Hearing Gosuke Shigure's question, Shutara Senjumaru's gaze flickered slightly.
As a member of the Royal Guard (Zero Division), she had long since gained access to secrets that ordinary Shinigami even captains would never come into contact with.
After a brief pause, she spoke.
"You should know that, long ago, there were no so-called seventy-two barriers separating the Soul King Palace from Seireitei. The connection between the two was far closer than it is now."
Gosuke Shigure nodded.
In his previous life, although the Soul King Palace (Reiōkyū) had always existed above the world, the interaction between the Royal Guard and Seireitei had been far more direct. He had even met Ōetsu Nimaiya, the creator of all Zanpakutō, during that era.
Senjumaru continued,
"In the past, the Royal Guard believed that the Soul King Palace stood beyond reach that no one could possibly threaten it."
Her tone became colder.
"Until one day… a Hollow broke through from Soul Society and reached the Soul King Palace itself."
"A Hollow?" Shigure's brows furrowed slightly.
He understood the nature of Hollows.
Although the world was divided among Shinigami, Quincy, and Hollows, Hollows were generally inferior in structured power. Even Vasto Lorde-class Menos while dangerous were not truly insurmountable threats to top-tier captains.
Only after undergoing Arrancarization did some Hollows gain abilities capable of rivaling high-level Shinigami.
Yet Senjumaru shook her head.
"This one was different."
"It was an ancient Hollow from Hueco Mundo, one that had existed since the earliest eras."
"It first fought its way through Soul Society, even defeating a younger Genryūsai Yamamoto. Then it ascended further… and invaded the Soul King Palace."
Shigure's eyes narrowed slightly.
A Hollow that could defeat Yamamoto even in his earlier years was no ordinary existence.
"In the end," Senjumaru continued calmly, "it was Ichibē Hyōsube who personally dealt with it."
"Afterward, Ōetsu Nimaiya forged its remains into a Zanpakutō."
She paused briefly.
"But that isn't the important part."
"The true consequence of that incident was this "
"The Royal Guard realized that the connection between Seireitei and the Soul King Palace was too direct."
"Relying on the Gotei 13 alone to guard the path to the Soul King… was insufficient."
"No one could guarantee that a similar enemy would not appear again."
"And next time, they might succeed."
Because of that realization, a decision was made.
"They began constructing the seventy-two-layer barrier system."
Senjumaru's tone remained steady as she explained.
"These barriers distort space itself. Without a designated method of access such as the Ōken it is impossible to reach the Soul King Palace."
"The palace is no longer simply 'above' Soul Society. It exists within a separate, sealed spatial layer."
Shigure listened quietly.
This aligned with what he knew entry to the Royal Realm required the Ōken, a key tied to the very bones of the Royal Guard.
"But at the time," Senjumaru continued, "the system was incomplete."
"The balance of the spatial layers was unstable. Over time, cracks would form within the seventy-two barriers."
"To solve that problem, Ichibē Hyōsube and Ōetsu Nimaiya created another object."
Her eyes settled on Shigure.
"The Ōin the Royal Seal."
Shigure immediately understood.
"The light emitted by the Ōin was used to stabilize space and repair the distortions within the barrier system."
"In the past," Senjumaru said, "the Royal Seal had to be transferred periodically throughout Seireitei. That process allowed its power to reinforce and maintain the barrier network."
"Of course, that truth was never revealed publicly."
Instead, Soul Society spread a different explanation.
The Ōin was described as a sacred treasure of the Royal Family an artifact so powerful that it could not remain in one place for too long, lest it distort the surrounding environment.
Through this narrative, the existence of a mysterious and supreme Royal Family was reinforced in the minds of ordinary Shinigami.
Shigure naturally understood the underlying intent.
It was all part of maintaining reverence toward the Soul King and the structure of the world.
As for the so-called Royal Family
That concept was largely a fabrication.
The only true "royal" lineage connected to the Soul King was that of the Quincy progenitor, Yhwach.
But that truth was something very few were allowed to know.
At this point, Shigure finally grasped the true nature of the Ōin.
It was not merely symbolic.
It had originally been a spatial stabilization device, used to maintain the integrity of the barrier between Seireitei and the Soul King Palace.
However, Senjumaru spoke again.
"Now, though… the Ōin is no longer truly necessary."
"Oh?" Shigure looked at her with interest.
"That's because the Royal Guard has already completed the Five Royal Guard Cities."
As someone close to Shigure, Senjumaru did not conceal this detail.
"With the establishment of the five palaces surrounding the Soul King Palace, the spatial structure has reached a perfect equilibrium."
"The seventy-two barriers are now stable."
"Cracks no longer form as they once did."
"In other words… the original function of the Ōin has become obsolete."
Shigure's thoughts shifted rapidly.
"So continuing to use it…"
"…could actually destabilize space instead," Senjumaru finished.
"The Ōin contains high-level spatial manipulation power."
"It is essentially equivalent to a large-scale, sustained spatial Kidō construct."
"In the past, it repaired distortions."
"But now, if activated carelessly, it may instead cause distortions."
Among the forbidden arts of Kidō, space-time manipulation had always been strictly restricted.
Even most captains could not freely employ such techniques.
The Ōin, in essence, functioned on that same principle only on a far greater scale.
Shigure immediately connected this to a possibility.
So that was why
In certain circumstances, exposure to the Ōin's light could result in unpredictable spatial displacement.
His thoughts turned briefly to the current day.
The transfer of the Royal Seal.
A rare event, occurring only once every several decades.
Which meant
Today was one of those critical moments.
After a brief silence, Shigure suddenly asked another question.
"Why is it forbidden… for two identical Zanpakutō to exist in Soul Society?"
This rule had existed for a very long time.
Central 46 strictly enforced it, yet never provided any explanation.
Even in his previous life, Shigure had known of the rule but never its origin.
And now
With the situation involving Hitsugaya Tōshirō and Kusaka Sōjirō unfolding
The question had become impossible to ignore.
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