In truth, yang chakra really does have that kind of effect.
Palm Sage Jutsu, the pinnacle of medical ninjutsu in Konoha, is built entirely on yang chakra. Strengthening cells, accelerating recovery, restoring vitality… the power to mend flesh and bone lies in that warm, vibrant current.
But if there's a way to heal, then naturally there's also a way to break.
Too much yang chakra doesn't just "heal faster."
It overloads the system, shatters the body's chakra flow, and sends the recipient collapsing into unconsciousness.
Kenichi Amamiya knew this very well.
He'd already almost killed that Hyūga bastard several times playing around with the dosage.
"What are you planning to research this time?" Orochimaru watched his disciple with quiet interest. He knew this boy—whenever Kenichi asked for materials, there was always some strange idea behind it.
"Nothing much, Sensei. I just want to study that previous case a bit more deeply."
Kenichi smiled innocently.
He really didn't have any bad intentions.
He just wanted his enemies to die of lung cancer, that's all.
Orochimaru paused for a moment, then gave Kenichi a sidelong glance.
Of course he knew which "case" Kenichi was talking about—the Hyūga with the strange growths in his brain and eyes.
This child's heart is… quite dark.
Leaving him in this "Will of Fire" village would indeed have been a waste.
On the other hand, Kenichi's mind was already running far ahead.
Since he'd decided to follow Orochimaru when he defected, there was no need to chain his future to Konoha. Building his own lab inside the village suddenly felt pointless.
Actually… why build it himself at all?
Sensei is loaded anyway, he thought. Wouldn't it be more efficient to just… let him sponsor one for me? A fully equipped personal research base. Yeah. That sounds about right.
Armed with that beautiful fantasy, Kenichi hauled his new "materials" into the lab and went back to work.
"Although… I'm almost out of cigarettes. I should stock up next time," he muttered, rubbing his chin as he crudely forced chakra into the test subject's body.
Because these weren't precious, unique samples like the Hyūga, and because—aside from missing a hand—this particular test subject was otherwise in decent shape, Kenichi no longer felt the need to be careful.
He was getting bolder.
".…What are you doing?"
When Orochimaru walked into the lab, he stopped dead, staring at the scene in front of him as if he'd just walked in on a genuine lunatic.
Kenichi turned around, completely serious.
"Doing experiments, Sensei."
What else would he be doing, if not that? Slacking?
Orochimaru pinched the bridge of his nose.
"I can see that. What I'm asking is… why is his chest cut wide open?"
The man on the operating table was still alive.
Barely.
His rib cage had been split cleanly down the middle and spread apart. His lungs rose and fell in ragged, shallow breaths, slick with blood and exposed directly to the air.
The man's eyes were unfocused, but full of raw, animal terror. He wanted to die. That much was obvious.
Kenichi blinked.
"Of course it's so I can observe more closely."
He said it like it was the most natural thing in the world.
If the skin blocked his view, he would simply remove the obstacle. Removing the obstacle required opening the chest. Simple.
And the truth was, it had worked.
Now he could guide nicotine precisely to the lungs and directly observe the changes—no more guessing through skin and muscle.
If the target happened to be a chronic smoker to begin with, it would be even easier. The battlefield would already be softened.
"…Fine," Orochimaru sighed, rubbing his temple.
Sometimes, his disciple really did make him feel complicated things.
Kenichi watched his teacher leave with a puzzled expression, then shrugged and returned to his work.
He could feel it.
Success wasn't far now.
[…Another sample consumed. But it was worth it. At this point, even without opening the chest, I can accurately direct the nicotine's effects to the lungs.]
[The remaining problem is how to make yang chakra affect specific cells. Every time I use it, the subject's lungs become unnaturally active.]
[Wait. When I intensified the stimulation just now, his heartbeat changed. Pallor, trembling lips, rapid breathing… respiratory alkalosis?]
Kenichi put down his pen and looked up, interest flickering in his eyes.
The test subject's heartbeat was racing. His chest rose and fell too quickly, breath shallow and chaotic. His lips twitched uncontrollably, and his face had turned the color of paper.
Hyperventilation syndrome caused by over-ventilated lungs.
Respiratory alkalosis.
"An unexpected bonus," Kenichi murmured. "But for a ninja, this probably isn't a serious problem."
Unfortunately, there was a catch.
He wasn't a trained medic. And his body, having adapted to the First Hokage's constitution, contained an excessive amount of yang chakra. Without precise control, any attempt to condense it turned into an overdose.
His "healing" touch was closer to a bomb.
"…If yang chakra can stimulate organs like this, then what happens if I focus it on the heart?" he wondered aloud.
The thought lodged itself in his mind and refused to leave.
Kenichi hesitated for exactly three seconds.
Then he turned, grabbed another test subject, and started a new experiment.
The result was exactly what he'd expected.
The heart began to pound wildly, skipping beats and then racing to catch up. The rhythm fell apart. The organ spasmed in overdrive and then—
Stopped.
Cardiac arrest. Instant death.
By the end of the afternoon, the mute cleaning staff had already dragged eight bodies out of the lab and buried them in the prepared pit outside. By then, they were completely numb.
They looked at the lab.
They looked at the young boy inside, standing there with a calm, focused expression, hands stained red.
And for the first time, they felt:
This kid is even scarier than Orochimaru-sama.
Kenichi, of course, had no idea how much psychological trauma he'd caused.
He was still lost in thought.
"Overloading the heart with yang chakra does exactly what I imagined," he mused. "Hyperactivity, arrhythmia, then sudden cardiac death. On a normal person, it's… perfect."
On civilians, it was terrifying.
On ninjas… not so much.
"A pity. Ninjas aren't idiots," Kenichi sighed.
In his previous world, this would have been a classic silent assassination technique—no poison, no visible wounds, just sudden death.
But here?
No shinobi would just stand there and calmly let someone pump a huge amount of chakra directly into their heart.
Unless they were already immobilized, heavily injured, or completely unaware. And if they were civilians, he didn't need such a complicated method to begin with. A single kunai would be cheaper and faster.
"Still too impractical. I'd need a way to inject a large amount of chakra into someone instantly…" he muttered.
Which meant one thing:
He needed more data.
Real ninja data.
"For the sake of science and the advancement of ninjutsu, I humbly request that you all… sacrifice yourselves," Kenichi said cheerfully as he went to find his teacher.
The test subjects Orochimaru had given him so far were mostly ordinary people. No chakra, no training, no resistance.
That wasn't enough anymore.
"I need shinobi samples, Sensei," Kenichi said earnestly. "Ordinary bodies just aren't adequate for testing chakra-based techniques."
Orochimaru stared at him for a long moment.
The speed at which this child consumed "materials" was honestly… astonishing.
Even faster than his own.
"…I have a genin here," Orochimaru said at last. "Use him. Sparingly."
He rubbed his brow again as he spoke, but there was curiosity in his eyes.
He, too, wanted to know exactly what that strange disease-like growth was—something that could resemble the First Hokage's cells.
Kenichi's eyes lit up.
"Understood, Sensei."
A living ninja test subject.
The next stage of his "cancer-style" ninjutsu… could finally begin.