For now, the most orthodox and practical method of evolving the Rinnegan was undoubtedly to have an Uchiha with an Eternal Mangekyō Sharingan transplant First Hokage Cells.
Simply put, it was the fusion of ultimate Yang Release chakra and ultimate Yin Release chakra.
Whether one was the reincarnation of Indra Ōtsutsuki was not actually a decisive factor. In the original timeline, Madara Uchiha and Sasuke, the two Uchiha who awakened the Rinnegan one after the other, happened to be reincarnations of Indra Ōtsutsuki. It was merely because Indra Ōtsutsuki's reincarnations possessed stronger Yin Release chakra than other Uchiha.
In other words, when it came to evolving the Rinnegan, reincarnations of Indra Ōtsutsuki, whose chakra quality was inherently far superior to others, had a higher chance of succeeding.
This theory was not Hyuga Kagami's random speculation.
The most direct proof was the Ōtsutsuki who possessed the Rinnegan. They did not carry the chakra of either Indra Ōtsutsuki or Ōtsutsuki Asura.
The pair of Rinnegan in Karin's eye sockets was also the product of Danzo's "Bloodline Fusion Ritual."
Therefore, as long as the Power of All Creation formed by the union of Yin and Yang was strong enough, even the Mangekyō Sharingan and Eternal Mangekyō Sharingan were not necessary conditions for evolving the Rinnegan. The Sage of Six Paths, for example, had evolved directly from the Three Tomoe Sharingan to the Rinnegan.
But in practical terms, fusing the chakra of Indra Ōtsutsuki and Ōtsutsuki Asura to evolve the Rinnegan was the most effective method in the Ninja World.
Because when it came solely to the strength of Yin Release chakra and Yang Release chakra, there were very few in the Ninja World who could surpass those two brothers.
Moreover, aside from enhancing bloodline power, there were currently few effective means in the