Soi Fon's sudden question caught Ailin off guard. He glanced at her and replied, "Well, have you considered that maybe Lady Yoruichi didn't bring you along because your strength is lacking?"

"Damn it!" Soi Fon clenched her fists, teeth gritted.

Though she hated to admit it, Ailin might have a point. Both Kisuke Urahara and Ailin surpassed her in strength by a significant margin.

Taking a deep breath, Soi Fon slowly composed herself, pushing down all other emotions. "Captain Ailin, let's begin."

"Alright." Seeing Soi Fon regain her composure, Ailin wasted no time and began explaining the theory behind Shunko.

"Shunko is a technique that combines Kidō and Hakuda, with power comparable to a Shinigami's Bankai. Before practicing Shunko, one must first identify the Kidō attribute that best aligns with their own."

With that, Ailin started instructing Soi Fon on the intricacies of Shunko within the underground base.

Over a century ago, Ailin had already acquired knowledge of Shunko from Soi Fon's Zanpakutō, Suzumebachi. However, due to Yoruichi Shihōin's departure, Soi Fon only had access to the basic theories Yoruichi left behind, leading her to believe that Yoruichi herself hadn't fully developed the technique.

Based on Yoruichi's incomplete theories, Soi Fon stumbled through the development of her own Shunko over the next hundred years.

However, since she followed Yoruichi's notes, and Yoruichi's attribute was lightning, the Shunko Soi Fon used when they met again a hundred years later was still the lightning-attribute Shunko with bursts of electricity from her shoulders.

After gaining Yoruichi's approval and receiving more insights into Shunko, Soi Fon completed her own unique wind-attribute Shunko during the Thousand-Year Blood War – the Mugen Shunko.

Naturally, Ailin wouldn't let Soi Fon take the same roundabout path. From the beginning, he had Soi Fon determine her own attribute and then taught her the key points of Shunko cultivation based on that attribute.

Since this was essentially Soi Fon's own experience from over a hundred years later, she learned quickly under Ailin's guidance.

It took her only three days to master the initial steps – converting her reiatsu into wind-attribute Kidō reiatsu and then merging it with her own reiatsu.

There was no helping it; having a cheat was just that convenient. Whenever Soi Fon made a mistake in her training, Ailin would immediately point it out and offer better suggestions. It felt as if he, not Soi Fon, was the one cultivating Shunko.

The fusion of two different types of reiatsu in the world of Soul Society brought exponential improvement to a Shinigami.

When a Shinigami released their Zanpakutō, their own reiatsu merged with the Zanpakutō's "spiritual pressure." Shikai could increase their reiatsu three to five times, while Bankai could boost it five to ten times.

Similarly, the "Hollowfication" of Shinji Hirako and the others involved merging the Hollow's reiatsu with their own, resulting in a surge in their spiritual pressure.

The technique of "Temporarily Placing a Soul Within the Body" created by Kirio Hikifune also involved merging a different reiatsu with one's own to enhance their spiritual pressure.

Shunko followed the same principle. Moreover, it shared similarities with Hikifune's technique.

However, Hikifune's completed research permanently increased one's spiritual pressure, while Shunko involved using one's reiatsu to create Kidō reiatsu and achieving fusion outside the body.

Within a week, Soi Fon mastered the basic Shunko. However, what gathered on her shoulders and back wasn't crackling lightning, but swirling tornadoes.

After using Shunko, Soi Fon's speed saw the greatest improvement, nearly rivaling Yoruichi Shihōin's.

Ailin then shared the theory and key points of Mugen Shunko with her.

With her experience in Shunko, it took Soi Fon half a year to finally complete the cultivation of Mugen Shunko.

***

"Hah!"

Boom!

With Soi Fon's shout, two azure tornadoes, like wings, spun behind her shoulder blades, causing her reiatsu to surge.

Her fists clenched, and her spiritual pressure erupted. Similarly powerful tornadoes swirled around her limbs.

Swoosh!

In an instant, Soi Fon vanished from her spot and reappeared beside Ailin. She compressed the massive tornado around her fist and threw a punch.

Boom!

The giant tornado exploded upon impact, unleashing the destructive force of wind-attribute Kidō and creating a powerful blast.

The resulting smoke and dust were instantly torn apart and dispersed by the raging wind before they could even spread.

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Within the Muken, Ailin concentrated his reiatsu in his hands and engaged Soi Fon in a Hakuda duel.

Every clash of fists and feet produced thunderous booms from the exploding wind. If this weren't the Muken, everything around them would have been reduced to rubble by the shockwaves of their battle.

Feeling the tearing and explosive power in Soi Fon's attacks, Ailin couldn't help but think of Kensei Muguruma, the "combat power level measuring unit" of Soul Society.

The reason Soi Fon and Ailin were fighting in the Muken was that after mastering Mugen Shunko, she immediately applied to Captain-Commander Yamamoto for the Captain Proficiency Test.

Since she hadn't completed Bankai training, she chose to challenge an existing captain. After much deliberation, she ultimately decided to challenge Ailin.

There was no helping it. Even with Mugen Shunko, she couldn't compare to the strength of the captains from other divisions. Since she would lose regardless of whom she challenged, she might as well choose the strongest. Thus, Ailin became the target of her challenge.

Typically, only the recommending captains and the Captain-Commander would observe the challenge for the Captain Proficiency Test.

However, ever since Soi Fon approached Ailin, everyone had been paying close attention to her progress, curious to see how much her strength had improved under his tutelage.

Before Soi Fon, Captain Ailin had already trained two captain-level disciples: Byakuya Kuchiki and Gin Ichimaru.

If Soi Fon also passed the Captain Proficiency Test, there would be three captains trained by Captain Ailin.

Of course, everyone knew that Soi Fon wouldn't have applied for the test without the confidence to pass. However, they were still very interested in how much she had grown under Captain Ailin's guidance in just half a year.

Therefore, almost all the captains came to the Muken to witness the battle between Soi Fon and Ailin.

During their fight, Soi Fon mainly attacked while Ailin defended, allowing her to showcase her strength before the numerous captains.

"Captain Soi Fon's current state involves converting her own reiatsu into Kidō reiatsu outside her body and then merging it with her own reiatsu, allowing her to infuse each attack with powerful Kidō damage... Captain Ailin is truly a genius..." After observing the battle for a while, 5th Division Captain Sōsuke Aizen adjusted his glasses and praised.

"Who can say otherwise?" Beside him, Shunsui Kyōraku joked, "When 3rd Division Captain Gin Ichimaru took office half a year ago, I said that Captain Ailin's teaching ability was already comparable to Old Man Yamamoto's. Now, looking at Soi Fon, the 2nd Division captain position is undoubtedly hers. It seems Captain Ailin has surpassed Old Man Yamamoto in the aspect of teaching students."

Among the Thirteen Court Guard Squads, Shunsui Kyōraku got along well with every captain. Additionally, if other captains encountered any difficulties, Shunsui would never refuse to help if he could.

After recommending Aizen as the 5th Division Captain, Shunsui often interacted with him, which didn't seem out of place.

No one could have guessed that Shunsui's actions were all to deceive Aizen and find his weaknesses.

If before hearing Ailin's words, Shunsui's suspicion of Aizen and Urahara was fifty-fifty, then after Aizen personally nominated Kaname Tōsen as the 9th Division Captain, it became seventy-thirty.

After all, Shunsui learned from Ailin that it was Aizen, Tōsen, and Gin who attacked Lisa Yadōmaru and the others that night.

Gin Ichimaru was Captain Ailin's disciple, personally recommended by Ailin as the 3rd Division captain candidate.

Sōsuke Aizen had a high reputation and was personally recommended by Shunsui as the 5th Division captain.

As for Kaname Tōsen...

If Ailin hadn't mentioned him and Aizen hadn't recommended him, no one would have imagined that the 3rd Seat of the 9th Division also possessed captain-level strength.

"Indeed, I've long heard of Captain Ailin's teaching ability. 3rd Division Captain Gin Ichimaru once worked with me for a while. Whether it's Kidō, Hakuda, Shunpo, or Zanjutsu, Captain Ichimaru is exceptionally skilled. Captain Ailin seems like an all-rounder, with no weaknesses... to possess such strength in just over a hundred years, Captain Ailin is truly fearsome..."

"Hahaha, if Captain Aizen hadn't mentioned it, I almost forgot Captain Ailin isn't even two hundred years old yet..." Shunsui pulled his hat lower, his mind racing with thoughts.

‘Sōsuke Aizen, was that carelessness or intentional? But regardless of whether it was careless or intentional, Aizen must be planning something, possibly related to Captain Ailin... Otherwise, he wouldn't have said that Captain Ailin is 'fearsome'. After all, a powerful ally brings peace of mind. Only a powerful enemy would inspire fear...

However, Captain Ailin's 11th Division already has two captain-level subordinates. With Captain Kuchiki, Captain Ichimaru, Captain Soi Fon, and Captain Ailin himself, their side would have six captain-level powerhouses. With this level of influence, if they had any ulterior motives, the Seireitei would surely fall into chaos again.’

At this thought, Shunsui suddenly realized something. No, Aizen's words weren't meant for him, but for his teacher!

As a disciple of Genryūsai Shigekuni Yamamoto, Shunsui understood his teacher better than anyone. Old Man Yamamoto was perhaps the most devoted follower of the Soul King.

It was precisely because of his admiration for the Soul King's actions that Yamamoto believed everything in the world unfolded according to the Soul King's will.

Regarding the betrayal of the Five Noble Houses, Yamamoto believed that since the Soul King hadn't acted against them, there must be a reason for their existence.

However, Yamamoto disagreed with the Five Noble Houses seizing the Soul King's authority to rule the Seireitei.

Therefore, two thousand years ago, Yamamoto founded his dojo to teach students. A thousand years ago, he formed the Thirteen Court Guard Squads under the banner of "protecting the Soul King's Seireitei," promoting the existence of the "Soul King," and ending the aristocracy's million-year dictatorship over the Soul Society and the Seireitei.

To avoid being purged by Yamamoto, many nobles joined the Thirteen Court Guard Squads and supported him.

In the current Seireitei, although the nobles still had the "Central 46 Chambers" to maintain their lofty status, everything they did had to be within the rules.

The true ruler of the Seireitei was the Captain-Commander of the Thirteen Court Guard Squads, Genryūsai Shigekuni Yamamoto!

The Thirteen Court Guard Squads were Yamamoto's life's work.

To protect the current Seireitei, Yamamoto would eliminate any existence that might threaten its stability.

The reason for Zaraki's current state was that Yamamoto saw his fanaticism for battle and the immense power hidden within him. Fearing that Zaraki might cause harm to the Seireitei in the future, Yamamoto disregarded even Retsu Unohana’s plea and used less-than-honorable means to dampen Zaraki's interest in swordsmanship.

If not for Ailin, Zaraki's strongest "swordsmanship" technique might have been simply holding a sword with both hands.

But because of his "ten-year promise" with Ailin, Zaraki was currently learning swordsmanship more suited to him from Unohana.

Presumably, in their next battle, Zaraki would surely bring Ailin an even greater surprise.

In the past, Captain Ailin rarely involved himself in affairs, so Yamamoto might not have thought much of it.

However, now that Aizen had brought it up, Yamamoto might have already started to develop a rift with Captain Ailin.

After all, Captain-Commander Yamamoto was wary even of Zaraki, who hadn't fully recovered, let alone Captain Ailin, whose strength was sufficient to receive an invitation from the Royal Guard of the Zero Division.

Sōsuke Aizen, this man, his schemes ran deep.

Shunsui buried his apprehension about Aizen deep within his heart and pretended not to understand Aizen's words, continuing their conversation.