Uchiha Kai's Mangekyō Sharingan had finally manifested the second stage of Susanoo—a skeletal upper body with arms and a weapon, a significant step forward from the initial ribcage form.
But until now, he had never used it in actual combat.
Lack of a worthy opponent had prevented him from truly testing its might. Pure destructive power wasn't enough—what Kai needed was a real gauge of combat utility.
Now, with an ANBU operative from the Yoru clan standing before him, Kai had finally been given that opportunity.
He knew this ANBU wasn't sent by Minato Namikaze. That only left one conclusion: the Third Hokage—or someone in the Council—had deployed him. In their eyes, Kai might be working as a political agent for Uchiha Fugaku. And naturally, it's easier to track an agent than their leader.
Normally, such logic would hold. But they had miscalculated. They were facing Uchiha Kai, a shinobi who had awakened the Mangekyō Sharingan.
Yoru Ryoma silently stared at the towering figure before him—a black Susanoo, its features chillingly sharp, resembling a wrathful Rakshasa from myth. Its chakra was oppressive—dense, malevolent, and hopelessly cold.
Ryoma realized that he might not survive this encounter. But still, he had a job to finish. Before doing anything, he had to pass on the information he had gathered.
Just as his chakra subtly pulsed to summon the insects hidden in his body, a voice cut in coldly:
"Trying to transmit intel?" Uchiha Kai's voice echoed, laced with disdain. "You think I can't see that?"
Ryoma narrowed his eyes. "You're... powerful. Frightening."
Bending into a fighting stance, Ryoma gave no reply other than a faint exhale. He was ready.
Kai tilted his head. "Is that your last line, then? I've got twenty minutes before I reach my limit... but I don't think you'll last that long."
As soon as he spoke, the eyes of the dark Susanoo flared scarlet.
In an instant, it drew the massive sword on its back. With a roar of wind and blinding chakra, the blade came down.
The shockwave alone flattened the trees in its path, cleaving a deep scar into the ground.
Ryoma tried to leap away—but his instincts screamed too late. The sword's speed suddenly doubled, then tripled, becoming a blur beyond visual perception.
It wasn't just power. Time itself had distorted.
By the time Ryoma understood what had happened, he was already airborne—his body sliced open, his awareness slipping away.
This... is death, he realized.
On the forest floor, Uchiha Kai stood still. The Susanoo dissipated slowly, vanishing like smoke under moonlight. A crimson trail of tears ran down from his left eye.
He exhaled.
"Still pushing it...," he muttered, wiping the blood. "Using 'Retrospect of Time' on Susanoo's full swing—it's way too much chakra."
That's right—Kai had activated his left-eye Mangekyō technique right after the sword strike. Even though Susanoo's slash was already deadly, he didn't want to risk even a one-in-ten-thousand chance of failure.
He had studied how Itachi, Sasuke, and Madara had integrated eye techniques into Susanoo's movements, and he wanted to try it for himself.
But the cost… was immense.
His Susanoo had reached the second stage—a half-body form, with torso, arms, and weapon, though still lacking full armor and legs. It was far beyond the ribcage stage, and every jump in stage exponentially increased strain on his chakra and eyes.
"Madara could form Susanoo without even activating his eyes..." Kai thought grimly. "I'm nowhere near that level."
He could barely sustain the full first stage without bleeding from the eyes. The second stage pushed his limits even further.
Still... the experiment worked.
Walking over to Ryoma's mangled body, Kai scowled. The man had been torn apart, and his insect-based ninjutsu had mixed grotesquely with viscera.
"These bugs really don't die easy," Kai muttered, igniting his chakra. Using Fire Release, he scorched the remains and began picking through the debris.
He had seen Ryoma write something before he died. That might have been a report.
Eventually, Kai found what was left of a note, but it was already reduced to shredded, unrecognizable scraps.
"...Useless," Kai sighed. With a small torch of chakra flame, he incinerated the last of the corpse and bugs. "No point leaving a trace."
Not even Tsunade would've been able to identify the body at this point—and if this had been Hashirama? Kai would've been the one in pieces.
As he returned to the underground lab, several Uchiha security members arrived—sent by Fugaku to secure the perimeter. From the look on their faces, they'd felt the battle's chakra surge.
Kai explained what had happened with minimal detail and left the rest to them. They were competent enough to manage damage control.
Back inside, Hyūga Aya had already sterilized the experimental blood samples and prepared for the next phase.
"Let's check the results," Kai said curtly, rolling up his sleeve for the skin graft test.
Aya examined the area carefully. "No immediate reaction. That's promising."
She hesitated for a moment, then asked, "That jutsu from earlier... was that a Mangekyō technique?"
Kai nodded. "It's a forbidden ocular jutsu. Extremely draining."
He knew she had watched from the lab's chakra window. With her Byakugan, she'd seen the entire battle.
Aya didn't press further, but the sheer scale of that chakra had shaken her. That Susanoo... If used on almost any shinobi, it would mean certain death. Unless they could escape in the first second—or were Hashirama.
She took a breath, shifting the topic. "That ANBU was strange. His tongue had a curse mark."
Kai's eyes narrowed. "A Cursed Tongue Eradication Seal?"
Only one organization used that—Root.
That meant this operative wasn't just from the Yoru clan… he was one of Danzo Shimura's men.
"Danzo... you're watching me?"
Kai recalled vague records—when Danzo summoned Nonō Yakushi, two Root guards had accompanied him. One of them might've been from the Yoru clan. He couldn't remember the name.
But this all pointed to one thing: Danzo was monitoring him—and possibly preparing to act.
"Interesting. So you don't want Fugaku watching me, but you want me watched yourself? That's fine. I'll deal with you eventually."
For now, Kai let the thought go. Vengeance could wait.
He turned to Aya. "Start the trial. Let's see if we've made any progress."
Aya nodded. Their work had only just begun.