"Cooperate?" Aizen narrowed his eyes and glanced at Tsunayoshi. "I do not believe there is anything between us worth cooperating over."
"Just grasping a little of the Hōgyoku's power has already made you this arrogant?" Tsunayoshi sat down heavily in the chair. He was somewhat puzzled. This Aizen differed a little from the elegant, composed image he had of him.
Aizen's act of creating chairs in front of them just now had been a display that he had already gained control over the Hōgyoku. He had used the Hōgyoku's ability to manifest the desires of its host to give Tsunayoshi and Urahara a small show of force.
"A little? What are you talking about? I have already become one with the Hōgyoku!"
Just as when Tsunayoshi had first arrived in Las Noches, a wave of Reiatsu pressed down from Aizen toward the two of them. Feeling the change in Aizen's Reiatsu, Tsunayoshi understood a little of Aizen's current mentality.
Before obtaining the Hōgyoku, he had needed to act cautiously and keep a low profile, always relying on schemes. If, after obtaining the Hōgyoku, he still had to scheme carefully, then what was the point of obtaining the Hōgyoku at all?
No wonder the final Hōgyoku-fused Aizen still could not defeat Ichigo. Was it the Hōgyoku that amplified Aizen's negative emotions, or had this been Aizen's true nature all along?
It had to be said that after obtaining the complete Hōgyoku, Aizen's days had indeed been quite pleasant. Aside from fusing with the Hōgyoku and gradually breaking through in strength, the process of casually using the Hōgyoku's power to enlighten Hollows into Arrancar brought him great physical and mental satisfaction.
That process of enlightenment had allowed Aizen to experience, for the first time, something akin to a god's authority. It made him crave the throne in the heavens even more: to ascend it, and then hold dominion over all living beings.
In response to Aizen's question, Tsunayoshi flipped his hand, and a small arrow the length of a chopstick shot toward Aizen. It was a spirit bow condensed by Tsunayoshi himself, only about the size of a palm. Ryūken had once jokingly called it a sign that he had no talent.
That Heilig Pfeil was not powerful, but its value lay in surprise. As long as it achieved Tsunayoshi's goal, that was enough.
"You think such a tiny thing can hurt me?"
Aizen caught the Heilig Pfeil between two fingers. It had been only a hair's breadth away from piercing his chest. Yet the moment his words fell, the Heilig Pfeil immediately exploded.
The force of the explosion could not injure Aizen at all, but it was enough to blow a large hole in his clothing. At his chest, a blue-purple sphere was embedded within a cross-shaped fissure. Along the edges of that fissure were fine, dense blue-purple lines resembling blood vessels.
That was the Hōgyoku. By embedding the Hōgyoku in his chest, Aizen had completed his initial fusion with it. He believed that, at this stage, his strength had already surpassed the combined strength of the Espada. Perhaps that was the source of his growing arrogance.
"Sawada Tsunayoshi, did you come all the way to Hueco Mundo just to look at the Hōgyoku?"
Aizen casually patted his chest. After brushing away the torn scraps of fabric caused by the explosion, he openly spread his clothes apart.
"If you want to look, just say so. The two of you are qualified to behold the power of the Hōgyoku."
Tsunayoshi wordlessly covered his eyes. He had merely wanted to see how far Aizen's fusion with the Hōgyoku had progressed. He had absolutely no interest in Aizen's body, let alone Aizen deliberately pulling open his clothes in front of him.
It was simply painful to look at.
Tsunayoshi did not mind cases like Ichigo, whose clothes were torn because of battle. What he could not stand was a grown man deliberately displaying his body in front of him. Well, technically, Aizen was only displaying the Hōgyoku.
Unlike Tsunayoshi, Kisuke Urahara did not even blink as he stared at Aizen. To be fair, he was extremely interested in the experiment of biological fusion with the Hōgyoku. If he had not feared that Aizen intended to destroy Soul Society, he would have long since joined him in studying it.
Tsunayoshi absolutely had to test the progress of Aizen's fusion with the Hōgyoku. This was information not even Sora Inoue knew.
But after seeing that Aizen had merely embedded the Hōgyoku into his chest, Tsunayoshi could not help feeling somewhat disappointed. This state of Aizen's might be very strong to others, but in Tsunayoshi's eyes, it was still far from enough.
"Your progress… is way too slow, isn't it?" Tsunayoshi's tone was filled with unconcealed disappointment.
For the first time, Aizen's expression stiffened. In order to maintain his dignity as the King of Hueco Mundo, he could only force his expression to remain composed. "What do you mean?"
"I deliberately left you alone for so long, and in the end, you've only reached the first stage of fusion. Are you really Sōsuke Aizen? Or have I been caught in Kyōka Suigetsu again?"
Of course Tsunayoshi had not been caught in Kyōka Suigetsu. Whether or not he was under an illusion was something he could sense himself. He was deliberately mocking Aizen. That genius Shinigami from Soul Society had only completed the first stage after all.
"What do you understand? I have already begun advancing toward surpassing the dimension of Shinigami!" Aizen said coldly.
The fusion with the Hōgyoku had allowed him to feel an increase in strength for the first time in a long while. That undoubtedly proved his path was correct.
"Yes, surpassing the dimension of Shinigami. Borrowing the powers of Hollow and Shinigami, evolving under the Hōgyōku's guidance, and ultimately becoming a higher-dimensional being who fully masters the power of a Shinigami."
"And the so-called power of a Shinigami is the Zanpakutō. The sign that this breakthrough is complete is fusion with the Zanpakutō. Where is your Zanpakutō? Don't tell me Kyōka Suigetsu is already inside your body."
This time, Aizen's expression completely failed to hold. Tsunayoshi had thoroughly condensed his future evolution into a mere two sentences.
"How do you know that? Fusion with the Zanpakutō?"
"Is it possible that I found another way for a Shinigami to fuse with their Zanpakutō?" Tsunayoshi looked at Aizen playfully. "Without using Hollow power or the Hōgyoku. As long as the Shinigami relies on themselves, they can obtain power stronger than what current Shinigami possess. Zanpakutō fusion."
"Impossible!" Aizen's immediate reaction was to deny it. But very quickly, he seemed to think of something. "Those two overwhelming Reiatsu readings in Soul Society… were they you?"
Even after coming to Hueco Mundo, Aizen had not forgotten to collect routine intelligence from Soul Society. He had detected the two times Tsunayoshi used power beyond Bankai.
The first time was when Tsunayoshi fought Yamamoto and used his Cloud-attribute power beyond Bankai. At that time, it had been sealed inside a barrier, so Aizen had only sensed a strange yet powerful Reiatsu.
The second time was when Tsunayoshi fought Yamamoto's duplicate and used his Rain-attribute power beyond Bankai. At that time, there had indeed been no barrier protecting the area. However, Aizen's surveillance Reishi insects had been crushed instantly by the ocean-like mass of Reiatsu. They had only had time to transmit information about that terrifying Reiatsu.
Because Aizen had not personally gone to the scene, he could only infer from the data. He did not know that Tsunayoshi had already mastered power beyond Bankai.
"Yeah. Zanpakutō fusion, that's all. It's already a common technique in Soul Society. Every captain-class Shinigami is training in it," Tsunayoshi said.
There was half a sentence he did not say. Training was one thing. Whether they could actually succeed in that training was another matter entirely.
Aizen's face twitched slightly. Since when had Zanpakutō fusion become a common technique among Soul Society's captains? When he was there, he had never heard of such a thing.
"You brought it to Soul Society?" There was only one possibility: Sawada Tsunayoshi had brought it there during his first trip to Soul Society.
"Not brought. It was a method that was feasible from the beginning. Gaining the complete acknowledgment of the sword spirit and allowing the power dwelling within the Zanpakutō to return to the Shinigami themselves—that is the so-called Zanpakutō fusion," Tsunayoshi summarized simply.
"Sword spirit? I didn't expect you to still possess such naïveté, Sawada Tsunayoshi." Aizen was disdainful of Tsunayoshi's explanation. He remained convinced that his path was the best way to obtain greater power.
"Lord Aizen, could it be that you do not have a sword spirit?" Suddenly, Kisuke Urahara, who had remained silent all this time, interjected. "Or perhaps you have already completely erased your sword spirit's intelligence?"
Having fought Aizen for over a hundred years, Kisuke Urahara had his own understanding of Aizen's personality. In his view, Aizen was unquestionably a forceful person, forceful enough to want to control everything, including an uncooperative sword spirit.
As everyone knew, in a Shinigami's Jinzen training with their Zanpakutō, the sword spirit was an extremely important existence. Only by listening to and understanding the name of one's sword could a Shinigami obtain greater power.
Once a Shinigami failed to satisfy their sword spirit, the sword spirit would hide part of its power, or even refuse to tell the Shinigami its name at all, let alone allow the use of Shikai or Bankai.
The Tenth dwelling within Tsunayoshi's Zanpakutō had already surprised Urahara greatly. He had never seen any sword spirit unconditionally trust its master and directly hand over even the power of Bankai to them.
"Heh. You guessed wrong, Kisuke Urahara…" Aizen laughed coldly twice. "I have never done anything to Kyōka Suigetsu. Unlike you and Mayuri Kurotsuchi, you two have never stopped modifying your Zanpakutō, have you?"
Aizen reached behind the back of his chair and took out Kyōka Suigetsu.
"The so-called sword spirit is merely a tool used by Shinigami during Jinzen training. Whether Shikai or Bankai, they are both only processes through which a Shinigami comes to understand themselves."
"As for Kyōka Suigetsu's sword spirit, it disappeared after I learned Bankai. Because I possess an absolutely clear understanding of myself, I no longer need any sword spirit to pave the way for me!"
A confident, almost arrogant expression appeared on Aizen's face. This was what set him apart from other Shinigami. From the moment he stepped into Bankai, Kyōka Suigetsu could no longer restrict him.
Hearing this, Tsunayoshi finally understood why Aizen had sought the Hōgyoku as a way to break through the limits of Shinigami. Kyōka Suigetsu, having lost its sword spirit, was nothing more than a tool in Aizen's hands. A cold, lifeless tool naturally could not synchronize and fuse with its master, nor could it unleash power beyond Bankai.
Perhaps this was Kyōka Suigetsu's limitation.
Although it was Aizen's powerful Reiatsu that had made Kyōka Suigetsu what it was, Kyōka Suigetsu's Complete Hypnosis was, no matter how one looked at it, simply too powerful.
Once Kyōka Suigetsu's master mastered Bankai and maintained absolute clarity, Kyōka Suigetsu's sword spirit would disappear. That meant its master would have no further room to rise.
Perhaps this was the window leading to a higher level that Kyōka Suigetsu had closed in order to open the door known as Complete Hypnosis.
If Tsunayoshi's guess was correct, then Aizen and Kyōka Suigetsu had made each other what they were.
It was just that Aizen's talent was far too great. The price Kyōka Suigetsu paid—having no sword spirit—had become a chain restraining Aizen instead.
For another Shinigami, having no sword spirit might not necessarily be a bad thing. For a Shinigami of average talent, surpassing Bankai was a power they could never touch in their entire life.
Through his research into the Hōgyoku, Aizen had walked another path of Zanpakutō fusion that belonged to him alone.
This path happened to reach the same destination as Tsunayoshi and the others' power beyond Bankai. However, what the Shinigami were currently training in was the use of Shinigami power and a small amount of Hollow power to reforge the sword within their inner world.
Aizen, on the other hand, directly combined Shinigami and Hollow powers to break through to a higher-dimensional power. He used himself as the material for forging the sword, and through the Hōgyoku's guidance, forcibly achieved fusion with his Zanpakutō.
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