The abandoned corridor trembled beneath the hum of machinery that wasn't supposed to exist. Nagisa's steps echoed softly in the dim light of the underground facility, the faint pulse of Erabus energy illuminating cracks in the concrete like veins of a living organism. Behind him, Hakumura trailed quietly, flashlight in hand, the silence between them heavy with failure and fatigue.
The radio—the one Koro-sensei had given Nagisa long ago—buzzed weakly in his hand. It had led them here, intercepting an encrypted transmission from a voice speaking about "Project Erabus." Nagisa didn't know whether it was coincidence or fate… but as always, fate had a strange way of finding him.
"Are you sure this is the place?" Hakumura whispered. His breath fogged in the cold, subterranean air.
Nagisa nodded. "The signal converged here. Whatever they're hiding… it's beneath this floor."
They descended another level, deeper into the bowels of the forgotten government structure. The walls began to change — less concrete, more metal. Machinery lined the corridors, faintly vibrating as if responding to something alive within.
When they reached the final chamber, the air felt different — not cold, but electric. Floating in the center of the room was a sphere of fractured light — a chunk of something glowing, suspended by invisible force.
Nagisa stepped closer, squinting through the shimmer. "That's…"
Hakumura's voice trembled. "A piece of the moon."
And then — from the other end of the chamber — a voice, smooth, resonant, with an age-old composure that carried the gravity of authority itself:
"Indeed. The very same moon your teacher destroyed."
They spun around. Out of the dark stepped a tall, slender figure in a spotless white lab coat. His silver hair reflected the blue glow of the Erabus core behind him, and his eyes, one artificial and one human, gleamed like polished glass.
He smiled, slow and deliberate. "So… you are Nagisa Shiota. I was wondering when you'd finally stumble into my little sanctuary."
Nagisa's hand instinctively brushed the knife holster at his waist. "Who are you?"
The stranger clasped his hands behind his back, unfazed. "Dr. Erabus Dalkuvinchi Mizukashi. Founder of Project Erabus. Former navy strategist. Current god of progress—" his tone sharpened, "—and the one who will cleanse Japan of its past mistakes."
Hakumura frowned. "You mean the moon? That wasn't a mistake. That was Koro's crazy situtation."
"Salvation?" Erabus laughed—a hollow, intellectual sort of laugh. "The death of one god does not free humanity. It only births another void. That void is what I filled."
He gestured toward the glowing sphere. "When your dear Koro-sensei shattered the moon, he didn't just scar the Earth's tides. He opened a wound in physics itself. The particles left behind—saturated with antimatter-like quantum residue—fused into a new mineral. We call it Erabus. It is divine entropy condensed into matter. Power, born from destruction."
Nagisa's voice hardened. "And you used it… to make weapons."
Erabus nodded proudly. "To create a new evolution of defense. You see, Japan stands alone among nations. We cannot afford weakness. The government entrusted me to turn this ruin into rebirth. And what better starting point… than the being who killed the greatest weapon in human history?"
Hakumura's eyes widened. "You… you mean—"
Erabus pointed directly at Nagisa. "You. You are the living variable. The one boy who killed what no army could. I have waited for years to dissect that miracle — to study what drives a mortal to slay a being such as Koro."
Nagisa felt his heart pound violently. His pulse echoed in his ears, like the rhythm of Koro-sensei's laugh. The same student once too small to even hold a weapon properly now stood in the shadow of science trying to play god once again.
"So that's it," Nagisa whispered. "You think you can rebuild what Koro-sensei destroyed. You think you can replace his lessons with machinery."
Erabus smiled faintly. "Not replace. Refine."
The moment hung, then shattered with a thunderous explosion — a blast of gravitational force erupting from the sphere behind him. Panels split, lights flickered, and fragments of lunar rock began to orbit the room like miniature satellites.
Dr. Erabus extended his hand. Metallic filaments spread across his wrist, forming a glowing device that pulsed in rhythm with the core.
"This," he said, voice rising, "is Erabus Energy in its purest form. Moonlight and chaos harmonized! I have learned to wield it—to control matter at the molecular level!"
The ground trembled. A wall of floating debris surged forward like a tidal wave.
Nagisa shoved Hakumura aside and rolled to the floor, barely dodging a spiral of spinning shards. His instincts—years of suppressed muscle memory from Class 3-E—took over. He reached for a broken metal rod, using it to deflect a blast, then dashed forward through the storm of floating wreckage.
"Don't you understand?" Erabus shouted, stepping through the chaos like a prophet through divine storm. "Your teacher's death was only the beginning! His destruction birthed evolution!"
Nagisa's eyes narrowed. "Koro-sensei didn't die for this!"
Their weapons clashed — Nagisa's improvised blade meeting Erabus's metallic glove, sparks bursting from the collision. For a moment, their faces were inches apart — one young and furious, the other old and obsessed.
"You mistake mercy for meaning," Erabus hissed. "He taught you love, and yet you kill without hesitation. You are not his heir—you are his error!"
Nagisa twisted his arm, forcing Erabus back, and slammed the rod into the floor, channeling his full weight. The shockwave shattered the tiles beneath them.
"Maybe I am!" Nagisa shouted. "But that's what makes me human!"
Behind them, Hakumura found cover behind a terminal, frantically typing commands. The radio on Nagisa's belt suddenly crackled, catching static feedback from the energy field.
"—sa… Nagisa!—the frequency… it's resonating with his energy!"
Nagisa ducked as Erabus launched a wave of magnetic force. "Then use it! If Koro-sensei's gift can reach this deep, maybe it's what he left behind for a reason!"
Hakumura rewired the radio's signal to match the Erabus field's frequency. The moment he pressed enter, the entire room screamed with resonance — the air vibrating with a familiar hum.
For a heartbeat, Nagisa heard a voice — faint, distant, but undeniably familiar.
"Don't let knowledge consume kindness, Nagisa."
Koro-sensei's voice.
It was gone a second later, drowned by the explosion that followed.
The feedback destabilized the Erabus sphere, its light fluctuating violently. Erabus hissed, clutching his chest as the energy around him spiraled out of control.
"You… insolent child!" he roared, regaining balance. "You think the words of a dead freakish disgusting teacher can save you now?"
He activated a panel on his arm, and from the walls, mechanical constructs emerged — humanoid forms forged from Erabus alloy, glowing with lunar blue energy.
Hakumura swore. "You've gotta be kidding—he built an army down here!"
"Stay behind me!" Nagisa yelled, grabbing a kitchen knife from his belt—the same kind he'd used years ago in his first assassination lessons. The contrast between the blade's simplicity and the weaponized moonlight that surrounded them couldn't have been starker.
Erabus watched, fascinated. "A kitchen knife against the next evolution of Japan. How poetic."
Nagisa exhaled slowly. "Poetic enough to kill gods twice."
He dashed forward — weaving through beams of plasma-like energy, slashing through machine joints, using every motion economy Koro-sensei had ever drilled into his body. His every movement was human — flawed, grounded, yet burning with life.
Hakumura joined in, wielding a stolen energy blade from one of the fallen constructs. The two fought back to back, just like students on a field exam years ago.
But Dr. Erabus wasn't just watching. He was studying. Recording. Smiling.
"You fight beautifully," he murmured. "Just as I hoped. Every scar, every hesitation—it's all part of the data."
Nagisa turned, eyes blazing. "We're not your experiment!"
Erabus lifted his arm, and the Erabus sphere condensed, glowing like a reshined star. "Then prove it. Survive what you were never meant to!"
The floor ruptured. Energy surged outward, engulfing everything in light. The screen flashed white — the roar of the explosion consuming their cries — and when the light faded, Nagisa was lying amidst rubble, his vision blurred, blood trailing from his forehead.
Erabus stood across from him, cloak torn but smiling serenely, untouched by rubble and dirt.
"Next time," he said, voice almost kind, "I will show you the true mirror of Erabus. The one that reflects not the world… but your soul."
He vanished in a storm of blue light, leaving Nagisa and Hakumura coughing amidst the ruin.
Nagisa's grip tightened on the knife still warm in his hand. The air smelled of ozone and dust, and in the silence that followed, the radio whispered once more—barely audible, like a ghost through static.
"The lesson isn't over yet…"
Nagisa closed his eyes. "I know, Sensei."...