Isshiki, feeling apprehensive about the change in Kaguya, realized he will have to up his game to stand a chance. He didn't want to do that but he had no choice. Not when facing... that.
Kaguya's eyebrow rose when she noticed an abrupt change in Isshiki. It was outwardly indiscernible but there was not much that could fool her Rinne-Sharingan. She could clearly see Isshiki's life force condense, granting his body a crazy power boost... but the lifespan of his vessel was instantly reduced to two years tops.
Naturally, if Isshiki had an appropriate vessel, like his own body, this technique could probably be sustained indefinitely. It was what made the Otsutsuki so dangerous, after all. They had bodies and chakra reserves strong enough that they could spam moves that would kill even S-rank humans after a single usage.
In fact, what Isshiki was doing was not all that different from Might Guy's fully opened Eight Inner Gates. It was simply a different, superior approach to a similar concept.
There was no new round of taunting after power-ups. Both combatants understood that taunting only worked when one had the upper hand and right now, neither knew the prowess of the other's boosted form so instead of spending brain cells on boasting, both Isshiki, and Kaguya carefully and patiently observed the other. Whoever attacked first would be at a small disadvantage.
Isshiki, however, quickly realized he was at a disadvantage anyway. The longer he gave Kaguya to analyze him, the more information about him would her Rinne-Sharingan gather and that wasn't a favorable situation for him.
There was only one way to nullify this advantage of hers. Attack.
Kaguya barely managed to shift one of her Truth-Seeking Balls into a shield to block the shockwave full of pressurized air thrown in her direction by Isshiki's punch. In retaliation, she shaped the other Truth-Seeking Ball into a spear and shot it at Isshiki...
The winds furiously picked up, the air pushed away and uncontrollably mixed by the shockwave rushing through it. The ground was not spared the catastrophe either as it heavily shook when Kaguya's spear hit where Isshiki stood just a second ago. Trees in the distance started falling down, uprooted by the shockwave and the massive cracks in the underground created by the impact of Kaguya's Truth-Seeking Spear.
The whole dimension was quivering under the massive pressure of Kaguya's and Isshiki's chakras and the fight between them was yet to start in earnest.
Kaguya spread her arms in front of her, her palms facing forward and balls of black and red chakra started to gather in front of them...
In a matter of milliseconds, a house-sized bijuudama was formed, instead of the usual purple coloration gaining more celestial white due to the influence of Kaguya's denser chakra.
Isshiki widened his eyes and instantly started to gather the life force around his palm, not having enough time because Kaguya was simply too fast. The bijuudama took barely one breath of time to reach him, but he was barely in time. His hand met the surface of the ball, his life force circulating in the exact manner needed to deflect it instead of clashing with it.
In front of Kaguya's impressed gaze, Isshiki slapped the bijuudama away, making it fly far away behind the horizon before a massive explosion occurred, a hot angry orange mushroom cloud rising in the distance, making the ground slightly shake some more.
Both Kaguya and Isshiki subconsciously braced themselves, knowing these light quakes were just the silence before the storm...
Kaguya from her position high in the sky could clearly see how something akin to an invisible force was mowing down the nature-chakra-enhanced trees as if they were just grass, slowly rushing towards them.
Then five silent seconds later, the shockwave reached them and Kaguya stood her ground, not budging even an inch from her position in the sky while her hair and clothes wildly flailed in the wind.
Isshiki's body was buckling, his feet digging into the ground as the shockwave swept over him, trying to force him to move while destroying everything around like some kind of invisible tsunami.
The shockwave passed and the world quieted down once again, both Kaguya and Isshiki uncaring, as if the country destroying force they had just resisted was nothing impressive.
And it really wasn't... as Kaguya was already charging another bijuudama while Isshiki sprang forth, leaping in her direction.
...
Rei watched with fascination as the two Otsutsukis wrought destruction and desolation upon his realm, wincing every time another patch of his forest disappeared.
Kaguya's bijuudama could be likened to nuclear blasts, erasing everything they hit and then some more with the resounding shockwave.
Kaguya's Shinra Tensei was no better, flattening everything in its path.
Isshiki shrugged whatever was thrown at him by the virtue of his physical might alone...
And the land in Rei's dimension was slowly crumbling apart.
Rei was safe in his space protected by seals from which he observed the epic battle. No attack of theirs could reach him since he made sure to spatially separate his hideout from his dimension much like Obito liked to do with his body in battle. In a way, Rei still was and was not present in the space of his dimension.
The only way to reach him was by using Spatial Abilities and while both Kaguya and Isshiki had a shitload of those, the space lock on Rei's dimension was so tight that neither of them could use even their basic space-related abilities.
As such, Rei was safe by default.
Not because he was strong enough. But because he was smart enough to set up countermeasures that left them no way of hurting him.
Thanks to that, Kaguya could completely let loose, not fearing injuring Rei at all.
And Sage was she letting loose!
The trees in Rei's dimension were a hundred times bigger than the trees in Amazon Forest and the forest in Rei's dimension was twenty times as big as Amazon Forest. As his dimension slowly grew, so did the forest due to the overabundance of the nature energy and somewhere along the road, it became as big as an entire continent, full of massive overbearing forest.
And yet, with another flick of Kaguya's wrist, a full one-tenth of the forest was flattened as the gravitational pressure descended upon it. A Shinra Tensei so strong it could erase an entire country... and Kaguya was not even winded as she threw another one of those at Isshiki who tanked it point-blank, barely faltering before once again sending a punch in her direction, sending enough force at her through the air that a city would be destroyed if it landed in the middle of it.
The surroundings were drowned in the deafening symphony of booming explosions, roaring earthquakes, and whistling rushing pressurized winds, Rei's dimension resembling more apocalypse with the previously lush and green ground now starting to resemble molten rock and fires from the explosions spreading far and wide while smoke reached the heavens in uncountable places all over his dimension.
Rei could only observe with a wry grimace, fully understanding why Kaguya gained the moniker of 'Goddess' all those years ago. She could destroy the world thrice over and call it a warm-up exercise. It was not something a person could believe without seeing it firsthand.
The pressure in the air was starting to grow heavier and heavier with every clash of the two Otsutsuki as their chakra filled it.
Isshiki was not as destructive as Kaguya but his resilience was not any less impressive. The man had a physique strong enough to swat bijuu like flies. A physique so strong that even Might Guy would shed manly tears of un-youthful jealousy upon seeing it.
But because of his limitations and focus on the body rather than techniques, his destructiveness suffered a great deal. In fact, Kaguya was the reason why 90% of Rei's dimension was erased of all semblance of life after but ten minutes of the fight.
Area three times the size of Elemental Nations was destroyed in ten minutes by her...
As yet another decimating blast of force swept through the dimension upon the clash of Kaguya's Shinra Tensei and Isshiki's punch, Rei's determination to never let any other Otsutsuki find his planet... or even his solar system... became stronger.
He didn't want nor need a clan of planet busters coming to his world!
Yes... he could fight Kaguya. He could beat Kaguya. In fact, he won all their spars so far.
But in all of those, he never once let her get going. This was the first time he witnessed her true prowess because he always restricted her with his seals before she could start throwing dangerous techniques around. Fuinjutsu was simply that broken. Even though Rei was a lot weaker than Kaguya, he could still restrict and seal her abilities enough to defeat her.
According to Kaguya, the Otsutsuki had a semblance of fuinjutsu but the way it was used in the Elemental Nations, the branch of Fuinjutsu created by Hagoromo Otsutsuki and passed down in the Uzumaki clan, was practically made to seal stronger beings.
That was the whole point of the craft, after all.
But sealing even one Otsutsuki was taxing as hell. The fight in front of Rei was a prime example.
The entire energy of Rei's dimension had to be focused on keeping the spatial lock going so the two monsters would not breach it.
Isshiki's main hard-hitting abilities were partially of the spatial nature and so were Kaguya's. All that decimation and the two were still under heavy restrictions and it was just mind-boggling how ridiculously powerful their race was.
Isshiki could not utilize his 'inventory'-like bullshit nor summon his sealing cubes and Kaguya's Rinne-Sharingan was mostly useless since she could not use her special eye ability and create dimensions due to the spatial lock.
Or rather, she could create them but transporting herself and her opponent into them would be impossible right now so the whole ability of her red eyes of doom became totally obsolete.
That's why she was spamming Shinra Tensei as if possessed.
Kaguya's dimensions were simply broken beyond belief and Rei believed it was Isshiki's fortune that the spatial lock was in place. Sure, it was to prevent him from fleeing but...
The capability to create a completely new dimension on the fly notwithstanding, Kaguya was basically 'The God' in her own dimension. Her Rinne-Sharingan ability let her dictate literally everything in her dimension and the whole dimension just bent over for her desires. As the dimension creator, she had supreme control of everything that originated from it.
It was why Rei was not awed at the prowess of his little rabbit as he watched her wreak havoc all over his realm. This... was nothing compared to what she could do with her Rinne-Sharingan special ability.
This was the reason why Rei would rather cloak and hide his planet than try to prepare for the Otsutsuki invasion. He had no idea what other busted abilities the other Otsutsukis had and from the little he knew, Kaguya was simply special but that didn't mean he would drop his caution.
Kaguya was all nice and cuddly, so much so that it was entirely too easy to forget she was an entity capable of creating new worlds.
The Elemental Nation was not ready for this. 'He' was not ready for this. For all the power Rei accumulated throughout his life, it was not enough to face a head-on invasion of the Otsutsukis. He could not depend on some stupid plot armor to give him the exact means necessary to win over the clan of world destroyers via some illogical twist of fate.
Rei knew he could develop the chakra practice of the Elemental Nations to heights capable of combating the Otsutsuki. For example, he had the Uzumaki clan. The clan full of seal masters who with the right training could go toe to toe with these monsters even if they were fundamentally weaker.
But for that he needed time. Two decades was not enough time to get a civilization up to snuff with a clan that was power-leveling by destroying other worlds for who-knows-how-many millennia.
If despite all Rei's efforts to hide, the Otsutsuki still show up? He will just accept his defeat and go to a different world with his family through Kaguya's Rinne-Sharingan abilities. Or maybe he would relocate all the ninja villages to a world created by Kaguya where she was the supreme authority. There were many solutions and paths he could take in that case. But until then, he was not giving up on his world, his village, or his accomplishments.
Not just yet.