"Oh? So Metastacia was really killed?" 

Sitting on the captain's seat, Aizen asked with surprise. A bewildered expression flashing in his eyes. 

He wasn't surprised at the fact that this hollow got killed. The hollow was too aggressive, so it was only a matter of time before he caught the attention of the 13th division. 

Aizen had initially created him because he wished to know if the hollow could possess and fuse with a vice-captain like he had theorized and potentially do the same with a captain-level shinigami. 

What's more, Metastacia had been installed with a failsafe that would transfer all his information and power to a new experiment of his back in the Hueco Mundo. 

An artificial Arrancar. 

Aizen had a deep understanding of the hollows. 

The normal path of a hollow was to first consume as much as possible. 

Then to consume each other. 

From this process, a Menos Grande of the first step, a Gillian, would be born. 

Gillians were an agglomeration of thousands upon thousands of souls. Each with their own memories, personalities, and all the baggage. 

After years of fighting between the different personalities. One soul would finally take control. 

This would usher in the birth of an Adjuchas. In terms of pure spiritual power, Adjuchas were already near the level of an ordinary captain. 

And the last step to Vasto Lorde was more a matter of luck than anything.

If an Adjuchas wished to become a Vasto Lorde, not only did it need to fight and eat even more Adjuchas, it also needed to avoid being eaten. 

If even the slightest bit of its body was taken out, then it would forever stay as an Adjuchas. 

From Aizen's study, he believed that this was because even if Adjuchas had one will, they were still the result of hundreds of souls fused in a whole. 

By having a part of their body eaten, it meant that they lost this perfect equilibrium, blocking all paths for further evolution. But at the same time, it stopped all paths from further devolution. 

Because unharmed Adjuchas could at any moment lose control and revert to being mindless monsters as Gillians. 

If an Adjuchas was lucky enough to fulfill all the conditions, it would become a Vasto Lorde. A monster that was closest to a human in appearance. 

All Vasto Lorde were equal to captains. Some of them were even slightly superior. 

This was the normal path towards power for the hollows. 

But Aizen had discovered that some hollows could take an alternative path. 

That of becoming Arrancar. 

Any hollows, even those inferior to Gillians, could become an Arrancar. 

What fascinated Aizen the most was that in essence, Arrancar were basically the closest existence to shinigami. 

Simply said, Arrancar were beings with shinigami-like powers. All they lacked was a zanpakuto and there would be basically no difference. 

Even to this day, Aizen had difficulty understanding how Arrancars worked. 

Natural Arrancars were born from hollows breaking their masks. 

But the mask was also the weakness of hollows. 

Death or evolution. This was truly something he could only observe with admiration. 

Even though those natural Arrancar were not necessarily stronger than their hollow counterparts, they had far higher potential. 

This was the greatest wish of Aizen. 

In the soul society, aside from a few monsters like the captain of the 1st division, Genryusai Yamamoto, Yachiru Unohana or the captain of division 0, Ichibei Hyosube. Aizen rightfully believed that he had no equal. 

In fact, if he could stop them from using their Bankai, he had nothing to fear from them. As he was now, he did not even need his zanpakuto if he wished to utterly destroy a captain.

He was strong. He was incredibly strong. 

But it was his limit. 

No matter how much he trained, he could never overcome this limit. 

All shinigami had a limit. Some had higher limits than the others, and his had been far higher than most. 

In the past, he did not mind that limit. He was happy to have reached the highest possible level available for a shinigami. 

'But after seeing that thing!'

A being of such level reduced to being nothing more than the linchpin between the worlds.

To think that such a disgusting being was the respected king of the Soul society. 

How utterly ridiculous! 

He loathed it. He wished to destroy that fake god and take its place!

But he had reached his limits.

All he had to do then was just to break it. 

This was how the creation of the Hogyoku came about. 

This was how his plan to fuse hollows and Shinigami powers in one was born. 

'Once I find the Hogyoku created by Kisuke… I will become the new god.'

But it seemed that his steps towards Godhood were being hampered. 

Had it been anyone else who killed Metastacia, Aizen would have believed that the failsafe not working was a result of his own mistake. 

Even though he was prideful, as a scientist he knew very well that one should never fear mistakes. 

But, the one who killed Metastacia was, 

"Satoru Gojo."

Aizen's narrowed. 

He was someone extremely distrustful by nature. 

He would rather doubt everything than blindly believe and fail. 

'Perhaps it's time to take care of him.'

Until now he had hesitated because Gojo had been too high profile. The reward for killing him did not cover the potential loss. 

Now though — light flashed as he adjusted his plans. 

'Firstly, let's make him go to earth for a mission.' 

Once Gojo was on earth, what would it have to do with him even if for some reason 20 or so Adjuchas decided to visit earth?

---

Back in Gojo's house,

"Achoo!" 

*Snort*

"I wonder who is talking behind my back. I hope it's a beautiful girl." 

(AN: okay so not much happened. But this chapter is necessary to understand what Arrancar and hollows are in general, while also understanding Aizen more. Anyway, we have the Espada and the Primera Espada. Basically first generation and second generation. The first generation were on average weaker since the way they were created was still experimental. The Espada 9 is the sole Gillian out of all the Espada. The reason he stayed as an Espada was mostly because he had absorbed Metastacia and by proxy Kaien Shiba. As for natural Espada and artificial ones, I think I already made an AN about it. But both Ulquiorra and Stark are natural Arrancar. There's even an entire short story about Ulquiorra evolution. Finally, for those interested in smut, I am happy to say that it happens at the end of Vol 4 and I already posted the chapter yesterday on patreon. I won't say with whom though. If you are curious go subscribe or wait. Today I post Epilogue 4 and vol 5 will have events I am sure everyone likes. Hint: Yoruichi will have an active part.)