The external training had served its purpose. It was finally time to attempt the third and final step: manifesting the Rasengan without a physical shell.
But not tonight. The grueling sessions with the solid iron ball had pushed his mental stamina to its absolute limit. Attempting the pinnacle of shape manipulation with a frayed mind was asking for failure. Sengoku ate a simple cold ration and immediately went to sleep.
He woke at dawn. His mind was sharp, his focus clear, and his chakra reserves completely replenished. He was at his peak.
Sitting in the center of his quiet stone house, he didn't hesitate. He raised his right hand and drew upon his coils. Chakra surged, pooling rapidly into his open palm.
'Control. Rotation. Stability.'
He mentally recited the core principles as a brilliant azure light flared to life. The moment the chakra materialized, it anchored itself to a central focal point and began to spin frantically.
A deep, piercing hum filled the small room. It sounded nothing like standard elemental ninjutsu; it was the mechanical, tearing sound of raw energy grinding against itself. The sheer velocity of the rotation whipped the stagnant air into a localized vortex, fluttering Sengoku's hair.
Right there, hovering just millimeters above his skin, was a highly condensed sphere of violently swirling chakra.
It was roughly the size of his fist, holding a perfectly stable spherical shape while its interior raged with chaotic, tearing currents. It pulsed with a dense, dangerous aura, casting an eerie blue glow across his face.
He had succeeded. On his very first attempt.
There was no backlash, no loss of control. The brutal, agonizing days spent forcing his chakra through rubber and solid iron had tempered his control to a razor's edge. He had already built the necessary framework; this was simply the natural conclusion.
He could feel the wild kinetic energy straining against its invisible boundaries, desperate to break free. Maintaining the sphere was a constant, heavy drain, but manageable. Based on the output, Sengoku quickly calculated his limits: with his current reserves, he could manifest the Rasengan exactly three times in a fight before hitting total chakra exhaustion.
He held it for a moment longer, memorizing its density and the specific rate of its drain. Then, with a simple thought, he severed the chakra flow. The sphere dispersed into harmless blue motes, and the piercing hum abruptly vanished, leaving the room ringing with silence.
He had mastered the form. Now, he needed to understand its actual lethality. He certainly couldn't test it inside his house, nor anywhere near the village center. Sitting back down, he closed his eyes to refine his slightly depleted chakra, mentally mapping out potential testing grounds.
Once his coils were full again, Sengoku slipped out of his house. He navigated quickly toward the absolute fringes of Sunagakure, making his way to a desolate, uninhabited sector dominated by towering, wind-eroded cliffs. It was a barren wasteland of cracked sandstone, perfect for a destructive test.
He approached a massive, unbroken section of the cliff face that rose several meters into the air. The stone here was thick and heavily compacted by centuries of desert pressure—an ideal target. Sengoku scanned the surrounding dunes. Complete isolation.
He widened his stance and extended his right hand.
Chakra surged. Rotation. Compression. Formation.
The vicious, tearing hum returned instantly as the dense blue sphere flared to life, shining brightly against the encroaching twilight. Without a second of hesitation, Sengoku lunged forward, driving the Rasengan directly into the solid rock.
The moment of impact was deafening.
BOOM!
It wasn't an explosion of fire or concussive force. It was the terrifying sound of hyper-compressed rotation making contact with solid matter. The dense sandstone folded like wet paper under the grinding vortex. There was no flying shrapnel or jagged debris; the sheer rotational velocity instantly pulverized the rock into microscopic dust. A massive, radial plume of fine sand erupted backward, swallowing the immediate area in a thick cloud.
As the jutsu's energy expended itself, Sengoku lowered his arm. The dust cloud slowly drifted away on the evening breeze, revealing the aftermath.
A perfectly spherical crater, nearly a meter in diameter, had been hollowed out of the cliff face.
Sengoku stepped closer, trailing his fingers along the crater's interior. It was eerily smooth, polished almost like glass by the absolute obliteration of the stone's structural integrity. The destructive power had vastly exceeded his expectations. This was the ultimate expression of grinding, tearing force.
And terrifyingly, this was only pure shape manipulation. If he ever managed to weave a nature transformation—like his wind affinity—into this matrix, the lethality would be catastrophic.
He silently visualized the effect of this impact on a human torso. The internal damage would be unsurvivable.
A brief spark of satisfaction warmed his chest. In the brutal world of shinobi, his odds of survival had just significantly increased. But he quickly forced the emotion down. The test was complete, and lingering here was a tactical error.
Glancing at the distinct, unnatural smoothness of the crater, he knew it was too suspicious to leave behind. He formed a second Rasengan, pressing it carefully along the edges of the crater to collapse the smooth walls and disguise the damage as an ordinary, jagged rockfall.
Once the evidence was thoroughly erased, Sengoku turned and vanished into the darkening desert, leaving only the whistling wind in his wake.