Naruto: Reborn as Kakuzu, the Immortal Assassin

"Chimera Technique."

The appearance of this forbidden technique was unexpected, but not entirely shocking.

"You've started researching bloodlines, haven't you?"

"Exactly." Tobirama Senju nodded slightly at Kakuzu. As top-tier shinobi of the ninja world, there was no need for pretenses.

The Sharingan of the Uchiha. The Byakugan of the Hyūga. The immense chakra reserves of the Senju and Uzumaki clans.

Their uniqueness boiled down to one thing—bloodlines. Or, in technical terms, kekkei genkai.

"You're trying to combine multiple kekkei genkai?"

"What, you've never considered it?"

The two exchanged a knowing look, smiling faintly without answering directly.

"Any progress?" Kakuzu asked.

"There is, but it's the hardest part," Tobirama admitted, biting the end of his pen as he glanced over a scroll covered in diagrams.

"I've experimented with fusing two different bloodlines, hoping to create something entirely new.

"But I found that the rejection between the two blood types is significant. They can't harmonize easily.

"Worse still, the mixture damages the host's body.

"At first, I refused to believe it and kept testing. After eight attempts, only once did I see a glimmer of success.

"Yet even then, I realized—complete fusion takes a minimum of ten years."

Tobirama stopped speaking, knowing Kakuzu could already guess the implications.

"Even if two bloodlines fuse, there's no guarantee they'll produce a new kekkei genkai or grant mastery over both. And waiting a decade? Not everyone has that luxury," Kakuzu remarked, his fingers tapping rhythmically against the table.

Tobirama wasn't an idealist. He'd seen the Warring States Era firsthand, where the average life expectancy barely exceeded thirty.

Even his older brother, Hashirama, couldn't escape the limits of mortality.

Tobirama wasn't sure how many decades he had left to pursue this experiment. And beyond time, there were obstacles—funding, his brother's disapproval, and countless other distractions.

He understood this was an experiment destined for failure.

Across from him, Kakuzu appeared calm, but inwardly, he was astonished. His gaze lingered on the incomplete Chimera Technique scroll.

The technique was unfinished, and its risks were unknown.

Tobirama had no intention of testing it on himself—recklessness could lead to disaster. But if he had been slightly more daring, he might have taken an Uchiha's Sharingan and hidden away for decades, letting the Senju and Uchiha bloodlines slowly meld.

Though he wouldn't awaken the Rinnegan as a reincarnated soul might, the true fusion of the two bloodlines could signify a sort of ancestral regression.

Still, this was just a fantasy.

Even Tobirama had admitted that combining two bloodlines was nearly impossible, especially with higher-tier kekkei genkai. The risk of failure only increased.

The birth of a single Rinnegan could render the strongest shinobi frail and aged.

Bloodline fusion wasn't something just anyone could achieve.

Kakuzu reflected that his own fusion with Hashirama's cells was likely a unique exception.

No other bloodline could retain such vitality even after being diluted over centuries.

That's why Hashirama's cells became the gold standard for the powerful in later years, with even Danzo clamoring for a piece of the action.

As for Kakuzu, adapting to this body would likely take him another decade before he could further strengthen the cells.

He glanced back at Tobirama, who seemed to have already dismissed the research. With a careless gesture, Tobirama tossed the draft onto a nearby pile.

The scroll fluttered in the wind, eventually landing in some obscure corner of the Senju estate.

If it were lost, Tobirama wouldn't bother looking for it. To him, it was just a scrap of paper.

Years later, a young boy named Hiruko would stumble upon it by chance.

What Tobirama saw as trash became a lifeline for Hiruko, stranded in his own personal desert of despair.

Hiruko poured all his potential and obsession into researching the feasibility of Chimera Technique.

When someone has nothing left but determination, they become a terrifying force.

One day, after observing a boy who had taken his friend's eyes, Hiruko had an epiphany.

The complete Chimera Technique was born.

Ah, Tobirama, the unacknowledged copyright ninja.

Then again, Hiruko's version differed from Tobirama's.

To put it simply, Hiruko prioritized speed and brute force, while Tobirama leaned towards gradual fusion.

Kakuzu preferred Tobirama's approach.

Rushing often led to failure, and Hiruko's greed for power turned him reckless.

According to Hiruko's claims, combining five kekkei genkai would grant immortality and spark a Fourth Great Ninja War.

Ridiculous. Immortality from five bloodlines? Did he consult the Sage of Six Paths first?

True immortality in this world belonged to people like Kakuzu and Orochimaru—not some cobbled-together bloodline.

As for starting the Fourth Great Ninja War? During that period, people like Rinnegan Nagato, Kamui Obito, Sage Mode Jiraiya, and the strongest of the Five Kage, Ōnoki, could definitely give him a run for his money. In the end, though, Hiruko's situation was rather pitiful. He shared a similar fate with Kakuzu, after all. Despite being so powerful, he was taken down by some random Wind Release jutsu. It made no sense—absolutely no sense.

Kakuzu's thoughts wandered far as he dwelled on this. Seeing him so lost in thought, Tobirama Senju assumed Kakuzu was deep in contemplation.

"Hey!"

"Hey!"

"Huh? What's up?" Kakuzu snapped back to reality, pulled out of his reverie by Tobirama's voice.

Tobirama handed him another set of notes. "Since you're so interested, consider this spare copy yours. Take your time studying it."

At that, Kakuzu inwardly smirked. He picked up the notes and began flipping through them at a leisurely pace. The contents were extensive, detailing the differences between integrating Kekkei Genkai and fusing ordinary bloodlines. After a quick skim, he reached a conclusion: merging Kekkei Genkai ultimately required time—decades, even centuries.

Waiting that long? Tobirama couldn't do it. Hiruko couldn't do it. But for Kakuzu? Time was hardly a problem.

Satisfied, Kakuzu tucked the notes away. "You're really generous, Tobirama. Giving me something like this? You're not worried I'll turn out like you, locked away by your older brother?"

Tobirama scowled. Hashirama hadn't gone after Kakuzu for a reason, and it wasn't because of some unpaid debts. No, in Hashirama's eyes, Tobirama was the true culprit and deserved to face the consequences alone. That was why he'd confined only Tobirama. But if Kakuzu decided to rebuild his lab, well, the story might change.

Kakuzu could already imagine it: Hashirama would not only use the excuse of unpaid debts but also confiscate his funds to fuel his gambling habits. Not an ideal scenario. Still, there was a solution—taking his research outside of Konohagakure. Problem solved.

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