When was Adult Sasuke at the peak of his strength?
The answer is: right after the Fourth Great Ninja War, during his second battle against Naruto at the Valley of the End.
That was truly a case of heaven and earth themselves lending him their strength.
A six-tomoe Rinnegan, an Eternal Mangekyō Sharingan, and Six Paths Yin Power.
On top of that, he had even dragged all nine tailed beasts over and turned them into living batteries.
Then, once the battle was over, fortune turned, and even heroes found themselves constrained.
In martial prowess, he failed to defeat his best buddy. In ideals, he lost to Naruto's Talk-no-Jutsu and voluntarily gave up on reshaping the world according to his own will.
Since he didn't win the battle, there was naturally no way he could keep the tailed beasts under his control. His batteries were gone.
The Yin Power that the old Sage of Six Paths had left in his left hand still had plenty remaining after sealing away Kaguya Ōtsutsuki. Using it in combat would have been a complete waste.
Its greatest value was allowing him and Naruto to jointly study and comprehend Six Paths-level Yin-Yang Power, gradually improving their mastery of Yin Release and Yang Release.
That was the proper way to use high-end power. The goal should have been to turn a temporary cheat into strength that truly belonged to them forever.
A bromance version of dual cultivation, so to speak.
Unfortunately, that sustainable-development mindset came from the soul of Makoto's previous life. Ninjas, on the other hand, only believed in the battle-first mentality of burning through everything for immediate gains.
You can have a thousand arguments and ten thousand philosophies, but in the end there was only one question:
Can you beat the other guy?
The final result was that, after one explosive clash, all their high-end power vanished. The temporary cheat granted by the old Sage of Six Paths logged out at lightspeed.
Not only that, Sasuke lost his left arm, while Naruto lost his right.
And those hands just happened to be where the Six Paths Yin-Yang Power had resided.
They started fighting before they had even finished their meal and ended up smashing the granary along with the rice.
Sasuke and Naruto had trained hard in their early years, but in the ninja world, what famous ninja hadn't worked desperately hard?
They possessed some of the greatest bloodlines in existence, but bloodlines had limits. Their cheat powers were the opportunity that could have taken them beyond those limits, yet they were too young to appreciate what they had.
After the war, Adult Sasuke had to support a Six Paths-level Rinnegan with a body that wasn't at the Six Paths level. He lived in a constant state of chakra deficiency, going from a man who could fight for three days and three nights straight to a three-minute man.
To solve Sasuke's awkward predicament, Makoto transformed into the old master living inside the ring and pointed him toward a proper system of advancement through training.
...
Early morning, at the dojo.
Today was the day to inspect the results of their training.
Adult Sasuke sat cross-legged on the ground, guiding secondary white natural energy into his body and using it to strengthen himself in every aspect.
After continuing for quite some time, he finally began to feel pressure.
He knew he had reached his limit for the day.
If he pushed any further, the power of annihilation would teach him a harsh lesson.
Next came Boruto's turn.
He was still a rookie who had only recently graduated from the Ninja Academy. His strength was far below that of his master.
However, thanks to the bloodline inheritance of the brothers Ōtsutsuki Hagoromo and Ōtsutsuki Hamura, the amount of time he could train with secondary white natural energy had already reached more than half of his master's.
That was a very respectable pace of progress.
After checking both of them, Makoto was quite satisfied.
Their aptitude for Senjutsu was firmly in the top tier of the ninja world.
"Not bad. You've officially stepped through the door of the True Sage training system. Once you return to your own world, you can continue progressing along this path step by step."
"However, during this period, you've devoted most of your attention to training and still don't fully understand the system I've created."
"So I think it's necessary for me to explain it clearly."
"Given your backgrounds and experiences, both of you already know that chakra originally belongs to the Ōtsutsuki Clan. From Urashiki, I also learned that chakra is the product of a God Tree devouring the natural energy of a living planet, bearing fruit, and then being absorbed and redistributed by the Ōtsutsuki."
"Chakra is the food of the Ōtsutsuki Clan. They possess the highest authority over it."
"That's why ordinary five-nature chakra ninjutsu is ineffective against them. The only things that can reliably harm them are taijutsu, sealing techniques, and Senjutsu."
"Therefore, learning Senjutsu through the absorption of natural energy is the true path for the intelligent lifeforms native to living planets to grow stronger."
Adult Sasuke listened quietly.
Everything Makoto had said so far was already familiar to him.
Boruto asked, "I know a little about natural energy and Senjutsu. My dad inherited the Senjutsu of Mount Myōboku, and my sensei is contracted with Ryūchi Cave. Makoto-sensei, how is your Senjutsu system different from the Three Great Sage Regions?"
"Of course there's a difference," Makoto replied. "Based on my observations over the years, natural energy itself comes in many varieties. In fact, they don't even share the same color."
"The natural energy contaminated by the Ōtsutsuki God Tree carries a faint red tint and makes up the largest proportion. The natural energy assimilated by the Sage Region of Mount Myōboku has a faint cyan tint, Ryūchi Cave's is faintly purple, and Shikkotsu Forest's is faintly green."
"These are currently the major types of natural energy found throughout the ninja world. There are quite a few other colors as well, but their proportions are insignificant, so there's no need to pay attention to them."
"As for the Senjutsu system I created, it uses the most primordial and uncorrupted natural energy in the ninja world. Its color is a pale white."
This was an entirely new version of Senjutsu theory that neither of them had ever encountered before.
Before today, they had no idea that natural energy could be divided into so many different categories.
Boruto asked curiously, "Natural energy has colors? Why have I never heard of that before?"
Makoto replied, "Natural energy is almost impossible to see, so it naturally doesn't possess a physical color. The colors I'm talking about are what can be observed only while in a special spiritual-body state."
"So far, I've only discovered that I possess this ability. No one else can enter that special spiritual-body state."
Boruto asked, "Then how are we supposed to tell them apart?"
"It's very simple. You can only rely on your senses."
"Absorbing natural energy contaminated by the God Tree will gradually make a person's emotions more violent. If they fail to control it, they'll mutate into a monster whose mind contains nothing but thoughts of slaughter."
"Orochimaru once told me that one of his test subjects was born with the ability to absorb this kind of natural energy. I think his name was Jūgo. I'm not sure whether someone like that exists in your world."
Boruto looked toward his master.
Adult Sasuke answered calmly, "He does. He's my friend. When he was younger, he really was exactly as you described and was prone to losing control. But he's much better now."
Makoto nodded.
"Right. That's the result of being influenced by red natural energy."
"If what you absorb is the cyan natural energy assimilated by Mount Myōboku, the first thing you'll notice is a change in your vision, along with an increase in strength."
"If it's the purple natural energy of Ryūchi Cave, you'll feel as though you've become one with the earth itself."
"And if you come into contact with the green natural energy of Shikkotsu Forest, you'll feel your body becoming more active while your mind grows sluggish."
"As for primordial white natural energy, any living thing that comes into contact with it will be directly annihilated."
"Primordial natural energy originates from nature itself. If you recklessly touch it, it will simply return you to nature."
"Fortunately, it is extremely inert. It doesn't react with living beings, nor does it affect human life. Likewise, humans are unable to make use of it."
"Even the Ōtsutsuki Clan doesn't dare touch it directly. They can only convert it by planting God Trees."
"As for me, because my soul is special, I became the only person currently capable of coming into contact with primordial white natural energy."
"Building upon that foundation, I created the True Sage system and developed a secondary white natural energy that can be widely taught throughout Konohagakure."
Boruto asked, "Makoto-sensei, what's the difference between your Senjutsu system and the ones used by the Three Great Sage Regions?"
"The difference is enormous."
"When you train in the Senjutsu of the Three Great Sage Regions, you can only use the natural energy they've assimilated. It's also a one-time-use resource, and its benefits are almost entirely limited to combat power. Its effect on continuously strengthening the body is negligible."
"However, the effect is completely different for the species native to those regions. The talented individuals living in the Three Great Sage Regions are all beings with extremely long lifespans."
"Yet their contractors remain short-lived species no matter how perfectly they master Senjutsu."
"There's no reason natural energy should work on animals while discriminating against humans."
"So the conclusion is simple: their Senjutsu was never truly suited for humans."
"You can tell just by looking at the appearance of its practitioners. Take Mount Myōboku's Senjutsu for example. No matter how perfectly you master it, you'll still develop certain frog-like characteristics."
"But my Senjutsu system is different."
"Secondary white natural energy is perfectly suited for humans. It can continuously strengthen our bodies and achieve effects comparable to those of the long-lived species in the Three Great Sage Regions."
"That said, regardless of which type of Senjutsu you're dealing with, the danger is essentially the same."
"Once control is lost, you'll either turn into a stone frog, undergo snake transformation, or become a slug and end up as part of Shikkotsu Forest."
"The Senjutsu I've developed is dangerous as well, but the risk of losing control is far lower. Once you've passed the initial fusion stage, all you need to do is stop training whenever you feel you've reached your body's limit."
Boruto's eyes lit up.
"That's amazing! In that case, it really does sound like a form of natural energy perfectly suited for us shinobi."
"But if I can't enter Sage Mode yet, that's one thing. Why can't my Sensei enter Sage Mode either?"
"I remember my dad and others talking about it. Back then, after he arrived at Mount Myōboku, he mastered it not long afterward and was already able to use Sage Mode."