"I wasn't certain before," the man answered, lowering his voice, "but judging from how they wiped out both Hunting God Squads... it's just as you suspected, Lord Tokinada. They're carrying Spirit King fragments inside their bodies—and a large quantity at that."
"That would explain their abnormal growth. In just one day of exposure to a Zanpakutō and Kidō, they achieved strength that defies logic."
Yamada Seinosuke nodded slowly. He no longer objected.
"There will be time later," Tsunayashiro Tokinada said coldly.
"For now, the fragments I have are just enough to keep things running. Let them breathe the reiryoku-rich air of the Seireitei a little longer. Consider it a gift—for bringing those precious fragments right to my doorstep."
Tokinada tapped the armrest of his chair as he made his decision: he would wait until Aiyan and Aizen left the Shin'ō Academy before striking. With the recent incident already attracting attention, infiltration like before was out of the question.
Once Yamamoto Genryūsai Shigekuni and Jūshirō Ukitake eased off their surveillance… then he would act.
Aiyan and Aizen attended class as usual, acting like nothing had happened. Not even a whisper about yesterday's ambush.
But unlike the previous Kidō class, today's Zanpakutō lecture was not being taught by Jūshirō Ukitake. Instead, a tall man with short black hair, a slim face, and wearing a sleeveless white haori marked with the insignia of the 10th Division walked into the room.
"Yo! Good morning, class!"
"My name's Shiba Isshin. You can call me Teacher Shiba… or just Isshin-sensei if you like."
He kicked open the door with gusto, used a burst of Shunpo to appear at the front of the classroom, and landed dramatically on the podium. His wide grin scanned across the stunned students.
The room fell into silence.
Aiyan, seated at the back, narrowed his eyes with interest.
A man who, in another time, could easily stand alongside legends—one who wielded Engetsu.
In the entire Soul Society, apart from Yamamoto Genryūsai, the man before him was someone truly worth noting.
"…Interesting," he murmured.
Aizen noticed Aiyan's subtle change in expression and his gaze briefly lingered on Isshin. Quietly, he made a mental note: Shiba Isshin. He would gather information later.
"That idiot… even after becoming a Division Captain, he's still the same as ever," muttered Yoruichi Shihōin with a sigh, palm covering her forehead.
As the childhood friend of Kūkaku Shiba, she knew Isshin's antics all too well.
"Alright, class! Let's get started!"
"This lesson can be summed up in two kanji."
Isshin turned, picked up a piece of chalk, and boldly wrote on the board behind him: 刀禅—Sword Zen.
Each stroke was sharp and clean, like the sweep of a crescent moon slicing through darkness.
"The path to Zanpakutō liberation comes in two stages: Shikai and Bankai."
"Both start the same way—by entering your own inner world and communicating with your Zanpakutō spirit."
"Hear your Zanpakutō speak its name, and you unlock Shikai. Confront and subdue your Zanpakutō's manifested form, and you reach Bankai."
Isshin's explanation was brief but to the point—impressive for someone known for his eccentric attitude.
Even students from noble houses—Byakuya Kuchiki, Yoruichi Shihōin—leaned in to listen more closely.
"Teacher Isshin," Komamura Sajin raised a pawed hand, "how do we enter the inner world?"
"You know that saying, right? 'Sincerity reaches the heart.'"
"Sit down, place your Zanpakutō in front of you, place your hand on the blade, and pour your Reiatsu into it. Use the sealed sword as a medium—let your Reiatsu guide the way."
"This is the practice of Sword Zen."
Isshin motioned for the students to draw their Zanpakutō and try.
As expected, only those who had already unlocked their Shikai—nobles like Yoruichi and Byakuya—showed progress. Tousen Kaname, Komamura Sajin, and others struggled to make any connection.
"Don't worry, newbies," Isshin said, waving off their frustration.
"Zanpakutō have personality. Sometimes they just don't feel like talking—especially if they think you're too weak."
"Take me, for example. My Zanpakutō's got a nasty temper. I spent an entire day doing Sword Zen, and just as my left foot stepped into the inner world, I got drop-kicked out by the spirit before I could say hello."
Laughter broke the tension. The students visibly relaxed, realizing even a Captain had struggled once.
Isshin suddenly shouted, startling everyone. Even Tousen nearly cut himself in the confusion.
"Oi, what's with the yelling?" Yoruichi snapped. "I was just about to enter my inner world!"
But Isshin didn't hear her. His eyes were locked on the back row—mouth agape.
Following his gaze, the class turned to look.
A giant crimson dragon claw had emerged behind Aiyan's seat, calmly resting on his shoulder.
A tangible, fully manifested Zanpakutō spirit.
"C-Concretization…?" Isshin gasped.
That was the first requirement of Bankai training. To manifest your Zanpakutō spirit into the real world was to begin the Bankai process.
But this—this wasn't resistance or conflict. The dragon claw didn't threaten, didn't challenge—it rested calmly, as if acknowledging its master.
"Wait… is that… one of the prerequisites for Bankai?"
"Is Aiyan already manifesting his Zanpakutō?!"
"He just got it yesterday!"
"Didn't Teacher Jūshirō say it takes decades for even a genius to master Bankai?"
Shocked voices filled the room as the implications dawned on everyone.
Yoruichi, Urahara Kisuke, even Byakuya and others from noble families, all looked at Aiyan—not with envy, but with stunned respect.
What kind of monster could master Sword Zen overnight, and then bring his Zanpakutō into the physical world the next day?
"…Kid's not human," he muttered under his breath.
Aizen, sitting beside Aiyan, smirked.
It had already begun.