Because Muten Natsu's Myōki was usually stored inside his left arm, and because he had not attacked all this time, Sōsuke Aizen assumed Myōki was still inside Muten Natsu's left arm.
But...
Sōsuke Aizen thought of a terrifying possibility.
If Myōki was not inside Muten Natsu's body, but outside it, and yet Sōsuke Aizen had not noticed at all...
Then that meant Myōki's dimension was above Sōsuke Aizen's own.
Snap!
Muten Natsu's crisp snap cut off Sōsuke Aizen's thoughts. Black ripples spread outward from Muten Natsu, canceling the white ring that bound him before radiating in every direction.
There.
The single eye on Sōsuke Aizen's forehead turned, his gaze piercing through his own skull. He saw Muten Natsu revealed as the black ripples swept past.
That was Myōki... No, it wasn't that he couldn't sense it. It had hidden itself, and he had failed to notice... No. That wasn't right.
The single eye on Sōsuke Aizen's forehead spun wildly, and he saw... even more Muten Natsus.
Ring after ring of ripples swept past. One Muten Natsu after another became increasingly solid, several of them even overlapping, each holding Myōki in hand.
Just as Sōsuke Aizen himself had said, he had put in countless efforts, explored many directions, and finally reached his current evolution.
In that process, he had also gained an understanding of how all kinds of powers were used.
Almost the instant the dense crowd of Muten Natsus appeared, he roughly understood everything Muten Natsu had done.
Sōsuke Aizen's earlier judgment of Muten Natsu had not been wrong.
Muten Natsu had indeed relied on high-level Rei'i and Myōki's power to reassemble his Reishi after death and revive himself.
But he had overlooked Myōki.
A Zanpakutō was a Shinigami's mirror. The abilities it displayed were abilities the Shinigami himself possessed.
Muten Natsu used Myōki's power to complete his own resurrection, while Myōki completed the resurrection of Muten Natsu's power.
This involved two phenomena Sōsuke Aizen had long understood.
The first was the soul splitting of the 1st Espada, Coyote Starrk.
When a spiritual body grew powerful enough, it would split off new individuals.
These phantoms of Muten Natsu were, in essence, extremely similar to Starrk's wolves.
The second was the spiritual body contraction of the former 3rd Espada, Nelliel Tu Odelschwanck.
When Nnoitra slashed Nelliel's mask, her Reiatsu leaked from the wound, causing her spiritual body to shrink.
Muten Natsu's body becoming smaller was, in essence, very similar to Nelliel becoming Nel.
As Sōsuke Aizen killed Muten Natsu, Muten Natsu's body scattered and reformed. Myōki, outside his body, repeated the same process.
When Muten Natsu revived, Myōki also reassembled the Reiatsu released at the moment of Muten Natsu's death into soul phantoms.
If that were all, it would not have been a concern.
The key was that Sōsuke Aizen sensed his own Reiatsu from those Muten Natsus.
The power released during Muten Natsu's death overlapped with Myōki's power. It not only gathered Muten Natsu's own power, but also reassembled the power Sōsuke Aizen had used to kill him.
Every bit of power Sōsuke Aizen had used to destroy Muten Natsu had been gathered by those phantoms, to be used in turn to destroy Sōsuke Aizen.
It was not that Muten Natsu did not want to attack. He was accumulating both his own power and his enemy's, waiting until there was enough to end the battle in an instant.
"Bankai." Muten Natsu reached toward Sōsuke Aizen. "Meiki (Return to the Underworld)."
In an instant, within the spreading black ripples, countless phantoms turned into blurred ghostly figures, igniting Hell Flames as they swung Myōki and flashed past.
Their already powerful force grew even more violent under the burning Hell Flames. Moving at impossible speed, their figures became straight lines of black blade-light.
Those lines of blade-light danced wildly through the twisted space like chaotic curves, refracting brilliant arcs before snapping straight in an instant, crisscrossing through Sōsuke Aizen's grotesque body.
The air, too slow to evade, churned between the blade-lights, and the shrill screams that erupted merged into a metallic roar.
Everything before him was brutally cut into scattered pieces and slid down powerlessly.
Sōsuke Aizen sensed his body falling apart. He understood the principle behind Muten Natsu's Bankai and the source of the power now ravaging him, but no matter what, he could not understand it.
"Why...? Why?"
Ignoring the pieces of flesh falling from his body, Sōsuke Aizen roared, "Why can you absorb my attacks? Those dissipated attacks are no different from something dead... Why can you control power that has already died?"
"You gave the answer a long time ago." Muten Natsu looked at the fragmented Sōsuke Aizen. "Because I'm already dead. I died a long time ago."
Sōsuke Aizen opened his mouth, and then lost his lower jaw.
Like collapsing building blocks, pieces of flesh and blood fell in disarray.
Muten Natsu's outstretched hand grasped the hilt formed from gathering black smoke. He stabbed Myōki into his palm and drew it back into his left arm.
To learn Bankai, one first had to materialize the Zanpakutō's true form and make it submit.
A Zanpakutō was a Shinigami's mirror. To materialize a Zanpakutō, the Shinigami had to possess a more concrete understanding of himself.
Muten Natsu had been able to master Bokukō's Bankai because, long ago, he had realized his dependence on the people around him.
He was like a fish swimming through another world, rarely coming into contact with anything around him.
Only when he was needed would he answer the call, shatter the barrier of the world, and descend with tremendous power.
Spirit Ink was Muten Natsu's desire to form a connection with an unfamiliar world. Every ink mark was proof of his existence. In essence, it was no different from writing [Muten Natsu was here].
Shadow Stealth was Muten Natsu's desire to shorten distance. When he was needed, he wanted to appear beside the person who needed him as quickly as possible. In essence, it was no different from a means of transportation in a long-distance relationship.
Forming connections and shortening distance. That was Bokukō's essence.
Rin'en Bokukō went one step further than that, taking and giving.
Muten Natsu revealed his own needs, took without regard for the consequences, and transformed everything he gained into power that shattered the barrier of the world, broke through the barrier of the heart, and revealed his true form. The more that revelation was suppressed, the more thoroughly it rebelled.
The power he revealed could be used to shatter a difficult situation, or to strengthen himself.
Taking with everything he had and repaying with everything he had. That was the essence of Rin'en Bokukō.
That essence was the materialization of Muten Natsu's desperate desire to connect with an unfamiliar world.
Muten Natsu faced that desire directly, shifting from passive to active, making Bokukō submit and learning Bankai.
The reason Muten Natsu had been unable to master Myōki's Bankai for so long lay in his mistaken understanding of the essence of Myōki's power.
For spiritual bodies, absorbing Reishi was a basic life-sustaining ability, as natural as breathing.
In Rukongai, souls with no aptitude for Reiryoku could survive by drinking water and absorbing Reishi from the atmosphere.
Souls with an aptitude for Reiryoku, however, needed to eat as well to make up for the extra consumption caused by that aptitude.
Myōki had formed inside Muten Natsu's body at that time.
The Soul Essence Stealing ability was, in essence, a further enhancement of Reishi absorption, allowing Muten Natsu, who possessed Shinigami aptitude, to absorb enough Reishi from the atmosphere to sustain himself without needing to eat.
Ink Fortress, which emerged after Myōki's release, was an enhancement of defense. Even if Muten Natsu did nothing, the spreading ink stains would help him block any damage he might suffer.
Both abilities reflected a powerful will.
The will to survive.
Soul Essence Stealing could absorb the Reishi left behind after a spiritual body's death, allowing Muten Natsu to read the lingering memories within it.
During the process of reading those memories, Muten Natsu would truly experience the deep despair and fear the spiritual body felt at the moment of death.
It was a reminder of how terrifying death was, meant to awaken his will to survive.
Myōki had been doing everything it could to show Muten Natsu's survival instinct.
The problem was... as someone who had transmigrated, Muten Natsu's understanding of death was flawed. He believed death was merely another journey to some unknown destination.
So whenever he faced a life-or-death crisis, his first thought was that after he died, he could transmigrate to another world, while for everyone else, death was truly final.
Since danger was unavoidable, then of course he, the person capable of transmigration, should be the one to take the risk.
That was why he showed such obvious self-destructive tendencies, and why he dared to sacrifice himself for the people important to him.
That seemingly fearless attitude toward death was a rational choice he made under the assumption that he would not truly die.
He was not actually unafraid of death. His survival instinct had always been working. He simply believed he would not die.
But precisely because he believed he would not die, every rational choice he made ran counter to his survival instinct. As a result, he could not face that survival instinct directly, and naturally, he could not complete Myōki's Bankai.
It was not until Hueco Mundo, because of Ririn's question, under the gazes of Rukia Kuchiki and Orihime Inoue, that Muten Natsu realized something.
If he died and transmigrated to another world, then to them, he would truly be dead.
It would be like an entire puzzle missing one piece.
Whether that puzzle piece was under the bed or in the gap beside a cabinet did not matter.
Even if that piece itself remained completely intact, it was still incomplete on its own.
And the whole puzzle would still be incomplete as well.
The pieces closest to the missing one would show that absence most clearly.
So even if he could still live on by transmigrating to another world after death, Muten Natsu did not want to die.
He truly began to fear death, and his survival instinct materialized with that fear.
But that still was not enough. To achieve Bankai, he had to make that materialized survival instinct submit.
For the survival instinct to submit meant facing death with resolve.
Not the way he had before, unafraid because he knew he would not die.
It meant knowing he would die, not wanting to die, yet choosing to face death without fear for the sake of something more important.
Just like the choice Retsu Unohana had made.
She had realized the power of Bankai before Muten Natsu did.
Back when Muten Natsu had been burned to the brink of death by the Hell Flames, it was Retsu Unohana who used Minazuki to continuously heal his body and preserve his life.
And it was during that process that Retsu Unohana sensed the hidden power inside Muten Natsu.
A power released between life and death, stronger than the power Retsu Unohana herself possessed.
That was why Retsu Unohana had acted in the Dangai, repeatedly cutting down and healing Muten Natsu, forcing him to walk the line between life and death until he truly mastered Myōki's Bankai.
For that, she gave everything she had to Muten Natsu.
She made Muten Natsu truly understand... what it meant to face death with resolve.
Meiki was a rather special Bankai. Muten Natsu could not release it on his own.
It would only trigger passively when he suffered a fatal wound.
After its release, Muten Natsu's main body entered a state where life and death overlapped, losing nearly all defensive power. Any attack would become a fatal wound, causing his own power to explode.
After the explosion, Soul Essence Stealing would fully activate. With the help of high-level Rei'i, it would absorb most of the Reishi to complete his resurrection, while a small portion of the Reishi would be absorbed by Meiki together with the enemy's power, forming residual souls.
When all the residual souls finished attacking and exhausted their power, Myōki's Bankai would be released as well.
"I... I was right!" Pieces of torn flesh merged into a pale humanoid shape. Sōsuke Aizen staggered upright and raised his broken Zanpakutō. "You only look like you haven't weakened. As long as I keep repeating the process of killing you, you will regress into an infant!"
"That's true." Muten Natsu, now noticeably shorter, opened his arms. "Let's see which comes first. Whether I regress so far that I can no longer release Bankai, or whether you step into death through my deaths."
"Death? I am immortal!" The Zanpakutō in Sōsuke Aizen's hand shattered piece by piece. "My Zanpakutō is disappearing. I have evolved into a new dimension again! Muten Natsu, your Meiki... is the Bankai best suited to helping me complete my evolution! Just as I expected, you truly are the finest tool!"
Fear was essential to evolution. That was why Sōsuke Aizen had kept Gin Ichimaru by his side, and why he had waited for Muten Natsu to grow.
Having evolved to such heights, Sōsuke Aizen could hardly feel fear anymore.
But Muten Natsu's Bankai could absorb Sōsuke Aizen's attacks. That meant Sōsuke Aizen could use Muten Natsu's Bankai to create fear capable of making even himself afraid.
"Before you disappear completely, you can still help me achieve more evolution!" Sōsuke Aizen's eyes widened in excitement. "Your regression will bring me evolution!"
Whoosh!
A sharp red flame suddenly burst from Sōsuke Aizen's chest. As he stared in astonishment, more sharp streaks of fire spread across his chest in the shape of a cross.
"This is... Kidō?! Kisuke Urahara!"
"Looks like it finally activated..." Kisuke Urahara appeared at some point. "That Kidō was something I fired into your body with Hadō #91: Senju Kōten Taihō at the moment you were most likely to overlook it, before your transformation was complete. After you fused with the Hōgyoku, I considered it almost impossible to kill you, so I developed an entirely new Kidō meant to seal you."
"Seal me? Wishful thinking!" Sōsuke Aizen raised his hand. "For me, now that I have evolved to a new realm, being seal-"