Inside the Sekkiseki Vein Laboratory.
"You're fast. I only mentioned it a few days ago, and you're already moving," Nanatsuki remarked with a hint of admiration.
"When the teacher gives instructions, it's only natural to treat them seriously. Besides, I'd already made some preparations in this direction," Aizen Sōsuke replied softly.
Before them stood a transparent polyhedral container. Suspended at its center was a bead emitting a faint, blue phosphorescent glow.
The Hollowfication vaccine developed recently was merely a byproduct of researching the Soul King's power. But now, Aizen had succeeded in fusing some of those fragments into a unified whole—the prototype of the Hōgyoku.
"This is the key to transcending the limits of the Shinigami... the Hōgyoku."
Nanatsuki lifted the polyhedral vessel containing the "Hōgyoku," sensing the subtle fluctuations through his spiritual perception.
"It hasn't reached its full potential yet. It likely doesn't have that kind of power just now," Aizen noted.
This "Hōgyoku" was still incomplete—merely a prototype. The real thing was far from realization. After all, this version had been created using only a portion of the Soul King fragments. The rest, confiscated from the Tsunayashiro Clan, remained unused.
"We're on the right path. Reaching the end is just a matter of time," Nanatsuki said, setting the Hōgyoku down and picking up a nearby data file.
"These are the test results using the Hōgyoku. Its amplification effect on Hollows is dramatic," Aizen said with a faint smile. "We now have the capacity to mass-produce Arrancars."
With the Hōgyoku in hand, their next step was figuring out how to use it to strengthen both Shinigami and Hollows. Aizen had taken a very direct approach—implanting the Hōgyoku straight into a Hollow's body.
Due to its limited power, the current Hōgyoku couldn't affect a Hollow externally. Implantation was necessary for it to work.
"Not bad… in just a few days, an Adjuchas-level Gillian evolved into an Arrancar Gillian," Nanatsuki nodded slightly. "Is that test subject still alive?"
He wanted to assess the Hollow's condition. If the evolution process proved stable, Harribel could undergo the same transformation. Though Harribel had a humanoid form, she had yet to achieve true Arrancar status.
"Still alive," Aizen confirmed. "I'm planning to set up more control groups and run further tests on him. After extracting the Hōgyoku, I locked him in a holding cell."
Aizen led Nanatsuki to the room where live subjects were contained.
"A Hollow this quiet is rare."
Nanatsuki glanced inside. A muscular male Arrancar sat calmly on a bed. The room looked perfectly ordinary, and yet the Arrancar made no attempt to flee.
"In his mind, he's still in the Forest of Menos."
"I see—you used Kyōka Suigetsu to hypnotize him."
Aizen opened the door, and yet the Arrancar didn't react at all.
A moment later, the Arrancar turned his head and locked eyes on the two intruders.
"Die, Shinigami!!!"
Having evolved, the Arrancar's strength—and ego—had both surged. Faced with the sudden appearance of two Shinigami, he didn't hesitate to attack.
The Arrancar charged, but from their perspective, he was merely spinning in place.
"Looks like evolution into an Arrancar doesn't come with added intelligence."
Aizen shook his head in mild disappointment.
"His Reiatsu is impressive. Even among the Menos, few possess pressure like this."
Nanatsuki narrowed his eyes. "He's grown considerably. I'd say his Reiatsu is close to Third-Class Spiritual Might."
Aizen nodded lightly and altered the illusion, shifting the Arrancar's perceived surroundings to a training ground.
Inside the illusion, the Arrancar began fighting imaginary enemies with full force, unaware of the dream he was trapped in. He freely displayed his power before the two observers.
"No visible side effects. Set up ten more control groups to verify."
Nanatsuki watched for a moment longer, finding no adverse reactions.
"No problem. I've already prepared plenty of Hollows," Aizen said with calm confidence.
After observing the Arrancar, Aizen moved on to present another project.
They entered a second room filled with several cultivation pods. Each one contained a slumbering Hollowfied being.
"These are byproducts from the previous Hollowfication vaccine experiments. They didn't survive the process and fully turned into Hollows," Aizen explained.
"Next, we'll test the compatibility between these beings and the Hōgyoku. If it can evolve Gillians into Arrancars, then... what effect will it have on these Hollowfied Shinigami?"
Aizen's eyes gleamed with curiosity as he gazed at the subjects.
"It might make them stronger, but I doubt it'll restore their sanity..."
Nanatsuki's gaze passed slowly over the pods—until it froze on one in particular.
Inside the pod was a creature clad in jet-black armor, its inner body stark white, and two horn-like protrusions rose from its head.
'White.'
The thought struck Nanatsuki like lightning.
The creature reminded him strongly of the Hollow White—the entity that once dwelled inside Kurosaki Ichigo.
"This project takes priority. Once the Arrancar control tests are done, focus on this one."
Nanatsuki remembered the mission titled "Path of Transcendence." To complete it, he needed to nurture disciples who had reached the peak of power as a Shinigami, Hollow, and Quincy.
On the Shinigami side, Aizen was his trump card.
But for the Hollows and Quincies, Nanatsuki had to find his own way.
For the Hollows, the Hōgyoku might be the key. If he could evolve Nelliel or Harribel into Hōgyoku-tier beings, he'd be on the right track.
As for the Quincy, the bar was set at Yhwach.
The four Quincy disciples he had taken in recently... none of them seemed likely to reach that level.
But Kurosaki Ichigo—he had the potential.
If Nanatsuki could make Ichigo his disciple, then perhaps this "Path of Transcendence" could truly be completed.
—Elsewhere, in the remote outskirts of the Ken District, Soul Society—
Urahara Kisuke, cloaked and unassuming, stepped into a grove of withered trees.
A single ordinary-looking tree blocked his path—but he didn't stop.
He walked straight through it like a phantom.
The tree vanished behind him.
Urahara was gone.