Following their disastrous encounter with one of the Hidden Sand's scouting teams, Naruto and Sakura were forced to reduce their speed as they came across more and more enemy patrols, forcing them to hide to avoid unnecessary fights. Moulding chakra vastly increased the range at which they could be discovered by enemy sensor-nin, making them as visible as a lighthouse on the blackest of nights, even when they were only using their chakra to bolster their jumping and running abilities…
"At this rate," Sakura sighed, "we'll never make it in time."
They were currently lying low in the grass as a scouting team of four Stone-nin passed in front of them. While they impatiently waited for them to leave, they could only grit their teeth in frustration as the sun continued its inexorable, lazy arc across the heavens; it had nearly reached its zenith, and when it did, Kakashi would be lost, and Hiruko would become truly unstoppable.
"There's no way I'm giving up now," Naruto said in a low voice. "I can't accept losing Kakashi-sensei this way. Not like this."
"I'm not saying we should give up," Sakura said quietly. "I…"
Sakura clamped her mouth shut as one of the Iwa-nin glanced over his shoulder her way.
Most of the shinobi from the Land of Stone were Genin, barely more than human, but due to their sheer numbers, Iwagakure still had a fair number of Jōnin… even after Namikaze Minato was done with them.
Sakura wasn't sure why that was the case, but if she were to hazard a guess, she would say that the Sage of Six Paths hadn't spent as much time in the far west as he had in the rest of the continent, resulting in the gift of chakra, which the Sage had imparted unto everyone he came across, becoming greatly diluted after generations.
After all, not everyone was capable of moulding chakra, even in the Land of Fire, which neighboured the territories of the Land of That, where Ōtsutsuki Kaguya had landed a thousand years ago. Well, to be precise, Kaguya had alighted in the Land of Ancestors, but thanks to the Land of That's liberal use of the Akatsuki for war, it had recently absorbed the territories of both that land and the Land of This.
Confusing?
Very.
"I think they're gone," Sakura whispered, squinting her eyes as the squad of Iwa-nin flickered away. "Are they gone?"
Judging by their trajectory, they were heading towards the scene of carnage Naruto and Sakura had passed on their way westwards: the huge corpse of a creature that looked like an amalgam of different beasts fused together, with a horn, a pair of wings, and two snake-like tails, all covered in plate armour… and surrounded by innumerable lifeless Iwa-nin bodies.
"I think so," Naruto murmured back.
Naruto and Sakura leapt to their feet.
Judging by the position of the sun in the sky, they had about another hour to reach their destination, and they were still quite a way from Mt Shumisen, where Hiruko was said to have set up his altar for his promotion to the Foundation Establishment realm.
Or something like that.
…
Thirty minutes later, Sakura finally began to notice changes in the landscape around them. The grassy plains receded, the earth giving way to dark basalt rock, with greyish chunks of tuff scattered about, and tufts of tough, windblown grass growing sparsely from the volcanic ash.
Mt Shumisen stood before them at last.
Naruto and Sakura nodded to each other in mutual understanding. Chakra flooded their bodies and burst from the soles of their feet as they repeatedly activated the Body Flicker Jutsu, their figures briefly vanishing and reappearing in quick succession as each landing propelled them onwards.
"The eclipse!" Sakura gasped, as they screeched to a halt in front of an immense stone gate hewn from the surrounding rock. The moon was already beginning to partially obscure the sun. "We're too late!"
…
Hiruko was perhaps the first person in human history to pioneer the cultivation of chakra as a means of increasing one's life level and attaining longevity, as the Ōtsutsuki did, rather than merely as a path to superhuman abilities, as the shinobi did.
Man, Earth, Heaven.
Traditionally, these were the three aspects of spiritual qi: celestial qi descends and telluric qi rises, which humans harmonise and metabolise within themselves to increase their cultivation… yet, even though the truth of this universe was chakra rather than qi, the principles seemed to remain the same: to defy the natural order and consume more than one's fair share to attain immortality in an act of supreme egoism.
For the ritual's purposes, the Man attribute was represented by the Five Elements: five sacrificial offerings corresponding to the five chakra nature affinities, which balanced each other out by feeding into each other in a beneficial cycle.
To cover the Man requirement, Hiruko doubtlessly already had the five human sacrifices accounted for, given that Naruto and Sakura had not seen a single trace of Kakashi so far. He also already possessed the Earth aspect in Mount Shumisen, whose Mountain Trigram neighboured that of Earth. Now, all he lacked was Heaven, the faint light spilling from the solar disc during the eclipse…
"Think, Sakura, think!" Sakura told herself as Naruto launched barrages of Shadow Clones armed with Rasengans at the stone doors, blasting craters into them. "There must be something you can do!"
Hiruko's ambition could not be satisfied by the mere five, basic chakra natures. He wanted more; he wanted invincibility, so he greedily sought out bloodline limit ninja and absorbed them into himself to fulfil the ritual's Man requirement… and in doing so, he completely forgot that the goal of the Foundation Establishment realm was to achieve the most stable foundation, not the strongest!
First was Kumogakure's Storm Style, a combination of the water and lightning natures.
Second was Sunagakure's Swift Style, a combination of the wind and lightning natures.
Third was Iwagakure's Steel Style, a mutated version of Explosion Style, internalising the earth and lightning natures instead of externalising them.
"That's weird…" Sakura thought out loud. "These all have something in common…"
Lightning Style was the central, guiding pillar for these three elemental Kekkei Genkai, around which earth, water, and wind continuously cycled to maintain equilibrium… but the overabundance of lightning and the absence of the fire nature naturally caused Hiruko's ChimericPhysique to remain unstable.
So, Hiruko's fourth Kekkei Genkai could only be Dark Style, taken from a certain red-headed individual, the diaspora of his clan having carried him across the sea to Kirigakure. In all likelihood, Dark Style was a degenerated version of Yin-Yang Style, or perhaps an atavistic revival of the Ōtsutsuki clan's ability to absorb chakra from external sources.
Yin-Yang Style fell within the realm of the Tai'in Taikyokuzu; the Taiji, or the Supreme Ultimate Yin-Yang Moon Diagram of the Eight Trigrams. Yin-Yang Style possessed the property of non-interference with the five chakra natures; perfect to absorb the excess lightning coursing through Hiruko's system.
And so, if Hiruko wished to complete his five-point elemental foundation without destabilising the entire arrangement, he could not choose any Kekkei Genkai rooted in water, wind, or earth, which he already possessed, as doing so would have upset his already delicate balance.
As far as Sakura knew, there existed no Kekkei Genkai that combined the fire and lightning natures directly, so Hiruko's decision must have fallen on the Dōjutsu that had inspired his Chimera Jutsu all those years ago: Kakashi's borrowed Sharingan, another product of Yin-Yang Style, tinged with the Uchiha clan's fiery chakra nature and Kakashi's own innate lightning affinity: perfect for the delicate tightrope act Hiruko was walking.
"With lightning as the core, earth, water, fire and wind in imperfect equilibrium," Sakura muttered to herself as Naruto continued furiously slamming Rasengans against the gates. "Heaven and Earth Trigrams in imperfect balance… Hiruko's greed for Kekkei Genkai has already pushed his ritual to its limits… so I just need to tilt the scales even further, don't I?"
Hiruko had chosen Mount Shumisen as the location for his ascension ritual, as it was an out-of-the-way, easily defendable place. However, this also meant that the stability of his foundation had been compromised from the very beginning; rather than being rooted in the Earth Trigram, his ritual was influenced by the adjacent Mountain Trigram.
In conclusion, all Sakura needed was the Thunder Trigram to overload Hiruko's formation with natural lightning to destabilise the Man attribute's elemental equilibrium. To make lightning, she would need thunderclouds, representing the Water Trigram, which would have the added benefit of obstructing the special light cast down by the solar eclipse and further tilting the Heaven and Earth Trigrams clockwise…
Sakura grinned. She knew the perfect jutsu to disrupt Hiruko's ritual!