There was a place in the Soul Society, reserved for those criminals that under Shinigami law couldn't be allowed to roam free, not even in the world of the dead.
The Central Underground Prison was an unbreakable bastion. Somber halls and grim walls, built of reiatsu-repelling stone; layers after layers of defenses, traps and unshakeable guardians. The prison was a shadowy kingdom of chains and closed gates, which measures of security trampled over anything else existing in the Soul Society. And all that security reached its absolute into the deepest level of that pit.
Muken was the eighth and final level of the Central Underground Prison, the most secure detention facility of all the Soul Society, and, probably, of the entire universe. Reserved for all the criminals that no executioners could kill, it was a dimensional space of its own. Unbreakable barriers separated it from the outside world and nobody could, no matter how powerful, hope to traverse them. Infinite, the black space of the Muken was impenetrable, both from within and from without.
All of this just to contain a single prisoner.
Aizen Sousuke, his forms barely visible under layers upon layers of black strips, sat on a stone throne at the center of Muken. The platform to which his seat was shackled was an island of bare floor amidst endless darkness.
The security that bound him was peerless, unmatched by anything else the Shinigami arts could muster.
And yet, a void silently opened before the prisoner's immobile form, a rip in the fabric of space that led elsewhere.
Three white figures stepped out of it.
The first, by far the tallest, was completely enveloped by stripes of white cloth, only a great green eye peering intently from where their head had to be. The tall figure looked around, sniffing softly.
"It's still here." She said, with a hoarse, scratchy voice.
The second figure, a tall, distinct man with a well-trimmed, short beard nodded solemnly and stepped towards the throne.
The third figure, a woman with long ebony hair and a fox mask covering her face, trailed after him with the slightest of hesitation.
"Sousuke Aizen." The man called, his voice strong and firm. He stopped at a respectful distance from the throne. "We've come here to take what you created. We respect your work, but you'll have to give it up to us!"
His words didn't resound far. The black space of the Muken seemed to swallow them as soon they were pronounced.
The man waited for an answer, and so his comrades.
Nothing.
Aizen Sousuke, which reiatsu was so massive that not even all the seals imposed on him could stifle it completely, remained completely motionless, both in mind and soul.
"The seals binding him must be stronger than we thought." The woman murmured, raising one of the long sleeves of her dress before her mouth.
"No." The man negated, his serious expression unchanging. "The shinigami don't have the power to seal him completely. He should at least be able to sense us and react with his reiatsu." He turned to the tallest figure. "It looks like it's just like you expected, Sol-sama".
The cloth-wrapped figure didn't answer. Without a word, she stepped toward the throne.
Great circles had been engraved into the floor, alongside lines upon lines of tight scriptures. As she came forward, they flared with red light.
Suddenly, the air around the masked figure seemed to come alive. A corona of red light enveloped her. As she stepped to a stop, her foot sank into the floor, cracking it. The contours of her silhouette buzzed and writhed like her body was struggling to keep its form. The air around her whipped into a storm as the seals put upon the prisoners tried to rip her to pieces.
The old man and the woman stepped back, covering their faces as the wind buffeted them.
The quantity of reiatsu rising up was immense, higher than Captain-level, an alarm mechanism meant to destroy anybody that managed to come there.
The cloth-covered figure grunted. Her figure snapped back into focus; the air around her swirled. A shockwave erupted out of her, blasting away the reiatsu oppressing her. The cracks radiating from her foot filled with red light, that then extended like water, covering the floor around her. The stone smoked and fizzled, then melted, all the scriptures running and mixing together into an indistinguishable mess.
When everything stopped, the masked figure stood at the center of a scene of devastation. Both scripture and circle were gone, replaced by a smoking, half-liquid mass that still flowed there and then like wax. The reiatsu, thick in the air until a moment before, was reduced to flickering straps of blue light fluttering away, only to crumble and disappear.
The figure stepped forward, her foot half-sinking into the lava-hot sludge. If it bothered her, she didn't show it.
The second layer of defense was even stronger. It encased its prisoner into a dome of coruscating energy even as it repeatedly blasted the intruder with lighting.
The masked figure just shrugged the blasts off before emitting another impulse of reiatsu, that shredded both barrier and seals like a soap bubble, leaving the floor a ruined wreck.
With that gone, there was nothing separating her from the prisoner.
The marked figure stepped inside the inner circle, advancing toward the throne.
Aizen remained motionless, not even his energy giving out any hints that he was conscious.
There was a soft rustle between the stripes of cloth enveloping the masked figure; a bony hand, with extremely long fingers, emerged from them.
She stopped in front of the throne for a moment, then she threw her arm forward.
Her arm sank into the chest of Aizen with a sound of broken bones and snapping seals.
There was no blood erupting from the wound, no reaction from the bound man. The man and the woman didn't even flinch. They just watched.
The masked figure rummaged inside with his long fingers for a while, filling the air with sickening sounds, before finding what she was looking for. Then, slowly, her closed fist re-emerged from Aizen's torso. Between the curled fingers, a cold light shone.
The fox-masked woman shrank back ever so slightly, while the man advanced, a feverish light in his eyes.
"It's…"
"Yes."
He stopped abruptly as Sol spoke. Her single green eye was pointed at her closed fist, an unreadable expression inside of it.
"It's the Hogyuku." She murmured. "It's finally complete."
"That mean that Aizen…"
"He's dead."
Sol didn't give another glance at the corpse on the throne. Keeping her closed fist before her, she turned and walked towards the waiting Garganta.
"His life was the last component that this thing needed to complete its growth." She said, traversing from the darkness of the Muken to the darkness of the passage. "Come. We have still much to do before it awakens completely."
The woman eagerly followed her, tarrying on the threshold just the time to glance at the man, that had remained behind.
She nodded to him and disappeared into the void, following their chief.
Left alone, the old man turned to the shinigami on the throne. Only in that moment he noticed the broken seals on the floor, the majority of which, he realized, had been already broken before Sol's intervention. He frowned. Had been the Hogyuku to break them? Had been Aizen, with his small reiatsu still free? Given their strength, it looked very much unlikely.
He looked at the still form of the Shinigami. Between the rent on the stripes on his chest, he could see only blackened flesh and charred bones.
Looked like the answer didn't matter anymore.
He bowed low at the dead shinigami on the throne. Like all the Arrancars that belonged at his faction, he resented him, but it was through his work, and, now, this last sacrifice, that their kind could finally find victory over the Shinigami.
Sousuke Aizen deserved at least this.
After straightening himself up, the man turned around and disappeared into the Garganta. The portal closed after him.
The darkness returned silent, his only prisoner now free.