Sera dropped her arm to her side, her chest heaving as if she had just sprinted a mile.
Daichi checked the scanner's final readout. "Mana level dropped."
"How much?" Hayama asked.
Daichi tapped the glass gauge. "Enough."
Sera frowned through her exhaustion. "That's not a quantifiable number."
"It doesn't need to be," Daichi said bluntly.
Hayama stepped toward her, his face deadly serious. "This is the primary lesson." He pointed his pen at the silver armlet. "Power isn't free."
The vast training hall became dead quiet.
"You absorbed residual mana from the first creature yesterday," Hayama explained. "You released a significant portion of it into the initial water projection. Now, you have spent more sustaining that formation. The stored level is critically lower."
Sera nodded, wiping a bead of sweat from her temple. "And if I use too much?"
"We don't know."
Ren's expression turned grim, all humor vanishing. "Could she collapse? Or worse?"
"Possibly."
Arata looked at the dark runes carved into his sword. "Then we need to learn the absolute limit before we actually need to use them in the dark."
Hayama nodded. "Exactly."
He turned to face the three teenagers, his voice echoing off the timber walls. "From today onward, you do not train to become stronger."
Ren frowned, shifting the heavy hammer. "Then what are we training for?"
"Control." Hayama pointed firmly at the ancient weapons. "Power without control gets people killed in the deep strata."
He turned and pointed toward the reinforced observation window, where the locked diagnostic laboratory sat beyond the glass. "Yesterday, that black dagger killed a stalker without you understanding how, or why. It activated on instinct, and it was completely unpredictable."
He looked back at the three of them. "Today, we learned something entirely different."
Hayama raised one finger. "Mana can accumulate."
He raised a second. "Mana can be directed."
He raised a third. "And mana can be depleted."
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He lowered his hand, his gaze meeting each of theirs in turn. "Those three rules will keep you alive when the rifles fail."
Ren looked down at the massive iron hammer, then up at Hayama. "What's the fourth rule?"
Hayama stared at him, his expression completely flat. "Don't break anything."
Ren glanced guiltily at the shattered cedar training post across the sand. "...We're already past that."
Mina laughed softly from the doorway. Even Hayama couldn't completely hide the small, weary smile that crossed his face.
For the first time since they had pulled the ancient steel from the subterranean tomb, the three teenagers didn't feel like they were simply carrying cursed relics. They had begun to understand them.
It wasn't enough. Not nearly enough. But it was enough to know that there were rules governing the ancient magic. And rules could be learned.
That afternoon, they returned to the sand.
The exercises became grueling.
Arata practiced drawing and striking continuously without relying on the sword's mysterious, perfect balance, building the raw muscle memory required to wield the heavy iron. Ren learned how to pivot and move the massive hammer using momentum, fighting the urge to let the weapon's sheer weight pull his body off-center. Sera practiced forming the glowing magic circle over and over, struggling to maintain the barrier without inadvertently releasing the stored mana.
Most of their attempts ended in failure.
Arata triggered nothing from the red crystals, no matter how hard he swung.
Ren knocked over two heavy timber posts by over-swinging.
Sera managed to create three unstable water spheres before losing concentration and soaking her boots. Then she managed five. Then six. She never achieved eight again. Each attempt left her breathing harder, a dull ache building behind her eyes.
But each failure taught them a fraction more about the limits of their own endurance.
By evening, Arata's palms were raw and blistered beneath his gloves. Ren's shoulders were burning with lactic acid. Sera's wrist throbbed beneath the heavy silver armlet.
They sat together against the reinforced timber wall of the hall, passing water flasks in exhausted silence.
Ren stared at the blackened iron head of his hammer resting in the sand. "I've decided."
Arata looked at him warily. "That's usually dangerous."
"I'm going to name it."
Sera looked up from rubbing her wrist. "No."
Ren frowned at her. "You don't even know what I'm naming it."
"I know you," Sera said flatly.
Arata nodded in agreement. "She's entirely right."
Ren looked deeply betrayed. "I thought friends supported each other in battle."
"We support you by stopping you from doing stupid things," Arata countered.
"That's cruel." Ren looked back at the hammer, completely undeterred. "I'm still naming it."
Arata sighed. "What?"
Ren smiled broadly. "Thunder."
Arata stared at him. "That is terrible."
"Thunder Hammer."
"Somehow, you made it worse."
Sera took a slow drink from her flask. "What if the violet crystals don't actually produce thunder?"
Ren shrugged, totally unbothered. "Then it's an aspirational title."
Arata laughed, his head falling back against the timber wall. Sera tried to suppress her own smile, pressing her lips tightly together, but she failed. Their laughter echoed through the vast, empty underground hall, a brief, human sound against the suffocating silence of the mountain.
Beyond the reinforced concrete walls of the station, the ancient, subterranean forest remained dark and utterly silent.
And behind several layers of steel and thick observation glass, inside the sterile, locked laboratory, something changed.
The black dagger remained perfectly motionless inside its transparent containment cylinder.
Then, one of the three pitch-black crystals near the hilt pulsed.
Once.
The sensitive monitoring terminal beside the cylinder flickered violently. For a fraction of a second, a single, jagged line of unreadable, ancient symbols flashed across the digital screen in bright green phosphor.
Then it vanished.
No proximity alarm sounded. No diagnostic researcher noticed the anomaly.
Not yet.