"This is the end! Kurosaki Ichigo!"
The monster, that facsimile parody of both a shinigami and a hollow, prepared what it believed to be the final attack of this battle. It used to be named Aizen, but having surpassed all previous barriers, it needed a name more befitting the god it had become. Surely it had grown stronger than the Spirit King himself. Surely no other being could possibly stand up to it.
Surely not even the transcendent Kurosaki Ichigo could stand in its way.
What a joke.
I will not die from such weakness.
Ichigo's eyes were filled with disgust and contempt at how badly he was being underestimated.
In truth, even Ichigo underestimated his own power.
"…The end? Such shit like this will end me?"
With a swing of his sword, all the power Aizen had built up for his "final attack" was dispersed. Gone. Even having evolved this far, Aizen was still a gnat before Ichigo. Aizen was out of his league
"I'm gonna put a stop to this Aizen. I've had all I can take of your logic."
Ichigo's hand formed the familiar claw above his right eye. Black wisps of power began to gather, and his unfelt reiatsu thundered forth.
It was a tactical decision. Aizen just kept regenerating, never dying, despite how far below Ichigo he was. There was a chance Mugetsu wouldn't kill him, just force another evolution. Ichigo just couldn't afford the risk of using Mugetsu. To do so would leave the world defenseless to a being beyond their comprehension.
Instead, he opted for a more familiar, safer path to power, one that wouldn't destroy his power and seal his fate if Aizen didn't die. One he had become intimately familiar with since his return from Soul Society. One that had thus far proven always useful.
With a swipe of his hand, the new evolved mask formed on his face
Human beings can be referred to as one-dimensional creatures. When they gain the power of either hollow or shinigami, they become two-dimensional creatures. Aizen 'transcended' both, reaching the third dimension, thanks to the Hogyoku.
Ichigo surpassed even that, moving, at least, one dimension beyond. In this state, his presence had no negative effects to those around him, unlike Aizen. His bankai was a weapon centered on condensing all of his power. It allowed no reiatsu to escape, allowed none to destroy everything around him. It was this that allowed his human friends, even his father, to survive his presence.
His hollow, however, does no such thing.
Aizen was suddenly forced to the ground. He could not tell what was causing it or why, just that Ichigo pulling out his mask brought about pain. It had felt like a sudden pulse, and what little terrain remained around them was blasted by the shockwave. Aizen could scarcely raise his head to view his doom.
"You were wrong about what happened to my reiatsu, Aizen. Getsuga Tensho." With an almost nonchalant swipe of his sword, Aizen found his body consumed by darkness.
In that moment, Aizen realized what the shockwave was, what had happened. And then, he despaired. His body began to tear apart. He was dying. And in death, he gained clarity once more, His mind returned from the madness it had entered, and he extended his senses one final time.
"…I see. The Hogyoku grants its wielder whatever they desire. And it seems now I desire death. Or perhaps more accurately, an escape." For a brief moment Aizen seemed almost remorseful, yet in a way content, as if to continue living would be as pointless as it was distasteful.
Glaring at Ichigo, who had begun to grin in victory, Aizen spat out, "Make no mistake, Kurosaki Ichigo. I am not dying from your admittedly impressive power. No. I die because I now see the consequences of pushing you this far. Perhaps it was foolish of me to mold you into a being that could compete with me. Because death is the only thing left for me. My only regret is that I won't be able to see your face when you realize what you've done."
By this point, most of his body had disintegrated. A grin of sadistic joy was plastered on his monstrosity of a face.
"I suggest you look for your comrades. And most importantly? Enjoy your stay in Purgatory. I do not envy you in the slightest."
With that Aizen disappeared.
A feeling of unease and foreboding filled him. Which was ridiculous. He had defeated Aizen. He had saved his hometown. His fight was done for now. He should be celebrating! The others needed to know who had one as well.
His partners, Zangetsu and that nameless hollow, remained silent.
He returned to Karakura, to find his friends.
All elation left him as he witnessed the destruction.
On an instinct that floated from his hollow side's desire to see him suffer, he opened a Garganta, entering Heuco Mundo.
The silence was even more oppressive than the featurelessness.
In a desperate hope, he returned to the Human world, and finally understood what Aizen had said, what he had done.
He snapped. His partners buried himself deep inside, unable to think of giving any advice, or really wanting to give any consolation. Instead they entered the buildings, and prepared for an unending monsoon.
For there was nothing. There were no survivors, anywhere, be they shinigami, hollow, human, or any combination of the three.
They had been crushed between the weight of the reiatsu emanating from a fourth-dimensional Ichigo.
And so he wept. Alone. Not even his hollow responded, not even trying to take over his soul. Because there was no point.
He had defeated Aizen. And in doing so, became the apocalypse.