The birth of the Hōgyoku allowed Aizen Sōsuke to glimpse a completely different path.
At the same time, the miraculous nature of the Hōgyoku fascinated him to an almost obsessive degree.
He clearly understood what this object represented.
If used correctly, the Hōgyoku could shatter the boundary that confined a Shinigami, allowing one to transcend into an entirely higher state of existence. However, Aizen was never reckless. He was a man who planned meticulously and advanced step by step.
Before fully understanding the Hōgyoku, he would never risk using it on himself.
Thus, experimentation was inevitable.
⸻
At first, Aizen used ordinary human souls.
Then, souls possessing latent Shinigami aptitude.
After that, Hollows from Hueco Mundo.
These became his earliest test subjects.
Over the course of many years, countless souls disappeared within his underground laboratory, becoming nothing more than data points in Aizen's pursuit of evolution.
However, all of this experimentation occurred under one crucial premise—
At that time, Aizen was unaware of the existence of Squad Zero.
⸻
Once he learned that above the Gotei Thirteen existed an even higher authority—
an elite guard sworn directly to the Soul King—
Aizen felt something he rarely experienced.
Urgency.
If such beings truly existed, then his current pace was far too slow.
The experiments had to escalate.
⸻
Ordinary souls could not withstand the power of the Hōgyoku.
Souls with undeveloped Shinigami potential were little better.
Even standard Hollows would collapse under the strain.
That left only one viable option.
True Shinigami.
A soul with Shinigami aptitude, before awakening its power, was not fundamentally different from an ordinary soul. Only after undergoing training and awakening its Reiatsu did it become something more.
So if he wanted reliable data—
He needed fully developed Shinigami.
Aizen already had an idea how to achieve this.
⸻
In recent years, Hueco Mundo incursionsinto Soul Society had become noticeably more frequent.
Menos Grande crossed the boundary with increasing regularity.
Although Squad Eleven was always the first to respond, even they could not handle every incident alone. Other divisions were frequently dispatched for support.
It was during one such operation that the captain of the Tenth Division died in battle.
When Aizen learned of this outcome, even he was genuinely surprised.
Originally, his intention had merely been to stir chaos—
to create confusion, opportunities, and cover for limited experimentation.
He never expected a captain to fall so easily.
Was the Hollow unusually powerful?
Or was the captain weaker than expected?
Aizen could not immediately determine the answer.
But one thing became clear.
If even a captain-level Shinigami could die under such circumstances, then perhaps they were not as invincible as once believed.
⸻
In the past, Aizen regarded captains as the pinnacle of power within the Gotei Thirteen.
But now, he knew better.
There existed a path beyond captaincy.
There existed Squad Zero.
Which meant that the Gotei Thirteen, for all its authority, was merely an intermediary force.
Aizen had always been bold.
Once the idea took root, he would naturally seek to realize it.
Of course, targeting captains was no simple task.
Patience was required.
⸻
Meanwhile, within the Seireitei—
The Shinigami Research Institute had officially been reorganized into the Shinigami Research and Development Institute, under the direct authority of Urahara Kisuke.
Upon taking command, Urahara immediately dismantled large portions of the outdated infrastructure, replacing them with newly designed devices.
No one knew when he had developed so many advanced machines.
But once the researchers began using them, they were forced to admit—
They were vastly superior.
⸻
Among the personnel was Kurotsuchi Mayuri, recently released from the Maggot's Nest.
Naturally, Mayuri only acknowledged that Urahara's inventions were superior for now.
He openly declared that he would one day create equipment far beyond anything Urahara Kisuke could design.
Urahara was well aware of Mayuri's ambitions.
But he paid them little mind.
⸻
With the Research and Development Institute fully operational, Urahara finally began verifying hypotheses that had once been impossible to test.
Not only had Gosuke Shigure repeatedly mentioned the abnormal "shadow" beneath Seireitei—
But the theory Gosuke had proposed long ago in Rukongai was deeply unsettling.
And it demanded urgent confirmation.
⸻
"All spiritual beings emit Reiatsu, because souls themselves are composed of spiritual energy."
"Shinigami, Hollows, and Quincy all utilize this energy—but in fundamentally different ways."
"Hollows survive by consuming souls."
"Shinigami rely on the Reiatsu generated by their own souls."
"Quincy possess comparatively little internal Reiatsu, but instead manipulate Reishi in the environment, freely constructing it for offense, defense, and even space itself."
"If such a hidden space truly exists… no matter how perfectly concealed it is, it must still emit spiritual fluctuations."
⸻
In the past, Urahara Kisuke lacked the manpower and equipment to confirm this theory.
Now, everything was different.
As captain of the Twelfth Division and director of the Research and Development Institute, he had access to continuous spiritual monitoring systems already used to detect Menos incursions in both Soul Society and the Human World.
With minor modifications—
Those systems could be repurposed.
⸻
Dididididi—
On the display screen, complex Reiatsu wavelengths pulsed at an abnormally high frequency.
When Urahara finally processed the data, his expression hardened.
"…So it really exists."
"Beneath the Seireitei… there is a vast, concealed spiritual space."
Previously, Urahara had only suspected irregularities within the "shadow."
Now, with precise readings in hand, the conclusion was undeniable.
⸻
Realizing the gravity of the situation, Urahara immediately compiled the data and sought out Gosuke Shigure.
The two left the Seireitei together and only examined the information in detail once they reached Rukongai.
⸻
"This is far worse than expected…"
After reviewing the findings, Gosuke Shigure returned the data to Urahara and let out a quiet sigh.
Although he had long known of the Wandenreich's existence, the scale revealed by Urahara's analysis far exceeded his expectations.
No wonder, in the future, the Quincy were able to completely overwrite Seireitei itself.
⸻
"Sensei," Urahara asked seriously,
"what should we do next?"
Gosuke Shigure answered without hesitation—
"Of course we report this to Captain-Commander Yamamoto."
And only then did the true storm begin to gather.
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