Katori Nanatsuki followed Unohana Retsu into the classroom of the Kaidō Advanced Class.
Women made up a slightly larger portion of the class, accounting for two-thirds of the students.
Unohana, the captain of Squad Four, came to the Shin'o Spiritual Arts Academy every year to recruit. Every student interested in learning Kaidō knew who she was.
The students of the Kaidō class looked curiously at Nanatsuki, who followed behind her.
Most of them didn't recognize him—only a few did, and those were students from the Sixth-Year Class he instructed.
Aside from the students, there were three Kaidō instructors present in the classroom. One of them, an elderly white-haired man, was the department head.
All four Kaidō instructors had transferred from Squad Four and were familiar with Unohana.
Word had already spread through the Kaidō Department that Unohana would be lecturing on advanced Kaidō techniques today. For the instructors, this was a rare learning opportunity, so they came to attend as well.
"Thank you, Captain Unohana, for taking time out of your schedule to lecture for us," said the white-haired department head as he stood up and began clapping.
"You're too kind, Department Head Yamada. Nurturing more Kaidō talent for the Gotei 13 is also part of Squad Four's duty," Unohana replied warmly.
"Haha, the moment we heard that Captain Unohana was coming, the rest of us dropped everything and rushed over," Department Head Yamada chuckled.
"Same here. I hurried over the moment my class ended. Luckily, I only had one lecture today, or I would've missed Captain Unohana's session!" said Instructor Kobayashi, clearly excited. She had heard from Yamada that Unohana would be teaching advanced Kaidō techniques today, and was so thrilled she couldn't sleep the entire night.
Unohana had come today hoping to see if the academy had any Kaidō prodigies. If she found one, she planned to bring them back to Squad Four early for training.
Those who excelled at Kaidō weren't always strong in combat. The Sixth-Year practical exam had a certain casualty rate, and she didn't want any Kaidō genius to be lost because of it.
Nanatsuki took a seat next to the Kaidō instructors.
"Isn't that… the Katori-sensei from the Swordsmanship Department?" Yamada looked at him for a moment, trying to recall his name, and was puzzled by his presence here.
"Katori-sensei is probably here to attend Captain Unohana's lecture too," Kobayashi offered, introducing him to the others. "He studied Kaidō under Captain Unohana for several years. I used to see him often back when I was still in Squad Four. Even though he's in the Swordsmanship Department, his Kaidō skills aren't any worse than mine."
Kobayashi had only recently joined the academy as a new instructor and taught the same class as Nanatsuki.
Since she had been transferred from Squad Four, she had known Nanatsuki from back when he was learning Kaidō under Unohana, so the two were relatively familiar.
"Oh? Impressive. Katori-sensei is still young, and yet he's made a name for himself in both Zanjutsu and Kaidō," Yamada said with a hint of surprise. Kobayashi's own Kaidō skills were solid—though still a bit green, she was expected to reach the level of a lower-seat officer in Squad Four given time.
"Captain Unohana is an excellent teacher," Nanatsuki replied with a polite smile.
Unohana began the lecture, starting from basic and intermediate Kaidō techniques and gradually building up.
But with her commanding presence and mastery, even the most ordinary techniques, when explained by her, offered profound insights. Even Instructor Kobayashi had sudden realizations, gaining new perspectives on techniques she had long thought she'd mastered.
An hour later, Unohana moved on to today's main topic.
"As you all know, Gillian-class Hollows often evolve unique abilities. Today's topic involves a particular type of injury related to such Hollows," Unohana explained.
"This type of wound is caused by Hollow poison. The toxins released by some Gillian-class Hollows are extremely dangerous. Even Captain-level Shinigami can't expel the poison with their Reiatsu alone. At best, they can suppress the poison to slow its spread."
"If untreated for long, even Captains will eventually succumb to Hollow poison."
She held up a translucent syringe filled with a pitch-black liquid. "This poison was extracted from an Adjuchas-class Hollow."
"For the sake of demonstration, I will inject this poison into my own hand to simulate a poisoned state."
As soon as she said that, the expressions of the Kaidō instructors changed drastically.
Adjuchas-class Hollows were evolved from Gillian-class and were rare and powerful beings, even in Hueco Mundo. The toxins produced by them were so potent that they could kill any Shinigami below Captain level in a short time.
"Captain Unohana, this is Adjuchas-class poison! Isn't it too dangerous to inject such a terrifying toxin into yourself for a demonstration? Can't you use an animal for the test instead?" Kobayashi said worriedly.
"Animal testing isn't viable," Yamada said, frowning. "The Hollow poison from an Adjuchas is enough to make most spiritual bodies collapse instantly. Even Vice-Captain-level Shinigami wouldn't last long. There simply aren't any animal spirits in the Soul Society suited for testing poison of this caliber. Still, for Captain Unohana to use her own body... that's far too risky."
It wasn't that he doubted Unohana's healing skills—he just feared that the poison might interfere with her ability to treat herself.
But Unohana remained calm and confident, smiling gently. "No need to worry. I can suppress the poison with my Reiatsu. As long as it's contained to a small area, it won't affect me much."
Seeing her so resolute, Yamada didn't press further. After all, he was also capable of dealing with Adjuchas-class Hollow poison.
If Unohana failed to suppress it, he was ready to step in.
The students, unaware of just how terrifying Adjuchas-class Hollow poison truly was, instead felt moved that Captain Unohana would risk harm just to teach them.
Nanatsuki, however, remained calm. From what he knew about Hollow poison and Unohana herself, he was sure she could control it.
Once the toxin was injected into the back of her hand, the skin rapidly turned black and swollen. Thick, black lines began to emerge and spread outward quickly.
The rate of spread was alarmingly fast—within moments, it had reached her wrist and showed no signs of stopping, already creeping up her forearm.
Nanatsuki's gaze sharpened. 'This rate of spread... seems too fast. Not like how an Adjuchas-class Hollow's poison would normally behave in a Captain-level Shinigami...'