Shikkotsu Forest.
Orochimaru told Tobirama all the information he knew, one piece at a time.
Tobirama mainly focused on Madara's Rinnegan.
It is said that the Rinnegan is the eye of the Sage of Six Paths; its appearance is a matter of destiny, marking the prologue of an upcoming transformation in the Shinobi World.
A thousand years ago, the Sage of Six Paths established Ninshu and saved a broken world.
A thousand years later, conflicts in the Shinobi World persist, and the reincarnation of the Sage of Six Paths will bring unprecedented stability to the chaotic Shinobi World.
And that reincarnation of the Sage of Six Paths, the child of prophecy mentioned by the Great Toad Sage, is very likely Madara.
Because he awakened the Rinnegan.
And the Rinnegan is the eye of the Sage of Six Paths.
If there were a prophecy for this, it would surely say—'When the Rinnegan appears, a Saint emerges.'
The Crimson Moon hangs high, the Six Paths appear, calming the chaos of the four seas and saving the people from their suffering.
"Nonsense!"
Tobirama couldn't help but shout angrily, "How could an evil guy like Madara be the reincarnation of the Sage of Six Paths?"
Orochimaru replied, "But Madara has the Rinnegan! This is proof that the Sage of Six Paths has acknowledged him!"
Tobirama snorted, "If that's true, then the Sage of Six Paths is an evil old devil. If he stands with the Uchiha, then he is an enemy of the Senju and an enemy of Konoha."
What a massive accusation!
Orochimaru looked at Tobirama with some surprise; he didn't expect Tobirama's resentment toward the Uchiha to be so great, like a bitter woman who had been played by an Uchiha.
Tobirama then calmed down and said to Orochimaru, "I have researched Forbidden Jutsus for many years and developed the Edo Tensei, which can summon dead souls from the Pure Land."
"I can tell you clearly that there is no soul reincarnation in this world. Once a person dies, they sleep in the Pure Land."
Orochimaru listened intently; these were extremely valuable points of knowledge that had to be noted down.
The Shinobi World has no concept of soul reincarnation; when a person dies, they fall into a permanent sleep.
In the original work, not a single soul summoned from the Pure Land to the living world could remember what the Pure Land was like!
Even Madara was the same; the moment they woke up, their brains were still stuck in the state they were in before death.
This proves that the Pure Land has no concept of reincarnation.
However, there is a concept of chakra reincarnation, though it's limited to special individuals—the sons of the Sage of Six Paths, Indra and Asura.
There is also the Karma reincarnation of the Ōtsutsuki Clan. Before dying, an Ōtsutsuki compresses their data into a black diamond-shaped mark and implants it into a chosen 'vessel'.
The Karma slowly dissolves within the 'vessel', rewriting their cells and completely turning the 'vessel' into an Ōtsutsuki.
When the Karma is 100% extracted, the 'vessel's' personality and body are completely overwritten, and the Ōtsutsuki is reborn through the 'vessel'.
But this only applies to the Ōtsutsuki; they are not accommodated by the Shinobi World's Pure Land mechanism.
Tobirama didn't know about the concept of chakra reincarnation; he only knew that Madara couldn't be the reincarnation of the Sage of Six Paths.
Orochimaru asked, "Then why does Madara possess the Rinnegan?"
Tobirama asked in return, "You mentioned before that Madara possesses Wood Release, right?"
Orochimaru nodded, "Yes, Madara has displayed the power of the First Hokage's Wood Release."
Tobirama snorted, "That's the key. If you set aside the Rinnegan and compare the current Madara to how he was in the Warring States Period, the only variable added to his body is Wood Release."
"In the battle at the Valley of the End, he wasn't there to attack Konoha, but to steal my elder brother's Wood Release cells."
Tobirama was very clever, possessing a sense of the big picture that far exceeded his peers.
Part of the reason Hashirama could lead the Senju Clan to defeat the Uchiha Clan was his powerful Shinobi stats.
The other part was Tobirama's support.
Tobirama possessed calm, pragmatic, and visionary political and strategic wisdom.
Although he didn't have Hashirama's overwhelming power, he used his intellect to single-handedly build the 'one country, one village' order for Konoha and the entire Shinobi World.
The Anbu, the Shinobi Academy, the Chunin Exams system, the procedures for the Hokage election...
These were the frameworks Tobirama built for Konoha; other hidden villages saw them and adopted them directly without even changing them.
Hashirama established the structure, and Tobirama laid the foundation upon it.
If the Hokage had temple names, then Hashirama would be the Hokage Gaozu, and Tobirama would be the Hokage Taizong.
In history, a Taizu founds the nation and settles the chaos, while a Taizong lays the dynasty's foundation and perfects its systems.
How far a dynasty can develop depends on how much land the Taizu conquered and how the Taizong integrates that land.
Relying on his powerful mind, Tobirama analyzed the key to Madara awakening the Rinnegan just by hearing the information once.
And that was Madara merging with Hashirama's cells.
Orochimaru was a bit surprised and asked, "Second Hokage, did the Rinnegan evolve from Madara's Mangekyo?"
Tobirama shook his head, "I'm not sure about that, but there's a huge possibility that the key to Madara obtaining the Rinnegan is my elder brother's cells."
"I once looked through ancient records; our Senju Clan and the Uchiha Clan share a common ancestor, the Sage of Six Paths."
"I originally thought this was just the family trying to glorify itself, but it seems it's true: both the Senju and Uchiha Clans are descendants of the Sage of Six Paths."
Orochimaru was shocked again: "Second Hokage, you mean—the fusion of Senju and Uchiha will awaken the Rinnegan?"
Orochimaru marveled at Tobirama's intellect once more.
Hashirama had great power but lacked brains.
While Tobirama had a great mind but was always grumpy.
The two complemented each other.
Could Orochimaru have analyzed these things?
Yes! But he would need time to verify his conjectures step by step, such as the inference that the awakening of the Rinnegan was due to Madara integrating Hashirama's cells.
Orochimaru only had conjectures; he wasn't even clear on the concept of the Eternal Mangekyo Sharingan yet.
In the original work, Kabuto's deduction—that the Rinnegan is an evolution of the Mangekyo combined with Hashirama's cells—was based on Orochimaru's years of research.
Orochimaru ran around, robbing graves by day and stealing dogs by night, studying ancient records in his spare time; it was only through such extreme diligence that he deduced the evolutionary relationship between the Sharingan and the Rinnegan.
But this theory isn't entirely accurate.
It was later overturned by the Sage of Six Paths; the Rinnegan and Sharingan are independent of each other, otherwise they couldn't be listed together as the Three Great Dōjutsu.
Orochimaru needed time, but with the recent drastic changes in the Shinobi World, the Third Hokage hadn't had a good night's sleep in days.
Orochimaru hadn't rested well either, worrying all day about how to deal with Madara or how to escape Madara's Infinite Tsukuyomi.
If he had the time, Orochimaru would certainly want to verify—what special properties arise after the fusion of Senju and Uchiha bloodlines?
Looking at the fanatical Orochimaru entering his research state, Tobirama suddenly felt a bit nostalgic; seeing Orochimaru was like seeing his former self.
However, there was still a difference between him and Orochimaru: Tobirama researched Forbidden Jutsus to better protect Konoha.
Whereas Orochimaru did it for himself.
Tobirama shook his head, negating his own words: "This is just an inference. Whether this theory is correct needs proof, and to complete this theory—"
Tobirama didn't continue, but Orochimaru knew—it was Human Experimentation.