A/N: I've already hinted that this power Hiashi uses is neither infinite nor easily replenished.
It is an extremely valuable, last-resort resource, so using it outside top-level, extinction-type hopeless threats would make little sense.
Moreover, in my setting, the Hyuga clan also has even deeper historical reasons to always want to keep a very low profile and avoid revealing this power to the world unless absolutely necessary (hint: The Moon clan fear).
On top of that, the power functions more like an external energy source. Hiashi needs to be aware of the danger before he can tap into it effectively.
Toneri's special puppets bypassed that condition by infiltrating the compound in the middle of the night and targeting Hanabi before anyone realized what was happening.
Taken together, this explains why the power was never used in the original series without contradicting or breaking the established setting.
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Hiashi's pupils constricted.
He had confirmed it.
The monstrous avatar was powerful, but it wasn't self-sustaining.
If the dragons ate enough chakra, they could collapse it entirely.
He pressed both palms forward.
The dragons coiled around each other, spiraling, accelerating, gathering momentum like twin storms converging into a vortex.
Kimimaro braced inside the titan.
He could feel the bone construct's foundation trembling.
He could feel the dragons' hunger gnawing at his armor even from afar.
He whispered to the sovereign.
"Hold."
The titan obeyed.
The dragons collided with it.
A shockwave ripped through the Uchiha ruins, pulverizing stone, shattering old walls, tearing tiles off rooftops, leaving the entire district glowing with pale white wind.
The skeletal giant staggered backward, its front limbs collapsing into dust under the devouring pull.
Hiashi saw his final chance.
He clenched his fists.
"Fall."
The dragons struck again, this time in perfect unison, sinking their jaws into the titan's spine from opposite sides, spiraling seals digging in like celestial leeches.
The White Sovereign shrieked without lungs, its entire frame vibrating under the drain.
Kimimaro felt the pull instantly.
A violent, hungry suction.
Not like chakra absorption techniques he knew, not like Preta Path, not like chakra leeches.
This was Hyūga.
A brutal, overclocked extension of Gentle Fist theory: chakra pressed so densely into a construct that it pulled chakra from whatever it touched through sheer dominance of flow.
The White Sovereign's spine buckled.
Its towering ribs flickered.
Kimimaro grimaced, not in pain, but in analysis, as he pumped additional chakra into the construct to stabilize it.
The giant skeleton shuddered, then roared soundlessly, bone re-forming in luminous white pillars as he force-fed strength back into its marrow.
It worked.
For a heartbeat.
Then the dragons' glow intensified as they drank deeper.
The more he fed the titan, the more they siphoned.
The more he reinforced the structure, the faster they consumed.
A perfect Hyūga trap.
A parasitic pressure-clamp.
A unilateral chakra equation.
He could sustain the titan indefinitely…
…but every second, Hiashi got stronger.
Kimimaro's Byakugan sharpened as he read the flow:
Just overwhelming superiority of chakra flow, reversed along his own construct and funneled straight back to Hiashi.
A pure Gentle Fist principle taken beyond human limits.
"Of course," Kimimaro murmured.
"Your clan's entire style is based on controlling the opponent's chakra. I suppose this is simply the… 'final form' of that."
The titan faltered again under the opposing pulls, cracks racing along its spine.
And worse—
The White Sovereign couldn't counterattack.
Its lances, claws, ribs—everything—passed straight through the dragons, which were chakra constructs, not physical targets.
A skeletal fist smashed one dragon… and its body simply rippled like white fire.
No effect.
Hiashi hovered above, arms extended, white aura blazing in spirals around him.
His expression didn't waver.
"Physical attacks won't reach," Hiashi said coldly, voice carrying even over the collapsing titan. "Your chakra has nowhere to go but into me."
It was true.
Kimimaro could pour more chakra into the titan.
He could hold it together for a long time.
But it wasn't a battle he could win by endurance.
He was, quite literally, feeding Hiashi.
His titan wasn't losing in strength.
It was losing in design.
Kimimaro clicked his tongue quietly.
"So it's an unsolvable equation," he thought behind the titan's ribcage.
"No matter how big my construct is… yours will simply drink it."
So, he slowly withdrew inward again.
Not retreating.
Refining.
He focused past the White Sovereign's cracking ribs, past the chakra storm outside, into the deepest architecture of himself.
His tenketsu, his marrow, the faint tremor in his soul that had begun the moment he implanted the Byakugan.
He traced every chakra pathway with surgical clarity.
Every pulse.
Every current.
Every microscopic branch.
And then the sensation hit.
A click.
A completion.
Like a lock turning after years of pressure.
He found it.
The evolution he had chased since the day he woke in this world.
He had finally mastered All-Killing Ash Bones.
Not a firework.
Not a transformation.
A realization.
The missing piece all along was Hyūga blood, the Byakugan's microscopic precision layering itself into his Kaguya physiology,
On top of the Yin–Yang control he'd spent a lifetime refining.
Together, they completed a circuit that had always been just one step out of reach.
All-Killing Ash Bones wasn't "weak" because the projectiles were small.
It was the biggest game-changer imaginable.
Because even a dust-grain was fatal.
A pure, certain-kill technique that bypassed durability, regeneration, ninjutsu resistance, everything.
A biological nightmare.
A DNA-level virus sculpted by chakra.
A microscopic death sentence.
One cell touched, and the victim's body cascaded into rapid disintegration, collapsing into pale ash-like matter.
Truth-Seeking Orbs erased jutsu and matter.
Ash Bones erased life itself.
Perfect biological tool for someone from Kaguya.
Perfect for someone walking the path toward that apex.
Kimimaro breathed once, slowly, and let the newborn essence of All-Killing Ash Bones seep into his skeletal network construct.
Quiet. Invisible. Still merging.
Then he made a decision with chilling calm.
He let a fraction of that essence bleed into the White Sovereign's spine.
Just enough.
Not to attack Hiashi directly…
…but to let the chakra-eating twin dragons swallow it along with his titan's chakra.
A trap disguised as nourishment.
Hiashi didn't notice at first.
He was too focused on draining the titan, too enraged, too certain of his dominance.
But then—
His Byakugan flared violently.
His pupils contracted.
A jolt of instinctive terror hit him like ice water.
He felt it.
A wrongness entering the dragons.
A foreign signature riding the flow of chakra back toward him.
Not poison.
Not corruption.
An extinction agent.
Something that would start breaking him down at the cellular level the moment it touched his physical form.
He severed the connection instantly in just a nickel of time.
The dragons recoiled in mid-air, serpentine bodies convulsing, white light shuddering along their length as Hiashi force-purged the foreign chakra, ripping them away from the titan with a booming shockwave.
For the first time, Hiashi looked genuinely frightened.
He had brushed death.
Real death.
"That… that was…" he whispered, voice cracking under the weight of comprehension.
He didn't have a word for it.
He had never seen anything like it.
But he understood one thing perfectly:
He could no longer siphon Kimimaro's chakra.
Not safely.
Not even for a second.
One wrong sip, and his body would corrupt beyond recognition
His advantage evaporated.
His greatest technique became unusable.
The sky-borne superiority he'd relied on flipped over in a single heartbeat.
The battlefield reset.
And now he was forced to fight Kimimaro head-on again—
With nothing but his baseline pressure waves, raw white chakra expenditure, and distance barrier manipulation.
All while Kimimaro's enormous bone avatar, ultra-dense and regenerating, still loomed beneath him like a death god waiting for permission.
Hiashi's expression hardened.
Kimimaro's Byakugan narrowed, glowing like a quiet star.
The situation had reversed again.
Even through the roar of clashing titanic forces, could feel it clearly now.
He had stepped fully into the Quasi–Six Paths bracket, matching Hiashi's current state.
But matching wasn't the same as parity.
Hiashi was still climbing, still charging, still drawing on that distant ocean of clan-fed chakra that dwarfed Kimimaro's comparatively meager external reserves stored in the Yang Seal.
A bottomless well versus a carefully rationed fuel tank.
Worse, Kimimaro realized Hiashi was not just attacking him.
That white pressure aura had been shaping the battlefield from the moment the fight started.
Hiashi had erected a wide, invisible dome around the abandoned Uchiha district, forcing the entire battle to stay contained, barring Kimimaro from retreating toward the Hyūga compound or from weaponizing the village as terrain.
That had been Hiashi's very first move.
Logical for a patriarch.
Annoying for Kimimaro.
But that strategic cage came with a price.
Hiashi was burning chakra at a monstrous rate, far more than he would have in a simple duel.
Every pressure blast, every gravitational crush, every moment of maintaining that boundary siphoned from his reserves.
Kimimaro, by contrast, was spending far less.
His bone constructs, once created, were semi-permanent.
His techniques, especially after his evolution began taking shape, were increasingly efficient.
Hiashi had to fire again and again and again…
Kimimaro only had to endure, adapt, and wait.
At this point, the real question became:
How much energy did Hiashi have in ancient stock?
How efficient is this entire conversion?
And more importantly, how much was he willing to spend to kill him?
That whole factor was out of his control.
So he wouldn't bet on it.
Kimimaro didn't know if the four girls could keep holding the line with the mysterious enemies approaching.
Instead, his mind kept turning, sharpened by the Byakugan's internal clarity, calculating paths, angles, timing.
Searching for a single opening.
A single moment where he could slip All-Killing Ash Bones into Hiashi's system.
A silent seed.
A death sentence masquerading as chakra.
If he could land even one…
The fight would end.
One way or another.