There are many unsolved mysteries in Seireitei.
For example, just how strong is Captain-Commander Genryūsai Shigekuni Yamamoto?
What is the source of 2nd Division Lieutenant Marechiyo Ōmaeda's wealth?
How many dried persimmons has the Third Division stockpiled?
Why is 4th Division Captain Retsu Unohana so gentle yet so terrifying?
How many talented members has the 5th Division sent to other divisions?
How many secret passages are there in the 6th Division's barracks?
What does 7th Division Captain Sajin Komamura actually look like under his mask?
Why does 8th Division Captain Shunsui Kyōraku wear women's clothing over his captain's Haori?
When will the serialization of The Path of Justice in the 9th Division finally end?
Has 10th Division Captain Tōshirō Hitsugaya grown any taller?
Just how strong is the 11th Division?
Are there monsters in the 12th Division?
Why is 13th Division Captain Jūshirō Ukitake in such poor health?
With the chain of events caused by Aizen's defection alongside Gin Ichimaru and Kaname Tōsen, many of these mysteries have been answered.
But for those who knew Muten Natsu back in their academy days, one question still remains unanswered:
Where exactly did he learn his Zanjutsu?
Muten Natsu slept through most of his classes, and he slept the most soundly during Zanjutsu lessons.
He was sharp and learned quickly.
But Zanjutsu requires more than intelligence. It demands long hours of practice and real combat, hammering each swing into instinctive muscle memory.
Even a genius like Tōshirō Hitsugaya, who mastered Bankai right after graduating from Shin'ō Academy, still needed extensive foundational training to refine his swordsmanship.
Yet Muten Natsu spent his days doing nothing but sleeping and eating, his body practically wasting away, so how did he come to possess sword skills far beyond others, even developing an incredibly destructive draw technique on top of that foundation?
He learned it in his sleep.
Muten Natsu walked to the fridge, pulled it open, took out a bottle of Calpis, opened it smoothly, and took a long gulp.
The sweet and sour taste spread across his tongue. He let out a quiet sigh, released his grip, and let the bottle fall as his hand moved to the hilt at his waist.
In the deathly silent room, a piercing shriek suddenly rang out, the scream of air being torn apart by a razor-sharp blade.
A chilling flash of steel descended. Before the severed glass bottle even hit the ground, Muten Natsu felt the clear sensation of his body being cut clean through at the chest.
Blood burst forth. His mind went blank for a moment. When he came to, the wound had already healed, and even his torn clothes had returned to normal.
He shifted to dodge, but before he could steady himself, a cold blade pierced up through his chin, punched through his skull, then split downward, cleaving him in two, leaving only a thin strip of skin barely connecting both halves.
Blood sprayed from his twisted face. He felt the wind brushing against his exposed organs. After a brief blur, his vision snapped back into focus.
The maddening pain still lingered in his mind. Muten Natsu's eyes widened, reflecting the figure holding the blade.
The figure in the willow-green cloak leapt upward, kicked off the ceiling, and dove down. The sharp blade fell once more.
Death. Death. Death. Death. Death. Death. Death followed like a shadow.
No matter how Muten Natsu dodged or hid, he was cut down in a single strike.
The cloaked figure moved like a ghost, so fast it bred despair. The long blade rose and fell again and again, killing Muten Natsu with effortless precision.
As if he were not a living person, but nothing more than an ingredient to be handled at will.
The Primordial Memory Core.
According to Myōki's analysis, this was a composite memory formed when the Soul Essence Stealing ability first awakened. The power, built up over time, ran out of control and absorbed large amounts of surrounding Reishi.
Otherwise, there was no way to explain how someone could command so many different schools and blade styles.
No. Not just command them.
Master them completely.
Different styles flowed effortlessly from his hands, blending with traces of others. There were no defining traits to identify any one school, but they all shared one thing in common.
An extreme efficiency in killing.
Shortly after Retsu Unohana brought Muten Natsu to Seireitei, the Primordial Memory Core began appearing in his dreams every time he fell asleep.
The willow-green cloak draped over the figure like a cloth, his face hidden in shadow. Through the gaps in the cloak, neatly arranged white, diamond-shaped plates—like armor—covered the exposed areas.
His face was unseen, and his form impossible to discern.
The katana was just an ordinary Asauchi.
It was only after learning about Zanpakutō that Muten Natsu realized this was not a dream, but his own inner world.
Whenever he fell asleep, the Primordial Memory Core pulled him into this inner world.
And he was slain, again and again.
If he could be cut down, it would at least be a form of liberation.
But…
Thud!
His throat was slit, blood spraying across the floor as Muten Natsu slumped forward, powerless.
The next moment, he steadied himself—only to have his head severed.
Then came the next time.
And the next…
Again and again, endlessly, his sleep became torture.
It was through these repeated deaths, through the pain of his own body, that Muten Natsu learned his sword techniques.
As for developing his draw technique…
Clang!
He raised his katana to block the Primordial Memory Core's swing, only for his heart to be pierced by the short blade in the opponent's other hand the very next instant.
Before the Primordial Memory Core, Muten Natsu had only a single chance to draw his sword.
Only when Retsu Unohana was nearby, releasing her Reiatsu to suppress Myōki, could he finally get a proper night's sleep.
It wasn't until he was imprisoned in the Nest of Maggots and Bokukō was taken away that Muten Natsu truly began to control Myōki's power, sealing the Primordial Memory Core so he could sleep normally.
Now, with Sōsuke Aizen's defection, no one knew when the next threat would arise.
Muten Natsu had to use every means possible to strengthen himself for the coming storm, even if the process was unbearably agonizing.
Struck down once more, he collapsed to the ground.
Everything around him, including the Primordial Memory Core, vanished, leaving only a pure white void.
Bokukō licked his cheek, and the Myōki Light Screen appeared before his eyes.
[Rest time is over]
"I can still go on…"
[You're already so delirious that you're deceiving yourself]
Muten Natsu: "..."
[The strain from the spiritual dream, combined with your excessive fatigue, increases the risk of your soul collapsing]
[You need time to reflect on the battle's details]
[You also need time to process what you've gained from the fight]
[And you need even more time to recover]
Muten Natsu hesitated for a moment, then closed his eyes completely.
Bokukō curled up beside him.
The pure white space gradually dimmed, and a gentle song echoed:
Sleep Sleep My dearest treasure~
In this eternal song~
Those eyes that know nothing~
Close them gently~
I am right here...