Cautious Eyes: An Uchiha's Path

The difficulty of transplanting Mangekyō Sharingan eyes was beyond anything Uchiha Kai or Hyūga Aya had anticipated.

It had taken them over four hours—four long, meticulous hours—to complete the delicate operation. Both of them were exhausted by the end. Kai had only played a supporting role, assisting and stabilizing the patient's chakra and blood flow, occasionally helping Aya when her focus wavered.

But Aya bore the brunt of it. She had maintained her Byakugan the entire time, monitoring chakra networks and optic nerves with microscopic precision. Sweat drenched her back, but she didn't complain. She gritted her teeth and pushed through, unwavering until the last suture was done.

Despite the bloody mess left on the operating table, the transplant was a relative success. According to Aya's Byakugan, the optic nerve integration was clean—no signs of rejection or leakage. The transplanted Mangekyō Sharingan were intact, though currently dormant.

"It's looking good so far," Aya finally murmured as she leaned back. "No signs of rejection... Maybe because the donor and recipient are brothers. Same blood, same chakra type."

Kai nodded, though he remained uneasy. "So it's true... bloodline compatibility plays a role, just like with Sasuke and Itachi."

That thought lingered in Kai's mind as he stood up and formed a hand seal, activating a subtle chakra pulse. Instantly, Uchiha Osamu's newly implanted eyes snapped open.

But something went wrong.

Blood immediately began to trickle from the corners of Osamu's eyes. His sclera turned red with bloodshot veins, and the overlapping patterns of two different Mangekyō designs began to shimmer oddly. Kai could feel it—an unstable, chaotic resonance brewing behind those eyes.

The sight was unnatural. Even Kai, who was no stranger to the grotesque, instinctively winced. Aya, wide-eyed, was frozen.

"What the hell are you doing?" she barked. "We just finished the transplant! Are you trying to ruin everything we've done?!"

Kai blinked. "I was just checking if they'd stabilized—"

Aya interrupted coldly. "If you want to waste four hours of our lives and destroy two Mangekyō Sharingan, go ahead. Otherwise, stop that."

He quickly cancelled the chakra pulse, and Osamu's eyes closed again.

"Don't just stand there. Stop the bleeding!" Kai barked.

Aya huffed and stepped in. Her hand glowed with emerald green chakra as she focused medical ninjutsu on the eyes. Slowly, the bleeding halted. She wrapped Osamu's eyes in bandages to block external light, allowing them time to stabilize.

"No more stunts," she said flatly. "Those eyes need time. If you crack the fusion prematurely, they might both degrade. You understand that, don't you?"

Kai sighed, raising his hands. "I get it. But did you feel that power surge? Something's happening. A resonance... but no sign of fusion yet."

Aya nodded warily. "Yes. Maybe it's the beginning of an Eternal Mangekyō reaction. But I can't say for sure—it's never been recorded clearly, not even in the clan archives."

And that was the problem.

The manga never described how long it took for Sasuke's Eternal Mangekyō to fully stabilize, only that he awakened it off-screen after receiving Itachi's eyes. Kai had nothing concrete to work with—only fragments of Madara's history and scattered Uchiha records. So he had developed two theories.

The first was direct transplantation—implanting a sibling's Mangekyō Sharingan and allowing natural resonance to awaken the Eternal version. This method had precedent with Sasuke and was generally safer, provided both donor and recipient were close relatives.

The second theory was more extreme: extracting the chakra essence or ocular power from one Mangekyō and fusing it into another using a sealing technique. He'd considered modifying the "Transcription Seal" (Izanagi's base mechanic) to forcibly transfer ocular energy. But this method came with a fatal flaw: there was a limit to how much "pupil power" the seal could store.

The technique couldn't preserve the totality of Mangekyō power, let alone merge it.

In the end, Kai had settled on the direct method, despite its instability.

"There may be better ways," he admitted, "but this is all I can manage for now."

"How long until they fully fuse?" he asked.

Aya shrugged. "I have no way to know. And honestly, I doubt you do either."

"Fair enough," Kai muttered. "Oh, and keep an eye on Hyūga Hiashi. He's likely to make a move in October. Let him cool off for now."

Aya raised an eyebrow but didn't argue. She knew Kai had his own web of politics to navigate, and as long as he upheld his side of their agreements, she wouldn't interfere.

Her thoughts drifted toward her own medical training. Genetic medicine, cellular regeneration—painful, deadly arts she'd begun to study more out of necessity than passion. The experiments she'd read about in old Root scrolls still haunted her.

And now...

"How far along are you with your cellular medical ninjutsu?" Kai suddenly asked.

Aya blinked. "Basic lab tests? I can do those. Why?"

"I need you to run a fusion test," Kai said. "See how compatible my cells are with another donor. Track mutation rates, rejection probability... and fatality rate."

His tone was calm, but his heart beat faster. He'd obtained samples of White Zetsu—modified cells infused with Hashirama Senju's DNA. Obito had survived using them, even enhanced his Mangekyō's stamina, but...

Kai didn't want to become a half-white monstrosity. He wasn't Madara. He didn't want anyone's face growing out of his chest.

"It's just a research test," he clarified.

Aya narrowed her eyes. "Whose cells are you fusing with?"

"You'll see. Just check viability."

Aya considered this. "Fine. But I'm not promising results, especially not if you want to do bulk tests. Do you even have enough cell samples for that?"

Kai chuckled. "I'm more prepared than you think."

He had two full White Zetsu samples stored—enough for extensive research.

He had originally planned to follow Madara's path precisely, awakening his Eternal Mangekyō first and later integrating Senju cells. But what he had just seen—the raw, chaotic power bubbling from Osamu's transplanted eyes—made him reconsider.

Obito never achieved an Eternal Mangekyō, despite having Senju cells. Perhaps the key wasn't just the cells, but precise genetic repair—something only Madara mastered. Without activating the relevant genes, maybe Obito had only gained the stamina, not the full ocular evolution.

"You can try," Aya said at last. "But if this experiment backfires, don't say I didn't warn you."

Kai gave a tight smile. "Understood. But if this works, it might just be the missing piece."