"Black Zetsu, you certainly made me work to find you."
Kagami slowly walked toward the shadowy figure, a confident smile on his face. Although Black Zetsu's combat power was negligible, its unique composition made it immune to all but Yin-Yang Release attacks. If it was determined to escape, not even Kagami could guarantee he could kill it. However, Kagami knew exactly what Black Zetsu wanted most and was not worried about it slipping through his grasp.
When Black Zetsu saw Kagami appear, his body trembled. A look of pure fear crossed his amorphous face, and he turned to flee. But Kagami's next words stopped him cold.
"Black Zetsu, after scheming for a thousand years, are you really going to give up on rescuing your mother, Kaguya Otsutsuki?"
Hearing this, Black Zetsu froze. He slowly turned back, his eyes wide with disbelief. "You... how do you know my plan? How do you know about my mother? Who are you?"
Black Zetsu knew Kagami was strong, but to have his deepest, most ancient secret laid bare was a shock that rattled him to his core. He had plotted for millennia, and not even Madara or Obito had known the full truth. How could Kagami possibly know?
"Who I am doesn't matter," Kagami said, his hands clasped behind his back, his eyes deep and confident. "What matters is that I can help you rescue your mother." He didn't believe for a second that Black Zetsu would abandon his lifelong mission. As long as that desire remained, he was confident he could turn the creature into his tool.
Sure enough, at the mention of rescuing Kaguya, a flicker of raw excitement appeared in Black Zetsu's eyes. But he quickly suppressed it. "Hah! Why would you help me rescue my mother? Do you think I would believe you?" he retorted, though he didn't move an inch. He stared at Kagami, trying to decipher his true intentions. He had witnessed Kagami's power; he had seen him kill both Madara and the Sage of Six Paths. If Kagami truly put his mind to it, he might actually be able to free his mother.
"I don't care if you believe me or not," Kagami said lightly, his aura flaring to life, enveloping Black Zetsu in an immense, crushing pressure. "Do you have a choice?"
In the face of this overwhelming power, Black Zetsu felt like a tiny boat in a raging storm, about to be annihilated at any moment. He had known Kagami was strong, but to experience it personally was something else entirely. He was completely powerless. A profound sense of futility washed over him.
He fell silent, weighing the truth of Kagami's words against the potential cost. What if Kagami was lying? After a long moment, he finally spoke. "Why would you help me? What is your purpose?" Black Zetsu didn't believe for a second that Kagami would help him out of kindness. There had to be some ulterior motive.
Kagami chuckled. "Purpose? Why would I need any reason that do whatever I want. Maybe it's because I want Kaguya to be my wife."
"What?!" Black Zetsu was stunned, looking at Kagami with utter disbelief. He never imagined that would be the reason. Didn't Kagami know that the ancestor of the Uchiha, Hagoromo Otsutsuki, was Kaguya's son? The audacity of the statement was staggering.
As if reading his thoughts, Kagami snorted. "You don't need to concern yourself with my purpose. You only need to know that I can help you, and the price is your absolute submission and service to me."
Kagami had his own theories. Hagoromo and Hamura were Kaguya's children, but so was Black Zetsu. Who was to say they weren't created in the same way, as living weapons born from Kaguya's genes and the power of the God Tree, much like the White Zetsu? Kaguya knew the strength of the Otsutsuki clan; she wouldn't rely on mere Zetsu to fight them. It was likely that Hagoromo and Hamura were just advanced prototypes. Therefore, their relationship with Kaguya might not be a true mother-son bond. In some accounts, they were simply born from the God Tree, with no blood relation at all.
Submit to Kagami?
Black Zetsu's form trembled, a flicker of struggle in his eyes. He, the mastermind behind a thousand years of history, the child of the Rabbit Goddess Kaguya, was being asked to submit to another? But then, he thought of his mother, still suffering in her endless seal. Compared to her freedom, what did his own dignity matter?
Gritting his teeth, Black Zetsu picked up a branch, snapped it in two, and declared with grim determination, "If that is the case, then there is only one answer! I, Black Zetsu, hereby declare my allegiance to you!"
Seeing him surrender, Kagami nodded with satisfaction. He had no intention of killing Black Zetsu. The creature had lived for over a millennium and surely knew countless lost secrets. A living Black Zetsu was far more valuable. Besides, as Kaguya's child, Black Zetsu would eventually call him 'father' after he and Kaguya were united. There was no need to kill him.
As for betrayal, Kagami wasn't concerned. He was confident he could suppress the creature. If Black Zetsu ever dared to cross him, he would learn the true meaning of despair. And given Black Zetsu's unwavering filial piety, a quality his two "brothers" sorely lacked, he likely wouldn't betray his mother's new husband.
"Since you have submitted, tell me how the Otsutsuki clan absorbs the pupil power of other eyes, and how to practice Yin-Yang Release," Kagami commanded. The Otsutsuki on the moon had a method for absorbing the power of other Byakugan. He was certain Black Zetsu knew this secret. With this knowledge, he might be able to develop a way to absorb and fuse other Sharingan and Byakugan, potentially granting his own eyes the abilities of both the Rinnegan and the Tenseigan.
For a thousand years, Black Zetsu had been unable to practice ninjutsu, but he had observed and recorded the secret techniques of countless clans. Not daring to hide anything, he immediately began to explain everything he knew.