A strange phenomenon was taking place in the white desert plains of Hueco Mundo - something this world or any of the other worlds that make up the Bleach Universe had never seen. A new Hollow was about to come into existence. That in itself is nothing new or unique, especially for Hueco Mundo. No, that wasn't strange. What was new and bizarre was what the Hollow was made up of. From Beyond, two things arrived at Hueco Mundo. One of them was a foreign soul from a universe much like ours. Filled with knowledge it shouldn't have, the future of Bleach will be unknown.
The second 'thing' that arrived in Hueco Mundo is, for lack of a better word, power. More specifically, the power of Ryomen Sukuna. The King of Curses, the Disgraced One. After Sukuna's demise in the Jujutsu Kaisen Universe, this creature's power was sent through the Multiverse, colliding with the lucky, or unlucky, soul that came from Beyond. After crashing, the soul was thrown off its course and finally landed in the Bleach Universe, where it fused with the creation of a new Hollow. And that is what is happening now.
Hollow are former Pluses, deceased Human souls who lose their hearts to despair or regret or remain in the Human World for too long. Any spirit not guided to Soul Society by a Shinigami via Konsō may eventually turn into a Hollow. Though most Hollows are transformed Human souls, a sub-group of Hollows known as Menos Grande, or simply Menos, exist. Menos are conglomerations of hundreds of normal Hollows, resulting in a single composite entity of far greater strength than any normal Hollow.
Menos are created when the void within an ordinary Hollow's heart becomes so substantial that human souls are incapable of sustaining it; at this point, it begins devouring fellow Hollows. These cannibal Hollows are attracted to one another, and a mass inter-devouring follows, resulting in a Gillian. Following this transformation, the Gillian can evolve twice more, first into an Adjuchas and finally into a Vasto Lorde, provided certain conditions are met. Meno's evolution is not only contingent upon the Menos eating other Hollows but also upon the Menos' ability to avoid being eaten. Suppose another Hollow succeeds in devouring even a part of their body. In that case, evolution becomes impossible for the one who was wounded this way. Menos-class Hollows are considered so dangerous Soul Society sends only highly trained Shinigami teams to handle them. However, these incursions seem rare, as Menos rarely leave Hueco Mundo unless attracted by large sources of spiritual power or encouraged by other forces. There are three categories of Menos: Gillian, Adjuchas, and Vasto Lorde.
As the story's main character emerged, it became clear that he doesn't truly follow the normal guidelines of normal Hollow evolution. This is because, despite him not being a Gillian just yet, his raw, unadulterated power dwarfed even the most powerful Gillians. And he still needs to finish. Something extraordinary was happening. The power of Ryomen Sukuna was already showing, and together with the added soul, the Reiatsu permeating the surroundings was staggering. Sukuna possessed vast levels of cursed Energy, likely the most transcendent throughout history, far surpassing special-grade cursed spirits and sorcerers alike. Cursed Energy was interesting. Right now, the foreign Energy is being ... translated, if you will, into Reiryoku and Reiatsu. All of Sukuna's Cursed Energy became Reiryoku and added to the ginormous pool already present in the Hollow. This was very lucky indeed because Hollows had a sinister or rather negative Reiatsu compared to Shinigami and regular souls.
The reincarnated soul from Beyond finally gained consciousness. He looked around himself, confused for a moment, but then ... he understood. He gained the memories and power of Ryomen Sukuna and understood what was going on. How could this happen? How could he experience what he had only read about in stories? Feeling his strange situation, the soul decided to embrace his new situation rather quickly. That was the influence of Ryomen Sukuna. No personality was attached to the power, but the experience of using the power was still somehow there, as if the main character had inherited the muscle memory to use the power of Sukuna.
"Ryomen Sukuna ... I like the sound of that."
The newly formed Hollow Sukuna took a moment for himself. He sat down and closed his eyes. He focused all of his senses inside and felt the change that was still going on. He could feel his already massive pool of Reiryoku expanding further and further. He could feel the Cursed Energy transforming and adding to his expanding reserves. He was somewhat unfamiliar with his current body, but as a Hollow, he was a creature of instinct. Due to his powerful mind and soul, he was in control of his actions and would not wander through Hueco Mundo deranged, searching for other Hollows to consume just yet. He would surely do that, but he would not allow his urges to control him. He would be in control of his new life; at least, he vowed that to himself.
Sukuna's body looked more like an Adjucas than anything already. He was still considered to be in the weakest category of Hollows, but he looks like an Adjucas. This should be impossible under normal circumstances. He had four limbs and was covered in dark red armour. His face was mostly covered, leaving just his eyes and mouth. His armour was whitish with a tint of pink, while his arms were golden. His chest and legs were armoured and protected him. He had only two arms, but he could feel that his evolutionary potential had not yet been exhausted.
.
Something was going on as Sukuna was getting used to his new body. For miles, the powerful Reiatsu released by Sukuna, the newborn Hollow, alerted many of his brethren. While the amount was humongous for a newborn, it wasn't yet as sinister as the original Sukuna's aura and Cursed Energy were. So, while the power of Sukuna's Reiatsu pushed the weaker Hollows away, it attracted the more powerful ones. So from all over Hueco Mundo, Gillian's and Adjucas were on their way to see what form of being tasted so delicious.
Sukuna opens his eyes to see hundreds of Gillians approach him. However, unlike what one might assume for a freshly reincarnated soul, Sukuna, true to his name, stays perfectly still and collected. He analyses his situation with narrowed eyes and recalls his knowledge of fighting groups and his 'meta-knowledge' of the Bleach Universe.
Gillian are the first and weakest of all Menos. In Human terms, they would be considered foot soldiers. There are many of them, and they all look alike. In Soul Society, they are the "Menos" generally referred to in textbooks. Gillians are huge, but they are slow and have beast-like intellect. Any captain-level Shinigami can destroy them with ease. Up to the point when Rukia Kuchiki met Ichigo Kurosaki, she had only seen them in such textbooks, despite having been a Shinigami for many years. Gillians commonly attack in a group, and one is powerful enough to be a challenge for a Lieutenant of the Gotei 13.
Though Gillians usually have no personality to speak of, which makes them dangerous since they have no mind of their own, one of the many Hollows which comprise it might gain control over the newly formed entity, reflected in the unique mask it wears compared to an average Gillian. Only these Gillian variants have the potential to evolve into Adjuchas-class Menos. The evolution is facilitated by the continued cannibalisation of fellow Hollows, even other Gillians.
The giant creatures finally reach his position and aim their noses towards Sukuna. A large red ball of energy appears in front of several of the Gillians' mouths. Then they open their mouths, firing a powerful blast of concentrated spiritual energy at Sukuna.
*VROOOOOOM*
*BOOOOOOOOM*
The Ceros land and cause a giant explosion. Sukuna opens his eyes in surprise as he realises that his body has moved independently. He didn't even realise that he had dodged the attacks; it just happened. He let his instincts take over and felt his body saturated with spirit energy. His Reiryoku was roaring, causing his Reiatsu to burst out and pressurise the surroundings. The Gillian were all hit with this powerful wave of spirit energy, causing them to buckle and the air to turn tremendously heavy. Their souls are pressured and compressed, attacked directly by Sukuna's Reiatsu. Not thinking about it any longer, the future King of Hollows shoots off into the distance, leaving a cloud of dust in his wake.
His body appears before the first Gillian as he pulls back his fist and punches the Menos in the mask.
*BOOOOM*
*Crack*
The punch lands before the Menos can do anything and starts to crack. The force of Sukuna's fist is powerful enough to crack their masks already in his newly born form. His hands reach into the cracked mask, pulling them apart with one powerful movement. The mask of the Gillian is ripped apart, destroyed, and caused to die and disintegrate into spirit energy.
As this happens, Sukuna lands on the sandy ground and doesn't know what it is, but he absorbs or rather consumes the spirit energy from the dead Gillian. Massive amounts of spirit energy fill his being, and he can feel himself growing even more potent. The energy mixes with his converting Cursed Energy and adds to the monstrous Reiryoku pool. Sukuna doesn't stop, however, and is already moving again. His figure blurs as he appears in front of the second Gillian's face. Another punch is handed out, and a Gillian's mask is destroyed, causing him to disintegrate.
Not a moment later, Sukuna attacks again, surprised by a large red orb being fired at him. There is no such thing as camaraderie amongst Hollows, so one of the Gillians decided to attack again, no matter who or what stood in his way.
*VRWOOOOOOM*
The Cero shoots towards Sukuna and hits him in mid-air. The impact blasts Sukuna through the body of multiple Gillians, and he lands far away from the dwindling group of Menos.
*BOOOM*
Sukuna kneels on the ground. Steam rises from his body. The heat he was exposed to has not caused him any damage. His eyes open, and a small smile appears on his face. Feeling his instincts and senses sharpen, the growing menace once again attacks. The speed is higher than it was just moments before, and his movements are sharper and powerful. The wind picks up as he dodges the Ceros fired at him and jumps into the air. Using his speed, Sukuna moves in circles and weaves through the tall trees that are the Gillians. Confused, the remaining ones start to fire their Ceros at one another, culling their hearing more and more. That doesn't matter, though, as the supply of cannon fodder doesn't seem to be depleted any time soon.
Sukuna moves up a Gillian and then jumps from one to another. As his body reaches a high enough altitude, he brings his index and middle fingers together with his thumb and points them at the Gillians. A small pinkish-red Cero appears, and Sukuna pumps it full of spirit energy. Seeing Ceros approaching from his adversaries, he fires.
"Cero ..."
*VVRROOOOOOOOOOOOOOOMM*
*BOOOOOOOOOOOOOMMM*
Sukuna's Cero swallows and overwhelms theirs, shooting forward and wiping out most of the large group of Menos Grande. The explosion shoots tons of sand into the air, obstructing everyone's vision for a second. But Sukuna doesn't need to see his enemies to kill them. As his figure once again shoots forward in order to slaughter all of the pool Gillians, something becomes apparent.
The slight smile on his face has transformed into a wide grin. He is enjoying himself ... who knows what this means for the poor world? One thing is certain: a powerful Hollow was born today, and the future was no longer safe.