Five hundred years ago, when the first Kenpachi, Yachiru Unohana of the 11th Division was searching for enemies to fight in the Rukongai District, she encountered the young Zaraki.

In that battle, Zaraki experienced the joy of fighting and slaughter for the first time. He believed that the woman he fought against was of the same kind as himself. They were both that type of person. However, in the end, Zaraki still lost.

After that, Zaraki never encountered Yachiru Unohana again. The next time Zaraki saw her was during his first meeting with Ailin, when he challenged him.

At that time, Zaraki realized that because he was too weak, this woman did not enjoy their battle to the fullest. He had been abandoned!

Over the years, although Zaraki recovered some of his reiatsu during his fights with Ailin, he still felt that there was a vast distance between himself and this woman.

It was not yet time for him to engage in slaughter with this woman. He needed to become stronger!

So, after being defeated by Ailin this time, Zaraki decided to take Ailin's advice and use the next ten years to learn the way of the sword.

In Zaraki's eyes, the only people qualified to teach him swordsmanship were Ailin, the old man Yamamoto, and Yachiru Unohana.

Ailin's swordsmanship was strong, but Zaraki preferred the intense close-quarters slaughter rather than fighting at a distance by unleashing slashes.

Yamamoto's style of swordsmanship made Zaraki feel very uncomfortable. He would rather swing his sword recklessly than learn from the old man. That left only this woman.

"What? You want to learn swordsmanship from me?"

"That's right."

Zaraki saw no problem in asking Yachiru Unohana to teach him swordsmanship.

"That man said my swordsmanship is too weak. If it's still like this in ten years, he won't fight me again. So I need to become stronger."

Saying that, Zaraki glared fiercely at Unohana, "After I defeat that guy, I'll challenge you again. I hope you don't disappoint me then, First Kenpachi!"

Hearing Zaraki's words, Unohana trembled all over, a glimmer of joy flashing in her lifeless eyes. From Zaraki's tone, it seemed she had not been abandoned yet...

"Okay. If you're willing, I'll teach you everything I know about the way of the sword!"

***

Time passed unnoticed by everyone.

That day, as Ailin was staying in the Kuchiki Manor, he heard the summons gathering the Captains once more.

On his way to the 1st Division, Ailin saw the delighted Kirio Hikifune.

"Congratulations, Captain Hikifune." Ailin stepped forward with a smile to congratulate Kirio Hikifune.

This Captains' meeting, once Ailin saw Kirio Hikifune, he knew the main purpose.

After all these years, Kirio had finally completed her research, fully integrating the "Temprary Soul" reiatsu into her own through culinary means.

At this moment, Kirio Hikifune’s reiatsu had reached a level comparable to Retsu Unohana and Genryusai Shigekuni Yamamoto in Ailin's perception.

In other words, by incorporating the foreign reiatsu into her body, Kirio Hikifune’s reiatsu had reached the limit for a Shinigami.

She had accomplished the great feat of "creating Soul Society history" and gained the recognition of the Soul King's will, passing the Zero Squad's test.

All that remained was for the Zero Squad members to ride the "Tenchuren" to the Seireitei and take her to the Soul King's Palace to complete the creation of the "King’s Key."

Sure enough, after all the Captains had gathered, Genryusai Shigekuni Yamamoto announced that Kirio Hikifune had passed the Zero Squad's selection and officially joined the Zero Squad.

Afterwards, Genryusai led everyone to an open area in the Seireitei. Since this was the legendary Zero Squad, those who came here were the Captains of each Division.

The only lieutenant present was likely Hiyori Sarugaki, Kirio Hikifune’s lieutenant from the 12th Division.

The awkward girl was both happy for her Captain's promotion to the Zero Squad, yet saddened that Kirio Hikifune would be entering the Zero Squad.

The Captains present all had solemn expressions. The Zero Squad, the legendary Royal Guards protecting the Soul King.

It was said that every member of the Zero Squad possessed strength exceeding that of the Gotei 13 Captains.

Although the Zero Squad did not have many members, their combined strength surpassed that of the Gotei 13.

This was largely due to the Gotei 13 having the thousand-year strongest Shinigami Genryusai Shigekuni Yamamoto, the unprecedented evil Yachiru Unohana, the 7th Kenpachi, Kuruyashiki Kenpachi, the 8th Sōya Kenpachi...

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Without these few, any single member of the Zero Squad could easily crush the entire Gotei 13.

The Gotei 13 had been established for nearly a thousand years, but this was the first time a Captain successfully joined the Zero Squad.

The 7th Kenpachi, Kuruyashiki Kenpachi had received an invitation from the Zero Squad before but declined.

Whoosh!

Without making them wait long, a rushing wind suddenly sounded from the sky. It seemed like something was rapidly descending.

"What's that?"

The Captains looked up and instantly saw a cylindrical object rapidly falling from the sky. Moreover, this cylindrical object passed through the great defensive barrier "Shakonmaku" of the Soul Society without obstruction, crashing into the open area in front of everyone.

Bang!

The massive cylinder struck the ground, creating a large crater and kicking up dust that obscured everyone's vision. Once the dust cleared, the cylindrical object was revealed before them.

A door opened on the cylinder, and a man with a pompadour hairstyle emerged.

There were only two ways to enter the Zero Squad, as the Soul King's Palace was separated from the Seireitei by seventy-two barriers.

The first was to possess the "King’s Key" capable of opening those seventy-two barriers and entering the Soul King's Palace from the Seireitei under its power.

In fact, the so-called "King’s Key" was the "Soul King's Bone" formed by the Soul King's power after joining the Zero Squad. Under the influence of the "Soul King's Bone," the reiatsu of each Zero Squad member surpassed the limits of Shinigami, reaching an terrifyingly powerful level.

This key relied on immense reiatsu to directly break through the barriers between the Seireitei and the Soul King's Palace, granting entry to the Soul King's Palace.

The second method was to hide within a reiatsu cannon and use the fireworks cannon to blast through the seventy-two barriers separating the Seireitei and the Soul King's Palace, entering the Soul King's Palace directly.

This fireworks cannon used to enter the Soul King's Palace was managed by the Shiba clan, one of the Five Noble Families.

However, after the Shiba clan fell from grace and was targeted by the other nobles, they moved the fireworks cannon out of the Seireitei to somewhere in the Rukongai District.

The one who came from the Soul King's Palace to welcome Kirio Hikifune was Tenjirō Kirinji, who used the "Tenchuren" to descend directly from the Soul King's Palace.

After exiting the "Tenchuren," the pompadour-haired Tenjirō instantly appeared beside Retsu Unohana, greeting her like an old friend, "Yo, long time no see, Retsu."

"Ah, long time no see."

"That appearance of yours is really... unfamiliar." Tenjirō clicked his tongue and spat out the toothpick in his mouth.

Tenjirō could be considered Yachiru Unohana’s friend of nearly a thousand years. He was familiar with that lifeless, murderous woman obsessed only with slaughter.

Well, back then, Tenjirō disliked her quite a bit. After all, the person who created "Kaido" and could condense reiatsu into healing hot springs like Tenjirō could be considered a "good person" in Soul Society.

When Unohana approached Tenjirō wanting to learn Kaido, Tenjirō wondered if the sky had fallen or the earth had split open. The unprecedented evil was not seeking opponents to slaughter, but learning healing instead?

With a "let's give it a try" attitude, Tenjirō taught Unohana the Kaido techniques.

Surprisingly, Unohana had great talent in Kaido. Apart from Tenjirō's method of condensing reiatsu into hot springs based on Kaido, Unohana’s mastery was not much inferior.

Moreover, after learning Kaido, Unohana abandoned her "Kenpachi" and “Yachiru” name, renaming herself "Retsu."

Once they became familiar, Tenjirō learned from Unohana that her change stemmed from being defeated by someone. Unohana studied Kaido to be able to fight that person again.

"Seeing you here, does that mean you won that battle?"

"No, we haven't had our second fight yet."

"Heh, I'd like to see what kind of guy needs you to use Kaido to defeat him."

Seeing the visitor from the Soul King's Palace only conversing with Unohana and ignoring everyone else, some of the Captains present frowned. They clearly felt this Zero Squad member was too arrogant.

After reminiscing with Retsu Unohana, Tenjirō's body instantly vanished and reappeared behind Ailin, reaching out to restrain Ailin's hands behind his back.

However, when Tenjirō grabbed Ailin's hand, intending to twist it behind his back, he found he could not bend Ailin's arm.

"Oh my, this Zero Squad gentleman is so enthusiastic, immediately grabbing my hand."

Ailin raised his hand, and the Captains then realized that in that instant, Tenjirō's movement had exceeded their perception range.

"Such swift Shunpo."

However, upon seeing that the one grabbed was Ailin, curious expressions appeared on their faces.

To be honest, among these Captains, Ailin had grown up before their eyes. But today, just how strong Ailin had become, they had no idea. The best reference was Zaraki, the lieutenant of the 11th Division.

Zaraki's reiatsu was far above most of the Captains. Even if they released their Bankai, increasing their reiatsu fivefold, they could still feel the vast gap between their reiatsu and Zaraki's.

One could say, excluding the abilities of their released Bankai, they had almost no chance of victory against Zaraki.

Yet this overwhelmingly powerful Zaraki, who challenged Ailin every ten years, always left defeated and injured.

They knew Ailin was strong, but not how strong. Could this Zero Squad member from the Soul King's Palace gauge Ailin's strength? Unfortunately, their expectations were not met.

"Tch, I see, no wonder you rejected the Zero Squad's invitation." Just that one action changed Tenjirō's expression.

Keep in mind, he possessed the "Soul King's Bone" and reiatsu surpassing the limits of Shinigami as a Zero Squad member.

Although he did not use his full power just now, being able to casually move his arm like that showed this man's strength was not to be underestimated. At the very least, on par with Unohana. His reiatsu had reached the limits of a Shinigami.

"What?"

Hearing Tenjirō's words, the Shinigami Captains were all stunned.

They did not know that besides 12th Division Captain Kirio Hikifune receiving an invitation from the Zero Squad, 11th Division Captain Ailin had also received one. And he had rejected it.

"Captain Ailin, what were you thinking?" The one asking was Kirio Hikifune.

When she recommended Ailin to become the 11th Division Captain, she felt that with Ailin's growth rate, one day he would reach the Zero Squad's standards and be recruited, giving her a familiar face in the Soul King's Palace. But she never imagined that Ailin would receive the Zero Squad's invitation before her and reject it.

As someone who had accepted the Zero Squad's invitation, Kirio Hikifune knew that apart from creating Soul Society history to gain recognition from the Soul King's will, there was a second requirement to join the Soul King's Palace.

First, one's reiatsu must reach the limit for a Shinigami.

Currently in the Seireitei, only the old man Yamamoto, Unohana, and herself met this condition.

The first two had reached this unmatched level of power many years ago. As for Kirio Hikifune, she had just raised her reiatsu to the Shinigami limit with this latest research success.

The second requirement was possessing formidable strength. For example, the Head Captain Yamamoto's Bankai flames reaching the temperature of the sun's core.

For example, the 7th Kenpachi, Kuruyashiki Bankai capable of destroying the entire Seireitei...

In terms of reiatsu, Unohana had met the first standard. Unfortunately, her forte was swordsmanship, lacking particularly destructive power.

Even after acquiring her Zanpakutō, its Bankai did not possess immense destructive power, instead fusing the blood-colored hot spring condensation method she learned from Tenjirō into her Zanpakutō's release.