On that day, Sosuke Aizen secretly came to Hueco Mundo to conduct early experiments on breaking the boundary between Soul Reapers and Hollows.
Zouken's massive and bizarre Spiritual Pressure attracted Aizen's attention.
Zouken, thinking it would be a simple hunt, roared and lunged at the seemingly frail Soul Reaper.
He experienced true despair.
Aizen did not even draw the Zanpakuto at his waist; with merely a high-level Kido that discarded its incantation, he crushed Zouken's proud, hard carapace into pieces.
That unfathomable Spiritual Pressure pinned him firmly against the white sand, rendering him completely immobile.
"What an interesting bug," Aizen remarked. That was the first thing he said to him.
Aizen stepped on his head, his calm eyes seemingly piercing through his soul. "You are clearly a Hollow... yet deep within your soul, there is a scent that does not belong to this dimension at all."
"What exactly are you?"
Under the fear of death and desperate to survive, Zouken did not hesitate to reveal everything.
He spoke of another world, of the Holy Grail War.
He explained the magi's pursuit of the Root, how he was banished by the Second Magic, and how he clawed his way up step by step in Hueco Mundo.
He thought this Soul Reaper would dismiss his words as the ravings of a madman.
But to Zouken's surprise, after listening, Aizen was not only unsurprised but let out a low, amused chuckle.
"I see... The Root? The Holy Grail? True Magic? What fascinating concepts." Aizen's eyes gleamed with a thirst for knowledge. "I once thought the design of the Three Realms was in poor enough taste, but to think there are such absurd other rooms beyond the canopy."
"This world is truly becoming more and more interesting."
From that day on, Zouken Matou completely submitted to Aizen's power and intellect, becoming a hidden and special pawn of his in Hueco Mundo.
Although under the rules of a different world, Zouken's once-proud insect magecraft had completely lost its effectiveness.
However, under Aizen's guidance and with the aid of the incomplete Hogyoku, Zouken began attempting to fuse the theories of Soul Reaper Kido and Hollow Reishi manipulation with the lingering concepts of insect magecraft in his soul.
After countless failures and attempts, they finally developed a brand-new Pseudo-Crest Worm magecraft that blended the spiritual power of Soul Reapers and Hollows.
Although these Pseudo-Crest Worms could not grant him true immortality like before, they could silently invade spiritual bodies and siphon their spiritual power.
They could even be used as a medium to construct special magical circuits in space.
And this was a crucial part of Aizen's plan.
During the earth-shattering turmoil at Sokyoku Hill, it was the Pseudo-Crest Worms he had given to Aizen in advance through Gin Ichimaru that played a vital role.
"For the sake of my plan, you have lurked in this sunless Hueco Mundo for so many years, even abandoning the form of a Vasto Lorde to willingly become an ugly insect."
"I have seen your loyalty and forbearance, Makiri."
Aizen stood at the edge of the pit, his voice echoing across the vast hollow with an intoxicating, beguiling charm.
"I once promised you that when the time came, I would grant you a perfect form that transcends both Soul Reapers and Hollows."
"Today is the day I fulfill that promise."
The giant worm in the pit trembled violently in extreme excitement and ecstasy.
Ever since he turned himself into a worm in pursuit of immortality in that other world... he had constantly yearned to shed this rotting, hideous shell!
And now, this man, as powerful as a god, was finally going to grant him a new life!
Aizen slowly raised his right hand.
In his palm, the Hogyoku—a crystal-clear, perfect gemstone radiating a prismatic halo—was slowly swirling with light.
As Aizen continuously poured his Spiritual Pressure into the Hogyoku, the air in the entire laboratory instantly became incredibly thick.
A suffocatingly massive storm of Spiritual Pressure began to spread outward with Aizen at its center!
Gin Ichimaru and Kaname Tosen had to step back, deploying their Spiritual Pressure as a defense in front of them to avoid being caught in the terrifying energy fluctuations.
"Evolve toward a higher dimension."
Aizen whispered softly, then aimed the Hogyoku in his hand at Zouken Matou at the bottom of the pit.
Hum!!!
A dazzling prismatic pillar of light spilled from the Hogyoku, enveloping the giant worm's massive body!
"Guaaaahhh!!!"
Zouken Matou let out a shrill, inhuman shriek.
The boundary-breaking might of the Hogyoku forcibly tore apart Zouken's fundamental structure as a Hollow.
His mountain-like insectoid body began to dissolve at a visible rate under the scorching prismatic light.
Black, foul-smelling bodily fluids evaporated instantly. Those hard carapaces, sharp appendages, and that abyssal maw were all reduced to the most primal Reishi under the Hogyoku's power.
"It hurts... It hurts so much!!! My body... my soul!! It's being torn apart!!!"
Zouken thrashed madly in the pit.
He was enduring pain a hundred times more severe than when he was banished by the Second Magic.
Old impurities were violently stripped away, and new rules were forcibly injected.
The white bone mask encasing the outer layer of his soul, symbolizing his Hollow instincts, began to form deep cracks under the impact of the Hogyoku's power.
Crack... crack, crack...
The sound of the mask shattering remained clearly audible even amidst the deafening storm of Spiritual Pressure.
"Do not resist this pain, Makiri," Aizen's cold voice pierced through the storm, echoing directly in Zouken's mind. "This is the necessary path to shed your mortal coil and step into the realm of gods."
"Recall your original desire. Take your greed for immortality and your obsession with the truth, and fuse them entirely into this new body!"
"Immortality... the Root... That's right, I am Makiri Zolgen!!"
"I will become a perfect existence that transcends everything!!!"
Accompanied by a final, resolute roar.
Bang!!
The Hollow mask covering Zouken's soul completely shattered into a sky full of white motes of light!
The prismatic pillar of light enveloping him contracted in that instant, transforming into a massive, dazzling cocoon of Reishi that wrapped around Zouken's soul.
The cocoon of light was like a beating heart. With every expansion and contraction, it radiated ripples of terrifyingly high-density Spiritual Pressure.
Time ticked by, minute by minute, second by second.
Finally, the prismatic light on the cocoon began to dim, ultimately dissipating into the air as countless streams of light.
At the bottom of the pit, that disgusting, massive giant worm had completely disappeared.
In its place stood a humanoid figure, quietly remaining in spot.
Aizen dispersed the Spiritual Pressure in his hand, and the Hogyoku vanished back into his palm.
He looked down at the figure from above, an extremely satisfied smile appearing on his face.
"Welcome to your new life, my Espada Zero."
The figure in the pit slowly raised his head, revealing the face of a human male.
He appeared to be around thirty years old, with a lean and gaunt face, deep-set features, and short, slightly curly blue hair.
On his left cheek rested a small fragment of a white bone mask, resembling an insect's carapace with serrated edges.
Below his collarbone, a fist-sized, pitch-black hole was strikingly prominent.
It was the proof of his existence as a Hollow, symbolizing that emptiness which could never be filled.
At his waist hung a dark green sword, its hilt wrapped in black thread.
This was an Arrancar.
A Hollow that had removed its mask and gained the power of a Soul Reaper.
Zouken looked down at his long, pale hands—no longer covered in age spots like withered branches—and clenched them tightly.
Feeling the pure, immense spiritual power surging endlessly within him like a mighty river.
He laughed maniacally.