Regarding the Soul King, very few people, whether in Soul Society or Hueco Mundo, are aware of the details.
The original Five Great Noble Families deliberately erased its existence.
Even the captains of the Gotei 13 know very little.
Aizen himself pieced together the truth from various records.
But based on the experiences of the other world's Aizen, he has to question whether his understanding of the Soul King is inaccurate.
"The Soul King, huh? It's an interesting hypothesis. It's a pity I'm currently confined in Muken Hell. Otherwise, I'd investigate further upon returning."
The Aizen who had crossed over was clearly intrigued.
The Aizen of this world said, "It's just a hypothesis. Our understanding of the Soul King is superficial. Even the Five Great Noble Families have lost many records over the long years. They may believe they know the whole truth, but even they might not be aware of the reality hidden behind the scenes."
"However, I have a different take on your situation."
"Oh?" The Aizen who had crossed over showed a look eager to hear more.
His feigned ignorance didn't faze the Aizen of this world: "It's simple. You've repeatedly claimed to have mastered a power that surpasses the heavens and that you've become one with the Hōgyoku."
"Do you really believe that, in your current state, you can be confined by the concept of hell as understood by Shinigamis?"
"Rather than being sealed in Muken Hell, it's more likely that you don't want to leave right now."
The Aizen who had crossed over gave a meaningful smile. "Who knows?"
The of this world continued, "Additionally, I have another piece of intelligence for you. Yu recently discovered a group hiding in the shadows of Seireitei, led by Yhwach, calling themselves the Wandenreich."
"I'm not sure about the situation in your world, but if it's the same, they'll inevitably make a move sooner or later."
"Yhwach, the Quincy king from a thousand years ago?" The Aizen who had crossed over questioned, lightly pinching his chin.
Visiting this world has certainly been full of surprises.
Many thoughts quickly flashed through his mind, and his lips curled slightly—it seemed that he wouldn't be too bored upon returning.
"Valuable information indeed; I'll keep it in mind when I go back. But for now, I'm more interested in taking a stroll through Soul Society," the Aizen who had crossed over said with a smile.
The Aizen of this world asked, "Leaving now?"
The Aizen who had crossed over nodded, extending his transparent arm in front of him.
"Yes, I'll go now. I've come here as a consciousness entity through the power of the Hōgyoku, not truly belonging to this world."
"Theoretically, people in this world shouldn't be able to see me, and I shouldn't be able to exert any real influence on this world."
"It's not a pleasant feeling, lacking genuine engagement."
The Aizen of this world raised an eyebrow: "Then why can I see you, and… what do you plan to do?"
The Aizen who had crossed over replied with a smile, "Perhaps because we are interdimensional counterparts, sharing a deeper connection."
"As for what I plan to do, it's quite simple. I hope you can help me craft a custom Gigai."
"Gigai?"
"Exactly, it's a soul container invented by Kisuke Urahara that allows Shinigamis to operate in the Human World. Since I am a consciousness entity and not just a soul, we'll need to tweak the formula a bit. That shouldn't be too difficult for you."
...
Soul Society, in a secluded area of Rukongai District 79.
Inside a house concealed by a stealth barrier.
The room is filled with neatly arranged equipment and various organized materials and biocapsules.
This is a laboratory established by Aizen in earlier years.
Suddenly, a dark fissure opens within the lab, expanding into a Garganta.
Wearing the attire of a Shinigami, glasses, and with a 5th Division Lieutenant's rank badge on his waist, Aizen appears from within.
He glanced around, ensuring the laboratory was empty before stepping out of the Garganta.
"Heh, a laboratory in Rukongai. How nostalgic."
As he stepped out, the Garganta behind him slowly vanished, and he revealed a nostalgic look.
In his early years, he had established labs in Rukongai, though that had been over a hundred years ago.
As his power grew and he became the captain of the 5th Division, many of the remote labs were abandoned.
Yes, he was indeed the Aizen who had crossed over.
At Las Noches, he taught his other self the key technology to create a Gigai and had the other separate some spirit particles to be used as materials to create a "Reishi Gigai," something only an existence like him would require.
Thus, he gained a body in this world as well.
The only downside is that this body is too fragile and cannot handle too much power.
But he was quite satisfied.
After looking around, he pushed open the door and exited the lab.
Outside, there wasn't a soul in sight, very quiet and desolate.
Adjusting his glasses with a slight smile, he mused, "District 79, truly cautious."
Once the foremost hidden hand in Soul Society, he was very familiar with Rukongai, able to determine his location solely by the surrounding scenery.
In those days, he needed many souls for his experiments, and Rukongai was the place he frequented the most.
However, in the other world, he wasn't as cautious, opting for District 79, an out-of-the-way place, to establish a laboratory.
He only chose District 78 instead.
Determining his direction, he headed toward Seireitei. After passing through several districts, the area gradually showed signs of life.
On both sides, there were dilapidated houses with emaciated souls languishing nearby. They were huddled weakly in front of their doorways, their thin clothing barely covering their grimy bodies.
It was the dead of winter, and the air was bitterly cold. Aizen saw some Rukongai residents huddled within drafty, broken buildings, trying to warm themselves with scavenged straw.
Many of them would not survive the winter.
Unmoved, Aizen continued walking steadily toward Seireitei.
In the other world, he had witnessed far more dire conditions, so much so that they no longer stirred any emotion within him.
Upon hearing his footsteps, many souls listlessly looked up at him, gave him a glance, and then returned to their despondent state.
A Shinigami appearing in such a remote district would normally be an astonishing event.
But they hadn't eaten in so long, and the cold threatened their lives at every moment.
What was a Shinigami to them amidst their own struggles?
Aizen pressed on.
These lower-ranked districts were all quite similar—lifeless souls, numb from the loss of self, resembling little more than walking corpses.
Even the lowest-tier Shinigamis typically avoided these areas.
And for a million years, nothing in Soul Society had changed. Souls here were left to fend for themselves, serving only as fodder to maintain the balance among the Three Realms.
Aizen's observations tethered closely to what he knew of his own world.
He sighed inwardly, realizing that the despair of Rukongai was consistent across worlds.
As he continued on, entering the higher-numbered districts of Rukongai, the surroundings began to grow livelier.