In the shadow of a collapsed cliff, a soft-bodied creature emerged silently, like a puddle of black mud, its shape shifting at will.
At what might have been the top or tail of its body, a pair of round, golden-yellow eyes opened. Beneath them, its mouth was lined with neat rows of triangular, bone-white teeth.
(The Rinnegan's birth couldn't be achieved through research alone.)
Black Zetsu was a child created through willpower and Yin-Yang Release by the Rabbit Goddess, Kaguya Otsutsuki, the progenitor of Chakra in the Ninja World.
After listening in the shadows to Uchiha Takeyasu and Madara Uchiha's conversation, he scoffed at Takeyasu's words.
He knew exactly what it took for the Rinnegan to be born—
The Rinnegan originated with his mother and had been passed down through the Ninja World by the Sage of Six Paths, Hagoromo Otsutsuki—his elder brother.
It wasn't enough to simply combine the powers of the Uchiha and the Senju of the Forest. The conditions were far more stringent.
Otherwise, for the sake of freeing his mother, Kaguya Otsutsuki, who had been sealed on the moon by those two beastly elder brothers, Hagoromo Otsutsuki and Hamura Otsutsuki, Black Zetsu would have acted long ago. He wouldn't have waited a thousand years for Madara Uchiha, the person most likely to awaken the Rinnegan.
The Uchiha bloodline traced back to Indra, the eldest son of Hagoromo Otsutsuki, who inherited the Sage's eyes—that is, the Sharingan.
The Senju of the Forest bloodline traced back to Asura, Hagoromo Otsutsuki's second son, who inherited the Sage's body.
Only by combining the powers of those two and reaching a state beyond life and death, of supreme enlightenment, could one draw out the remnants of the Sage of Six Paths' Chakra left in the world and awaken the true Rinnegan.