"Once you're in Soul Society, you'll need to make your way from Rukongai to the Seireitei. That shouldn't be a problem for you, Captain-Commander."
"Indeed. Let us depart."
"Yes, Captain-Commander!"
Led by Yamamoto, the Shinigami entered the Senkaimon.
Gin Ichimaru, Rangiku Matsumoto, and a few barely conscious Arrancar were left behind in the replica Karakura Town.
…
"Las Noches' canopy… is shattered… And Nnoitra, Zommari, Szayelaporro, Grimmjow, and Yammy's spiritual pressures… are gone."
Aizen stepped out of the Garganta, looking up at the gaping hole and the network of cracks in the artificial sky above Las Noches.
Only Ulquiorra Cifer and Grimmjow Jaegerjaquez, the Cuarta and Sexta Espada, still registered faint spiritual pressures.
He was surprised. Before returning to Hueco Mundo, he had exchanged blows with Ichigo Kurosaki. While he had effortlessly blocked Ichigo's Getsuga Tenshō and sent him flying with a single swing, he acknowledged Ichigo's strength.
He had only achieved such a decisive victory due to his own overwhelming power. Without the Hōgyoku's enhancements, he doubted he would have been so easily victorious.
Grimmjow and Ulquiorra's survival suggested a level of strength that warranted a second look.
However, to Aizen in his current state, they were insignificant, unworthy of his attention.
He dismissed them, raising his hand to tear open a Garganta to Soul Society.
But…
A hand emerged from the fabric of space, slamming down on Aizen's head, sending him flying.
He crashed through several of Las Noches' cylindrical towers before regaining control.
"Who?" The only person capable of such a feat, he reasoned, was Captain Ailin. "Captain Ailin, do you intend to fight me now?"
He hollered hotly. He hadn't reached his final form yet, hadn't created the Ōken. Why would Ailin attack him now? He had shown Ailin respect, hadn't he, by sparing Gin?
"Very well! Let's see if you can defeat me before I evolve further!" While he wasn't confident of victory, he believed he could survive long enough to trigger another evolution, surpassing even Ailin.
"I'm afraid you're mistaken, Aizen. I'm not Captain Ailin."
Ichigo Kurosaki flashed before him.
"You?" Aizen began to retort, then paused, his attempt to sense Ichigo's spiritual pressure met with… nothing. "Impossible! I can't sense your spiritual pressure?"
"Who knows? Perhaps… there's a dimensional gap between us. Your senses can't comprehend my power."
"That's absurd! I can sense even Captain Ailin's spiritual pressure. You can't be stronger than him!"
Only two individuals in this world commanded Aizen's respect. No, only one. Kisuke Urahara's intellect, once a source of intrigue, was now deemed insignificant, their perspectives separated by dimensions. Only Captain Ailin, whose power Aizen hadn't yet surpassed, remained a potential threat.
Aizen, through the Hōgyoku, had transcended the limitations of both Shinigami and Hollow, achieving near-limitless evolution. However, he sensed no trace of Hollow or Quincy influence within Ailin's spiritual pressure. Ailin had achieved his power through purely Shinigami means, a feat that intrigued and unnerved Aizen. Even Captain-Commander Yamamoto, wielding the most powerful fire-type Zanpakutō, Ryūjin Jakka, hadn't been able to surpass the limits of a Shinigami.
Ailin's power suggested three possibilities: exceptional talent surpassing even Yamamoto's, a Zanpakutō far more powerful than Ryūjin Jakka, or a combination of both.
Ichigo didn't answer. He raised Zangetsu, and swung.
Aizen raised his own Zanpakutō to block.
BOOM!
The force of Ichigo's attack sent Aizen flying through several of Las Noches' towers.
Aizen emerged from the rubble disheveled, his spiritual pressure flared as he healed his injuries.
Ichigo reappeared before him, and Aizen instinctively retreated. They both recognized the familiar dynamic. Their previous encounter in the World of the Living had followed a similar pattern. The only difference was their relative strength.
In the World of the Living, Aizen had easily overpowered Ichigo, dismissing him as unworthy of his attention. Now, the roles were reversed.
"Aizen, you seem… weaker than you were in the World of the Living. Back then, despite the difference in our power, I didn't retreat. Are you admitting defeat?"
BOOM!
Ichigo's words struck a nerve. Aizen's pride was his greatest weakness. Aside from Urahara and Ailin, even Sōya Azashiro, whose abilities countered his own, hadn't been considered a true threat. To be mocked by Ichigo, a mere plaything he had manipulated, enraged him.
Truth, unlike lies, could wound. And this truth, spoken by the one he had dismissed as an insect, stung.
"Ichigo Kurosaki… does defeating me truly give you such satisfaction? Do you find such pleasure in humiliating me? You may be a hybrid of Shinigami, Quincy, and Hollow, but you're still just a human. Don't get ahead of yourself."
"Raaaaaar!"
His spiritual pressure exploded outwards. A Hollow hole appeared on his chest, the Hōgyoku visible within, protected by a cross-shaped membrane. His wings multiplied, from two pairs to three.
A blinding light enveloped him. Then, he reappeared before Ichigo with his Zanpakutō descending.
Clang!
Ichigo blocked.
BOOM!
The shockwave shattered everything around them.
…
"Has the battle begun?"
As soon as he entered Hueco Mundo, Ailin sensed the clash of Aizen and Ichigo's spiritual pressures. He, too, had wondered which version of Ichigo was stronger: Mugetsu Ichigo or dual-wielded, Hollowfied Bankai Ichigo.
Mugetsu Ichigo's reputation stemmed from Aizen's inability to sense his spiritual pressure, suggesting a dimensional gap, and from the sheer power of his attack, which had even silenced the background music.
However, having met Ichigo after his training, Ailin understood. It wasn't that Aizen couldn't sense Ichigo's spiritual pressure. It was that Ichigo, in his Mugetsu state, was constantly leaking spiritual pressure. Instinctively, he contained it, preventing further loss. As long as he didn't attack, he could maintain that state. But once he fought, his spiritual pressure would rapidly deplete. And his attack, while powerful, was uncontrolled, a consequence of his lack of experience with such immense power. His continued existence as a Shinigami, even after defeating Aizen due to his sheer spiritual reserves.
Ailin arrived above the battlefield, observing the fight. Even after evolving, gaining near-light-speed movement, Aizen couldn't match Ichigo's power. After nearly being bisected, Aizen evolved again.
Ailin watched, shaking his head.
A black flash streaked across the sky. Even Ailin was impressed. Ichigo's Mugetsu… Only Yhwach, after absorbing the Soul King, could withstand such an attack. Even Yhwach, in his earlier state, would have been defeated. It was a technique far more powerful than even Yamamoto's Zanka no Tachi, North: Tenchi Kaijin.
"Hahaha! Ichigo Kurosaki, I win!" Aizen, having regenerated, looked down at Ichigo, whose spiritual pressure was almost completely gone.
"Damn it!" Ichigo struggled to stand, his remaining spiritual pressure barely enough to sustain him, let alone continue fighting.
"Aizen, are you certain… this is the form you desire?"
Just as Aizen was laughing wildly, Ailin's figure slowly descended from the sky and questioned him.
"Hmm?" Seeing Ailin, Aizen's eyes narrowed. He had forgotten about Ailin.
"Captain Ailin…"
His body was encased in a Hollow shell and a black mask covered his face, six wings sprouted from his back, each adorned with a grotesque, tumor-like growth, he spread his arms wide.
"Yes, Captain Ailin. This is my ultimate form. In this state, I am the strongest being in existence. No one can defeat me. Not even you."
"No one?" Ailin shook his head. "Aizen, I'm disappointed. If you want to boast, at least do it before someone who doesn't know better. I watched you being toyed with by Ichigo Kurosaki, nearly cut in half. How can you claim such power before me?"
"So what?" Aizen retorted. "Just like my fight against Yamamoto, I'm still standing. That makes me the strongest! I can crush Ichigo Kurosaki with one hand!"
As if to demonstrate, one of the growths on his wing opened its maw, a small, purple sphere of energy manifested and arced towards Ichigo.