The air in the underground training halls of Kukuroo Mountain was always heavy.
Not from dust, not from smoke, but from aura.
The Zoldyck family trained here for generations. The stone walls absorbed the strain of screams, the clash of weapons, and the thrum of Nen pressure so intense it made the torches flicker. For Kaien, this place was a crucible.
He sat cross-legged in the middle of the hall, his white eyes half-closed, his hair falling loose around his shoulders. Zeno's voice cut through the silence.
"Ten. Hold it. Do not let it slip, boy."
Kaien inhaled slowly. He pulled his aura tight around his body, wrapping himself in that invisible skin. It was like pressing water against himself, forcing it to cling without spilling. Sweat dripped down his jaw.
Zeno's cane tapped once against the stone.
"Good. Better than yesterday."
From the shadows, Illumi watched. His brother's dark eyes were unreadable, but Kaien could feel his gaze. Not envy. Not admiration. Just calculation. Illumi never looked at people — he weighed them like tools.
Kaien exhaled, letting the aura disperse. His chest rose and fell, exhaustion creeping in. He had been at this for hours.
"Again," Zeno ordered.
Kaien obeyed.
The sessions blurred together. Ten, over and over, until his body screamed. Then came Zetsu.
"Extinguish yourself," Silva commanded one evening. His massive form loomed above Kaien, a mountain of muscle and menace. "Not a drop of aura should leak. Become nothing."
Kaien closed his eyes and tried. He drew his aura inward, shrinking it, smothering it. It felt unnatural, like holding his breath underwater. Darkness swam at the edges of his vision. His muscles trembled.
Then — a flash of pain.
Silva's hand shot forward, grabbing Kaien by the throat and lifting him effortlessly.
"You're leaking," Silva said calmly. His grip tightened. Kaien's vision blurred. "An assassin who leaks dies."
Kaien's teeth clenched. His hands went to his father's wrist, but he forced himself to focus inward instead. Pull it in. Smother it. Be empty.
The moment the last trace of aura vanished, Silva released him. Kaien collapsed to his knees, gasping.
Illumi's voice came, flat and cold.
"You almost died. That means you almost passed."
Kikyo's laugh echoed from the balcony above, sharp and shrill.
"Pathetic. Always on the edge of failure. Perhaps we should throw him to the beasts and see if his aura saves him."
Kaien wiped the blood from his lip and stared up at her with those pale, unyielding eyes. He didn't answer. He never gave Kikyo the satisfaction.
Zeno's cane tapped again.
"He learns faster when pushed. Continue."
And so they did.
The tests grew crueler.
One night, Kaien was thrown into the underground tunnels. The walls dripped with moisture, and the air stank of old blood. Silva's voice thundered from somewhere unseen:
"Hide your aura. Survive until dawn."
The first footsteps came within minutes — Illumi's. His aura was controlled, suffocating, coiled like a serpent waiting to strike.
Kaien pressed himself into the darkness, smothering his aura with Zetsu. He felt Illumi glide past him once, twice. Each time, his brother's presence scraped against his nerves like knives.
But on the third pass, Illumi's head turned.
Kaien bolted.
The chase that followed was silent but brutal. Illumi's needles flashed in the dark, embedding themselves in stone. Kaien's longsword sang as he deflected one, sparks lighting the tunnel for a heartbeat.
"You leak when you run," Illumi murmured behind him. His voice was like silk stretched over blades.
Kaien's chest burned. He forced himself to pull the aura tighter, even as his lungs screamed for air. His footsteps slowed. His body grew lighter. For a single, terrifying moment, he felt invisible.
Illumi stopped walking.
"…Better."
The sound of retreating footsteps echoed into the dark, leaving Kaien alone, trembling but alive.
By the time dawn came, his clothes were torn, his muscles raw, but his aura control had sharpened.
Silva met him at the exit of the tunnels, arms crossed.
"You didn't die. That's something."
Zeno nodded slightly.
"His Zetsu is taking form. Next — Ren."
Kaien's head tilted up, sweat dripping from his hairline.
"Ren?"
Zeno's eyes glimmered with something that might have been approval.
"Yes. The true flood of aura. Learn to release it — or it will drown you."
The next days were hell.
Kaien sat in the center of the hall again, veins bulging as he forced his aura outward. It roared from him in waves, rattling the torches, cracking the stone floor beneath him. His longsword quivered at his side, the steel humming as if it felt his aura too.
But every time, the aura burst too wide, too wild. It left him drained, staggering, nearly unconscious.
Silva never showed pity.
"You are a blade. If you cannot hold your edge, you are useless."
Illumi's voice cut in, monotone.
"He's getting closer. His aura is heavy… it makes the air thick."
Kaien wiped blood from his nose, forced himself upright, and tried again.
Each failure chipped away at him. Each success — brief, fleeting — lit a fire inside.
For the first time, Kaien felt something strange: the thrill of control.
He wasn't just enduring the training. He was beginning to crave it.
The final test of the week came suddenly.
Zeno stood before Kaien with his cane, and Illumi at his side.
"Fight him."
Kaien blinked. "…Illumi?"
Zeno nodded.
"Your brother will not kill you. Unless you force him to."
Illumi's eyes were flat as he drew out a handful of needles.
"Don't disappoint me."
The spar exploded instantly.
Illumi's needles flew, whistling like vipers. Kaien's sword met them, steel flashing, sparks dancing. His aura flared — uncontrolled, wild — but it gave him speed. He lunged, blade cutting air where Illumi had stood a heartbeat earlier.
Illumi's hand touched his shoulder. A needle pricked his skin. Kaien snarled, aura bursting to shove him back. The needle clattered to the ground before it could bury itself.
"Ren," Illumi observed quietly. "Rough. But strong."
Kaien panted, chest heaving. His vision swam — but his blade never lowered.
Zeno's cane tapped once.
"Enough. He lives. He learns."
Silva's gaze lingered on Kaien a moment longer before he turned away.
"He'll be ready soon."
Kaien stood alone in the silence of the hall, every muscle screaming, aura flickering like a storm around him.
He touched the hilt of his sword, pale eyes narrowing.
Soon.