Hueco Mundo, Las Noches.
Aizen was conducting experiments on the Hogyoku here.
Because he still didn't have enough Soul King Fragments, Aizen hadn't immediately begun researching how to turn Menos Grande-class Hollows into Arrancar.
Although the King of Hueco Mundo had lent him a hundred battle-hardened Adjuchas to experiment on as he pleased—and promised there were more if he needed them—
Aizen was still working on something he knew better: the Hollowfication Experiment on Soul Reapers.
The process involved driving a Hollow's soul into a Soul Reaper's body, thereby breaking down the boundary between the two souls. It was a trial that destroyed the original subject and elevated it into a higher form of soul.
This was something Aizen hoped to accomplish himself in the future. But he hadn't yet mastered the technique well enough to use it on himself.
What he had confirmed so far was that only Soul Reapers with Vice-Captain-level Reiatsu or higher could temporarily withstand the destruction of their own boundaries caused by Hollowfication.
However, remaining in a "Hollowfied" state would cause the two souls to gradually merge into one, turning the subject into a completely mindless monster. The boundary between their Reiatsu and the Reishi around them would disappear as well, resulting in "soul suicide"!
It was a problem even Aizen couldn't solve... though Urahara Kisuke had.
The solution was to inject a soul opposite to a Hollow's—namely, a human soul—restoring the balance that had collapsed when the boundaries vanished and establishing a new one.
That was how the Visored had been saved.
But this meant the boundaries reappeared, which ran counter to Aizen's goal of continually elevating his own limits.
So the conclusion was clear: more experiments were needed to find the perfect answer.
Inside the barrier before Aizen, a female Soul Reaper struggled endlessly, her face entirely covered by a Hollow mask. Though the one-armed Soul Reaper had clearly lost her own will, her fighting style was remarkably violent.
She even fired a Cero, but still couldn't break out of the cage imprisoning her.
After some time, the Soul Reaper's body suddenly swelled and burst apart.
"Failure!"
Aizen gathered up the shattered soul. As expected, the Soul Reaper's and Hollow's souls could no longer be separated!
Tosen Kaname took the soul and recorded the changes that had occurred within it.
Tosen was no less interested in Hollowfication than Aizen, so he cared deeply about the results of each experiment.
After all, Hollowfication also meant gaining tremendous power.
Tosen didn't need to reach the extreme that Aizen sought. All he needed was enough strength to cut down evil.
Still, though these experiments were important to both of them, they couldn't conduct them constantly.
Soul Reapers with Vice-Captain-level Reiatsu weren't easy to find.
Each one took some effort to obtain.
By comparison, experimenting on Hollows was much easier.
Every Gillian had spiritual pressure equivalent to five Soul Reapers.
After all, they were formed when hundreds of Hollows devoured one another, their Reiatsu merging together.
And compared to Soul Reapers, the signs that a Hollow had broken through its boundaries were far more obvious.
That was the shattering of its mask—which was why they were called Arrancar!
The opposite of Hollowfication, artificially turning a Hollow into an Arrancar, required injecting it with a Soul Reaper's soul.
Aizen's Hogyoku contained a large number of Soul Reaper souls. There weren't quite enough to turn a large number of Adjuchas into Arrancar, but there were more than enough for experiments on Hollows that weren't Menos Grande.
Aizen hadn't completely conquered Hueco Mundo yet. He was working with the King of Hueco Mundo, so he couldn't act as recklessly as he otherwise might have.
Besides, Aizen was certain that if he succeeded in changing the Soul Society, he would eventually have to fight the King of Hueco Mundo!
A sound rang out beside Aizen, and Gin Ichimaru appeared.
Aizen asked warmly, "You've worked hard, Gin. Did you confirm it?"
Gin never opened his eyes. Smiling, he nodded and said, "I did! The war between Hueco Mundo and Hell has been going on for over a thousand years! More than a third of Hueco Mundo's current territory was taken from Hell."
"That's truly remarkable." White light glinted across Aizen's glasses. "He really is the King of Hueco Mundo."
Tosen found it even more ironic.
Hueco Mundo had been waging its "just war" for over a thousand years, while the Soul Society hadn't merely failed to challenge it—it hadn't even heard about it.
A deep sense of loathing surfaced in Tosen's mind.
He even began to feel a certain kinship with Hueco Mundo.
Aizen's wariness only grew. "Prepare for the Arrancar experiments! If we want to overthrow the Soul Society as it is now, then even if Hueco Mundo becomes our enemy someday, for now, it's an ally we need to work with!"
Tosen and Gin nodded together.
But unlike Tosen, who was somewhat stirred into action, Gin remained as calm as ever.
At least, on the surface.
Beneath his smiling mask, the turmoil in his heart was impossible for even Aizen to discern.
Gin had just encountered the King of Hueco Mundo!
On his way back from the Hell Battlefield, the King of Hueco Mundo had seemed to be waiting for him.
And the King of Hueco Mundo had said only one thing.
"I'll help you retrieve the soul Rangiku Matsumoto lost. You'll work for me in return. What do you say?"
The words sent Gin's thoughts into turmoil.
But his expression didn't change.
He didn't dare show what he was feeling, because he feared it might be something Aizen had created with Kyoka Suigetsu.
But the King of Hueco Mundo easily dispelled that suspicion by telling Gin something he didn't know yet, but could later confirm without raising any doubts.
Kyoka Suigetsu didn't create illusions. It caused complete hypnosis.
In effect, Aizen gave the target a beginning and an end.
Aizen didn't know what happened in between or the details along the way; the target's own mind filled them in.
But a target couldn't imagine something they had never seen.
That meant Gin could confirm afterward whether he had been under Kyoka Suigetsu's influence.
"If you confirm it and accept my offer,
"then continue following Aizen's orders. Don't change anything unless I tell you to."
With that, the King of Hueco Mundo vanished before Gin's eyes.
There was no trace of Reiatsu, which made Gin feel even more as though he had fallen under Kyoka Suigetsu.
But it was also entirely possible that the King of Hueco Mundo had used Past Reenactment to leave behind an echo of the past.
The abilities of those two were beyond Gin's ability to grasp, even though he excelled at direct combat.
He truly couldn't get a handle on them!
But Gin's customary smiling mask never changed.
He returned to Aizen's side as calmly as ever.
He continued with his work, as though nothing had happened.