Obito knew his limits better than anyone else.
As he only had enough chakra to sustain one of Madara's Rinnegan, he had already lost a few tugs-of-war with Sakura over the Demonic Statue of the Outer Path. The Sixth Hokage would periodically summon it to check whether he'd fed it the One-Tail, but no matter what either of them did to try to seal it away on their side, the other would always summon it back.
The Rinnegan, it seemed, trumped all fūinjutsu when it came to summoning the Gedō Mazō. Only when one of them became the Ten-Tails' jinchūriki would this finally stop, but since Obito's heart had been marked with a cursed seal stopping him from doing just that, he dared not place the final piece of the puzzle until he rid himself of Madara's heart demon.
Thanks to the Yin-Yang Style ninjutsu he had learned from Madara, Obito had managed to stabilise his White Zetsu half a long time ago, but deep down, he knew he was still too weak to control a creature as powerful as the Ten-Tails for long… and so, until he found a way to free himself, he had elected to hide in the far west, in the Land of Swamps…
…and there he had remained for two years, confident that Madara and Black Zetsu would never find him this far from the shinobi world. But unfortunately for him, his hiding spot held no secrets before the senses of a man whose inner sight enveloped the entire planet.
One fine day, as the world teetered on the edge of total annihilation, trouble came looking for him. However, it was not those he had expected who sought him out in his frozen-over swamp; it was neither the Hidden Leaf nor the Akatsuki, but rather a bunch of autonomous flying puppets.
…
For as long as humans could remember, the Divine Tree had always been there. The weak and feeble inhabitants of this star had always possessed a healthy amount of respect for this tree, which would flower every thousand years and bear a forbidden fruit. Despite their innate foolishness, humans still had enough wisdom to avoid touching what must never be touched.
Alas, this state of affairs could not last…
"Who are you?" Obito said coldly, standing his ground even as puppets fell from the sky and encircled him, each projecting a man's soft voice. "What do you want from me?"
One day, a visitor from beyond the stars descended upon this world and consumed the Divine Tree's fruit. Princess Ōtsutsuki Kaguya gained vast stores of chakra, and using this power, she brought peace to this war-torn star, becoming the origin of chakra. Ōtsutsuki Kaguya would go on to give birth to two sons, the first humans to inherit the gift of chakra: Ōtsutsuki Hagoromo and his brother Hamura.
For a time, the peace continued, but alas, this state of affairs could never last.
"Where are you?" Obito asked, peering at his surroundings with his scarlet eye. "Show yourself if you aren't a coward, puppet master, or don't blame for what happens next…"
Obito was rapidly losing his patience, but he did not attack just yet. The puppets were animated with chakra of the same rank as his Rinnegan, and he did not know enough about his opponent, who had visibly got the drop on him.
Princess Kaguya's taboo act could not go unpunished.
Enraged by the theft of its fruit, the Divine Tree uprooted itself and took the form of a terrible beast, the Ten-Tails, before swallowing her whole. Yet the death of the fruit's thief proved insufficient to quell the Ten-Tails' rage. It continued to rampage out of control until Hagoromo and Hamura battled it for seven days and seven nights, finally separating its body from its chakra.
"How did you come to learn these secrets?" Obito asked, finally unnerved. "Only by reading the Uchiha Stone Tablet with the Rinnegan could you possibly have uncovered the world's true history, so who are you, really?"
Hagoromo would go on to become the Sage of Six Paths by absorbing the Ten-Tails' chakra, sealing away the monster's discarded corpse on the moon. As for younger brother Hamura, he would go on to emigrate to the moon with his clan, to keep an eye on the Ten-Tails' husk… what you would call the Gedō Mazō, the Demonic Statue of the Outer Path.
"In the end, you're after the Gedō Mazō as well," Obito said darkly. "Farewell."
Obito vanished from the spot in a spiral of twisting space-time, only to reappear a few kilometres away. He had no interest in having a big battle with his mysterious opponent, as using his chakra would greatly increase his chances of being detected by a passing Zetsu.
The Sage believed in the hearts of humans, and seeking to unite humanity, he founded the Sect of Ninshū, believing that granting chakra to humanity would lead to common understanding and eternal peace… but humanity squandered his gift, using it instead to fight and kill each other!
Obito's eye twitched. Flying puppets swarmed over the horizon, making a beeline towards him while loudly broadcasting their master's message… the enemy could still track him after he'd teleported across half the country!?
Hagoromo's very own descendants would continue squabbling amongst themselves for the next thousand years, claiming an uncountable number of lives! And you, Uchiha Obito, along with Uchiha Madara and Uchiha Sakura, are the worst criminals of them all! You even stole the Gedō Mazō from under my care and attempted to revive the Ten-Tails for your own selfish purposes!
"You would blame me for your own failure to properly guard the Gedō Mazō?" Obito said coolly.
The Sage of Six Paths' vision failed. His world is a failure, so now the time has come for the descendants of Hamura to fix the elder brother's mistake. I will crash the moon into this star, wipe the slate clean, and start anew!
Obito's eyes narrowed. Without the moon, there was no Eye of the Moon Plan, no Infinite Tsukuyomi! With no planet beneath their feet, there would be nowhere for Rin to stand on! The destruction of either of the two was unacceptable!
None of you Uchiha devils who would abuse the power of the Sage of Six Paths' eyes can escape my senses. You will relinquish the Ten-Tails' husk to me; if you refuse, then I will simply move on to the next, until all three of you have been sentenced to death.
"I see, so that's how it is…" Obito laughed darkly as he connected the dots. "This power of Nagato's and I, Chibaku Tensei, or Catastrophic Planetary Construction… it all makes sense. The Sage of Six Paths didn't just seal the Gedō Mazō on the moon; he created the moon to seal the Gedō Mazō, keeping it out of humanity's reach in outer space!"
The mysterious voice remained silent.
In the process of aura farming, Toneri had accidentally let spill the moon's greatest secret and the reason for his descent. It wasn't just to punish the three interlopers, it was to prevent them from banding together and using the Ten-Tails' power to save this corrupt world!
"You're afraid that if one of us obtains the Sage of Six Paths' full power, we'll become powerful enough to create another moon; more than powerful enough to keep the existing moon from falling," Obito went on. "As long as one of us is holding on to the Gedō Mazō, your plan has a chance of failing… that's why you bothered to come down from the moon when you could have just as easily waited out the apocalypse in orbit!"
Without warning, the puppets attacked.