
He had originally planned to use the Sound of Hell discreetly—hence his silence on the matter.
But now that it was unavailable, it made more sense to ask Hikifune about that former Third Seat of the Twelfth Division.
Before that, though, he wanted to visit Muken to see Saitō Furuofushi.
Saitō had been in possession of the Sound of Hell for quite a long time and should have some knowledge about Hell itself.
Although Muken Prison was heavily fortified, Nanatsuki's Shadow Space could pierce even Seireitei'sShakonmaku, so slipping into Muken wouldn't be difficult.
He returned the scrolls to their original position and left the archive.
The Shin'o Underground Prison was the largest detention facility in the Soul Society—used to imprison Shinigami criminals.
Its defense system was formidable, protected by a spiritual barrier that even Captain-class Shinigami couldn't break quickly.
Yet a shadow silently slipped past that very barrier.
Nanatsuki had condensed his Shadow Space to just cover his body—a black veil cloaking him like a shadowy mantle.
This layer allowed him to pass through nearly all spiritual barriers, including the one protecting the underground prison.
After breaching the first layer of defense, Nanatsuki merged into the shadows cast by the building itself, quietly observing his surroundings.
The exterior of the underground prison was entirely constructed with sekkiseki, a material known for blocking Reiatsu.
Thanks to that property, this was one of the most secure locations in Seireitei.
There were plenty of guards on patrol, rotating every four hours.
Nanatsuki waited for half an hour until the next rotation. Seizing the moment, he slipped into one of the guards' shadows and infiltrated the prison interior.
Once inside, the guards headed back to the resting quarters.
Nanatsuki didn't make any rash moves. He had gathered information from Muguruma Kensei—there were still patrols even within the prison itself.
All he needed to do was follow a patrolling guard, and he'd eventually find Muken.
"Sasaki, you take the next round of patrol, alright? I'm beat—I need a nap," said an older guard in the tone of a senior addressing a junior.
The younger guard had no objections. After resting for a bit, he got up and began his patrol.
Nanatsuki immediately slipped into his shadow and left the resting area alongside him.
The underground prison had a total of eight levels.
The upper floors held inmates with lighter sentences.
Each prisoner was held in isolation. The structure resembled an ordinary prison, and one could even see the inmates through the iron bars.
Inmates on the first level all wore Reiatsu-suppressing collars.
Such equipment was mandatory for anyone held here.
The lower the floor, the more restraints the inmates wore—and the worse their condition.
Long-term Reiatsu suppression severely damaged the soul body.
The young guard took a casual approach to his patrol—he didn't believe infiltration was even possible.
After all, the underground prison was one of the most heavily guarded locations in the entire Soul Society. There was no way anyone could sneak in undetected.
Eventually, Nanatsuki followed the guard to the lowest level—Muken.
The guard didn't enter. Inmates here were high-level criminals, and even prison guards were prohibited from direct contact.
Nanatsuki swiftly exited the guard's shadow and slipped along the wall, using Shadow Space to pass through solid stone.
Inside Muken, all was pitch-black.
But that didn't hinder Nanatsuki's vision. Ever since he acquired control over shadows, he could see in complete darkness.
The interior resembled a massive, boundless cavern.
'Judging by the footprint of the underground prison above, there's no way such a vast space should exist down here... Soul Society still hides many secrets. Muken… could it be related to the actual Hell?' Nanatsuki mused.
According to historical records, this so-called Endless Hell (Muken) had existed long before the underground prison was ever constructed.
The Shin'o Underground Prison was merely built on top of it.