Chapter 29: Tobirama Senju's Darkest Secret
A hundred meters beneath the Hokage Rock.
Hiruzen used Earth Style to phase through the solid bedrock, bypassing layer after layer of complex security seals until he reached a narrow, elongated underground vault.
The air was stagnant, carrying a faint, metallic scent of decay. The walls and ceiling were made of ink-colored stone, hardened by jutsu to withstand even a direct blast. Rusty lab tables were lined with preservation jars, each containing various "specimens."
At the far end of the room stood a cultivation tank that immediately drew the eye. It was covered in a dense lattice of sealing formulas, with a thick, translucent hose running from its base into a secondary life-support chamber.
This was Tobirama Senju's Sanctum.
The original Hiruzen had been here once, decades ago. But at the time, his teacher's research was so disturbing and advanced that he had deemed it uncontrollable. Lacking the confidence to continue the work, he had simply reinforced the seals, taken the research notes, and buried the entire lab under the village.
But Arata was different. He had been waiting for the right moment to crack this vault open.
He remembered his first visit twenty years ago—he had nearly been vaporized by one of Tobirama's defensive arrays. He allowed himself a small, wry smile at the memory.
Holding the Second Hokage's research notes, Hiruzen began cross-referencing the "half-finished masterpieces" in the room.
"This black mass... this must be the prototype for the monster that absorbs negative human emotions," Hiruzen mused. "The Zero-Tails. Pity, it seems to have lost its vitality."
Hiruzen marveled at Tobirama's imagination. The Second Hokage had proposed a theory that human malice was an untapped energy source. He had intended to create a man-made Tailed Beast that fed on war and hatred to act as Konoha's ultimate deterrent.
He moved to the next shelf. "And the Sharingan collection. A few single-tomoe... one pair of three-tomoe..."
These were likely "battlefield acquisitions" from the Warring States period. It was a smaller collection than Hiruzen expected, but Tobirama's notes on the eyes were terrifyingly clinical. He had mapped out the ocular structure, the chemical triggers in the brain that awakened them, and the neurological cost of their use.
If an Uchiha elder came down here to debate the Sharingan with Master Tobirama, the Master would leave them speechless, Hiruzen thought. The man understood the Uchiha better than they understood themselves. It's almost obsessive. If he hadn't been so busy killing them, people might have mistaken his interest for love.
Finally, he walked up to the large tank—the "Black Coffin."
Based on the notes and his own deductions, Hiruzen knew what he was looking at. Before the First Great Ninja War, Tobirama had been obsessed with using "Bio-Engineering" to solve the village's manpower issues.
His first goal had been noble: limb regeneration for wounded veterans. It was a project that fit the Will of Fire perfectly. But as his research progressed, he had moved toward 'Shinobi Cloning'.
The results were mixed. Tobirama had discovered that cloning a human with a high chakra capacity was a nightmare. The more talented the donor, the more likely the cells were to collapse into a chaotic, cancerous mess of tissue during gestation.
Tobirama hypothesized that a clone required "Spiritual Nourishment" and a specific "Gestational Environment" that he couldn't replicate in a tank. He believed that without a soul to anchor the body, the cells simply didn't know how to organize themselves.
Unless... the donor had a very specific genetic trait.
Hiruzen unlatched the exterior seals of the tank. As the mist cleared, a body was revealed.
It was a boy, roughly fourteen or fifteen years old. He had a shock of messy black hair and handsome, aristocratic features. He looked vaguely like Tobirama around the eyes, but the rest of him was unmistakably Uchiha.
"Heh... Master," Hiruzen whispered, his lip twitching in amusement. "I've never seen him in person, but based on your notes, I can guess who the donor was."
The boy in the tank was a clone of Izuna Uchiha.
Hiruzen looked at the secondary tank connected by the hose. It was filled with wood-like cellular structures that hummed with a vibrant, familiar energy. Hashirama Cells.
"So you used the First Hokage's cells as a 'stabilizer' to keep the Uchiha tissue from collapsing? And then added your own DNA as a 'bridge'?"
Tobirama might not have kept a diary, but every scientist keeps a log. After he died as a decoy, he hadn't had time to "clear his browser history." Hiruzen, as his successor, had inherited it all.
He could see the story now. On the day Tobirama struck Izuna down with the Flying Raijin Slice, he had taken a "sample" of the dying man's flesh.
Years later, as Hokage, he had tried to grow that sample. He discovered that by adding his own cells and his brother's, the failure rate dropped. After thousands of attempts, he had actually succeeded in growing a complete, functioning human body.
He had locked it away in this tank, intending to finish the research after the war. But he never came back.
Hiruzen leaned in, examining the boy's joints. He noticed several "nodules" along the chakra pathway system—blockages that looked like severed chakra nodes..
He flipped through the notes until he found a scrawled, barely legible entry:
'How to ensure smooth Tenketsu conduction?'
'Sister-in-law Mito might know... but if I ask her, she'll tell Big Brother.'
'Too risky. He told her to keep an eye on my "unethical" projects.'
Hiruzen gave a sharp, bark-like laugh.
Tobirama had been too afraid of his older brother's lecture to finish his masterpiece. The "God of Research" was a coward when it came to family drama.
"The things you did to Izuna Uchiha... 'Hate' doesn't even cover it, Master," Hiruzen sighed. "If I didn't know better, I'd say this was the plot of a very tragic, very twisted romance novel."
"You were afraid to ask Lady Mito for help. But I'm not."
Hiruzen slowly resealed the tank.
"I was looking for a vessel for you, Master. Now, I have one that's perfect. An Uchiha body stabilized by Senju cells, with the face of your greatest rival."
"Master... it's time for the Hidden Leaf to accelerate. And I'm going to need your brain to do it."