The clock turns back to the beginning of the siege of Konohagakure.
Uchiha Obito and Uchiha Madara had just retrieved Shimura Danzō, the One-Tail Jinchūriki, and beaten up the Third Hokage, who had come to his rescue. Using Kamui, their party had then warped to the nearest Akatsuki safehouse, a large, gloomy cave whose every entrance was sealed tightly shut.
Obito watched with a bitter expression as Madara began performing the summoning jutsu to call forth the Ten-Tails' husk from their secret hideout in the Mountain Graveyard, a few kilometres north of the Land of Iron.
It should have been me, not him! It's not fair!
He had always found the circumstances of his own darkening to be quite suspicious, so he had never truly trusted Madara. Even though he agreed with his ideas and plans, he had never intended to revive his master… but since Madara had found a way to restore himself to life without him, he could only stand back and accept a supporting role.
"Summoning: Gedō Mazō!"
Madara gritted his teeth, his knees buckling as the summoning instantly consumed all the chakra he had moulded in preparation for the jutsu, then began forcibly drawing out vast amounts of his physical energy to compensate for the deficit, far more than he could ever have expected. For an intra-dimensional summoning, that was an enormous amount of chakra, enough to kill the average Gokage ten times over… this wasn't normal.
BOOM!
A column of white smoke erupted in the middle of the cavern as the summoning finally and thankfully succeeded. It had taken longer than expected and cost him quite a lot of chakra, but the Demonic Statue of the Outer Path was finally here…
"At last, the Eye of the Moon Plan is within reach!" Madara said, his eyes gleaming with excitement as he waited for the summoning smoke to clear. "Soon, the world will know true peace… what the...?"
An enormous tentacle suddenly emerged from the roiling cloud of summoning smoke, smashing down upon the spot where Madara had stood mere moments before. The white smoke rapidly dispersed as the gigantic, shadowy figure concealed within unleashed an earth-shaking roar, revealing its monstrous appearance… that of a very angry, one-eyed monster.
It had a mouth filled with rows of sharp teeth and ten swaying tails; it was a Ten-Tails, without a shadow of a doubt, and it was gigantic, all right… but it was rather on the smaller side, wasn't it? Even the shrunken, deflated and dehydrated Demonic Statue of the Outer Path was slightly bigger than this monstrosity…
"What's happening, Madara?" Obito asked coldly as he phased through a swipe from one of the creature's massive tails. "Why has the Ten-Tails revived already? We haven't even fed the statue the One-Tail yet, and it's still missing quite a lot of chakra from the Three-Tails, Eight-Tails and Nine-Tails, too."
Madara's mind raced. Was this what happened when the correct order was not followed?
The Fourth Hokage remained a flight risk even from Obito's Kamui dimension, so they'd had no choice but to strip the Yin half of the Nine-Tails from him and seal it inside the Demonic Statue of the Outer Path, even though it was meant to be absorbed last due to the sheer volume of chakra it contained…
"Cease your rampage at once!" Madara shouted, clapping his hands. "Restrain yourself, Ten-Tails!"
The Ten-Tails (?) gradually quieted down before it could bring the cave's ceiling crashing down on top of their heads. Thankfully, it still obeyed him… or rather, his Rinnegan.
The beast peered down at Madara, its enormous eye fixed upon a certain part of his body, where he had hidden a certain item… Could it be interested in the shiny pebble that had fallen from Danzō's clothing during their battle, which he had picked up and pocketed on a whim?
'Ten-Tails? Without the Great Me? No, that's impossible! But… could it be…?'
A small, annoying voice echoed in Obito's mind. Danzō was still stored safely within the Kamui dimension, paralysed by his Mangekyō Sharingan genjutsu… and through this connection, Obito could just barely hear the One-Tail's voice.
'No, that's not our Ten-Tails! It's another one!'
Obito's eyes widened in shock.
Could it be that Madara's Rinnegan were linked to a different Ten-Tails? If there were more than one, then perhaps each Ten-Tails was imprinted upon a specific pair of eyes, and wherever Madara had obtained these Rinnegan, they were not associated with the one they had been restoring all this time? If so, then as the holder of Madara's original pair of Rinnegan, this meant that Obito now possessed exclusive rights to the correct Ten-Tails!
Obito began slowly backing away from Madara.
…
Indeed, the Ten-Tails Madara had just summoned was actually Ōtsutsuki Urashiki's pet.
The reason it was so diminutive was that it was merely a seedling, the first stage preceding its transformation into a God Tree via the sacrifice of an Ōtsutsuki clan member, which would then be followed by the flowering and fruiting stages. It had not yet laid its roots and burrowed into a planet rich in nature energy, so its chakra remained lacking, even though it could just barely sustain itself by absorbing ambient energy from the atmosphere.
As such, this blank slate of a Ten-Tails was far inferior to the Ten-Tails the Akatsuki had been striving to restore, which had been feasting upon the shinobi planet's dragon veins for a millennium. It also lacked the genetic material of the high-ranked Ōtsutsuki clan member Isshiki, whom Kaguya had sacrificed to her God Tree, as well as her own low-ranked DNA, which she had contributed when she fused with the tree, after going mad due to the corruption of the Chakra Fruit's will.
…
"Something's wrong," Madara muttered to himself. "The Ten-Tails is far too weak… its chakra is no greater than that of the Two-Tails, or that of the Three-Tails at most…"
Such a Ten-Tails was nowhere near powerful enough to power the Eye of the Moon Project, let alone empower its Jinchūriki to true Six Paths level; the genetic template to achieve that transformation was missing. At best, it was just another chakra battery like any of the ordinary Tailed Beasts.
"Madara," Obito said suddenly. "If you wish to obtain the true Ten-Tails, remove the cursed seal you placed on my heart."
Madara slowly turned to face him.
Unbeknownst to him, Obito had been slowly inching away from him over the past few seconds, taking advantage of his elation and confusion while he inspected his prize. Obito now stood far enough away that Madara could not flicker across the gap and kill him before he could activate his teleportation jutsu, which took a heartbeat longer to initiate than his phasing.
Madara's face darkened. His disciple knew his limits well. "When did you realise…"
"Not one more step… I won't ask again," Obito said coldly. "In case you've forgotten, I've still got both your original eyes and the One-Tail Jinchūriki…"
In other words, Obito now had everything he needed to revive the original Ten-Tails, except permission: the cursed tag Madara had placed on his heart the day he saved his life, which prevented him from becoming the Ten-Tails' Jinchūriki. He could not remove it himself, nor would death free him from its hold… not that it would allow him to commit suicide, either.
Killing intent radiated from Madara as he took a step towards Obito, raising one arm to pluck out his eye…
"So be it," Obito said coldly as his silhouette spiralled away into nothingness.
Madara instantly reappeared before Obito, but his fingers closed around nothing but air.
…
"That was too close!" Obito gasped as he collapsed onto the ground a hundred kilometres away, one hand flying to his right eye. Three deep grooves had been scratched under his eye, in his artificial Zetsu flesh. "Cunning bastard tried to go for my Sharingan, too!"
Badump.
Obito clutched his heart as a tremor shook his body. He began writhing in agony on the ground as his Zetsu half went haywire, losing its human shape and growing tentacles…
"Bastard!" Obito gasped in pain. "What… have you… done to me!?"