Renji Kurogane, a completely normal high school student, was on his way back from a convenience store late one night when he was suddenly enveloped in a blinding light and lost consciousness.
When he awoke, he found himself in a decaying ocean world ruled by 'Hakos Baelz.' The sky was shrouded in murky clouds, and the sea was black and polluted. The world was slowly being devoured by colossal monsters known as 'Kegareshin' (Corrupted Gods).
Renji manages to reach an underwater 'Safe House,' wh
Elpis of the Corrupted Sea -A God's Despair Game- - Frequency of Awakening
In his private room at the Kaleidoscope, Kurogane Renji lay curled up on the hard bed. A full day had passed since that patrol mission. The image of Gant being devoured, the sight of Isak's Urca slamming into the rock wall and shattering, and Luca's scream—these replayed in his mind over and over, tearing his spirit to shreds. The faint orange glow of the Lumen stones embedded in the wall traced the gaunt outline of his cheeks.
His right fist was wrapped in gauze. He'd punched the wall too many times before, splitting his nails and tearing the skin. The gentle sensation of Kirsche tending to his wounds was the only warmth that barely kept him tethered to reality.
(I couldn't do anything here, either.)
It was a repeat of middle school. He knew his best friend was being bullied, but he was too afraid of becoming a target himself, so he pretended not to see. In the end, his best friend left the school, and a deep scar remained in Renji's heart. Coming to another world hadn't changed a thing.
He was nothing but a powerless existence, just watching his comrades die before his eyes.
Bam, bam, bam!
Suddenly, the sound of someone pounding violently on the metal door echoed through the room. It was less of a knock and more like someone trying to kick it down.
"Hey, kid, you're in there, right?! Open up!"
Renji's shoulders flinched at the voice from outside. A familiar, low voice that was light yet resonated deep in the ear. It was Volg, the helmsman.
Lacking even the energy to respond, Renji remained silent. A sigh came from outside.
"If you won't open it, I'll open it from this side."
A heavy clunk sounded as the lock was forcibly disengaged from the outside. He must have used an emergency unlock code. The heavy door swung open with force.
Standing in the doorway was a tall man with dull blond hair tied back carelessly. Across his entire left arm, a tattoo of an old nautical chart—copied from a relic predating the Volak—faintly shimmered in the light of the Lumen stones. His grayish-blue eyes narrowed just slightly as they caught sight of Renji huddled on the bed.
"...You look like hell, kid."
With those words, Volg strode into the room and grabbed Renji's arm.
"Come on. This isn't the time to be rotting away in a place like this."
"W-Wait...!"
Pulled with an irresistible force, Renji was hauled up, practically tumbling off the bed. Without even a moment to protect his aching fist, Volg was already walking briskly down the corridor. Left with no choice, Renji followed after him.
A low groaning sound came from the pipes running along the corridor walls. Just inside the outer hull that withstood the pressure of the seawater, the plumbing trembled. Over the past few days, Crawler activity had intensified, and the entire safe house felt perpetually on edge.
"Stop it... I'm..."
"I won't."
Volg declared without even turning around.
"Don't just sit there sniveling forever. It won't even make for a good story to take to the afterlife."
They descended the stairs, heading for the hangar on the first level. Passing through a massive steel hatch, their view suddenly opened up. In the high-ceilinged space, five special submersibles—Urcas—lay silently, their enormous forms at rest. Their streamlined bodies, twelve meters in length, had almost no seams to withstand the deep-sea pressure. Powered by Lumen stones, these craft were the very lifeline in this sea of despair.
The hangar was filled with the distinct smell of engine oil mixed with seawater. A few mechanics were climbing the outer hull of an Urca, inspecting its armor. Volg stopped in front of one of them—his beloved machine, the "Strom." The craft's flank bore numerous vivid scratch marks. Traces of near-misses with Crawlers.
"Look at these scars."
Volg gently stroked one of them with a calloused finger.
"I screwed up once, too. A long time ago."
He held out his left forearm, the one with the nautical chart tattoo, right in front of Renji's eyes. Looking closely, beneath the tattoo ran several large, stitched-up scars.
"Seven years ago. My piloting error caused a collision with a reef in the deep zone. I survived. But the two crewmates in the rear seats were caught in the crushed bulkhead. It was over in an instant. One of them was still a rookie, a kid of sixteen."
Renji's throat tightened. The usual levity had completely vanished from Volg's voice.
"Back then, I was just like you, kid. I shut myself in my room and just trembled. I couldn't move. I thought, 'They died because of me.' But then, three days later, Commander Mira said this to me."
Volg pulled a worn piece of notepaper from his pocket. On it, in Mira's neat block handwriting, was a short message.
"Stand up. For the sake of those who died. Your hands can still hold the helm."
"You've got no choice but to stand up. For the sake of the dead, too. We're creatures who can do nothing but keep moving forward, even so."
Renji felt a burning heat behind his eyes.
"...I couldn't save my best friend back in middle school, either. I pretended not to see. He told me, 'You didn't do anything for me'... This time, it's the same. I just watched Gant-san die in front of the monitor."
His voice trembled, and he desperately fought back the tears threatening to spill.
Volg listened to Renji's words in silence. Then, he placed a hand on Renji's head—roughly, but warmly.
"That was hard. Thanks for telling me. ...That wound of yours, kid, it's definitely yours. But giving up here is the same as telling the dead Gant and Isak, 'Your deaths meant nothing at all.'"
Renji couldn't say anything. He just felt a single tear roll down his cheek.
"But, kid. That 'normal' of yours... it might just become a weapon."
As if to change the mood, Volg picked up a weapon leaning against the Urca's outer hull. It was shaped like a spear, about a meter long, with a Lumen stone embedded in its tip. A Sonar Lance.
"You've seen this before, right? A sonic spear for anti-Crawler use. The principle is like a crude copy of the Purification Song. It uses the resonance of the Lumen stone to fire a directional shockwave. Against something like a Crawler, it can intimidate and drive them off. But against a Kegare God, it won't even leave a scratch. However—"
He clicked the output adjustment dial on the side of the Sonar Lance.
"There's an interesting phenomenon. When you fire this thing at maximum output, the target it hits sometimes stops moving for just a moment—a mere fraction of a second. Until now, everyone just thought, 'Oh, it must have been stunned by chance.'"
At those words, something clicked sharply in Renji's mind.
"...Huh? Wait, tell me more about that."
(It stops moving for a moment? When hit by a specific sound wave?)
A phenomenon he'd learned about in science class back in Japan vividly resurfaced in his mind. Rubbing a finger on a wine glass produces a specific sound. And if you hit that glass with a sound at the same frequency as its natural frequency using a large speaker, the glass resonates and shatters into pieces. Every object has a "natural frequency" at which it vibrates most efficiently. It's the principle behind bridges collapsing from soldiers' marching footsteps, or washing machines shaking violently at specific spin cycle speeds.
"If the Kegare God also has a natural frequency, and the Sonar Lance just happened to emit a frequency close to it...!"
"Huh? What's that, natural frequency?"
"It's physics from my world! Every object has a specific frequency at which it vibrates most efficiently. If the Black Tide Miasma that makes up Crawlers and Kegare Gods has such a frequency, and we can precisely match the Sonar Lance's output to it—"
Renji spoke rapidly, as if trying to convince himself. The tears were gone from his eyes now.
"It wouldn't just stop them for a moment. It might be possible to disintegrate their very bodily tissues. Back then, Kirsche's song had no effect on the Kegare God at all. That's because the enemy's miasma layer was too thick, causing the 'frequency' of her song to scatter and not reach it. We were just diffusing the sound waves. But with a focused sonar pinpointed to the enemy's natural frequency, we can pierce through the miasma layer and destroy its core structure from within!"
The words tumbled out, driven by sheer momentum.
The levity vanished from Volg's face. He folded his arms and stared intently into Renji's eyes. Those gray eyes, which were usually laughing, now coolly appraised Renji with the stern, professional gaze of a helmsman.
"...Kid, are you just saying that off the top of your head? Or do you seriously have a theory?"
"I'm serious! Of course, I'm not a physicist. But the theory is sound. Please, Volg-san. Let me do this. Let me handle the frequency adjustment of the Sonar Lance!"
Renji's voice trembled, but his eyes alone held a strong light as he looked back at Volg. It was a forceful tone that surprised even himself.
Volg was silent for a few seconds. Only the high-pitched clang of someone hitting metal with a hammer far away echoed through the hangar.
Finally, the corner of Volg's mouth slowly lifted. It was his usual friendly grin again.
"Heh, fine. Let's take a gamble on this 'natural frequency' thing of yours. Come on. I'll swipe some parts from the tech crew for you."
—-
Volg was capable. Before anyone knew it, he had "borrowed" a spare frequency modulator for a Sonar Lance, crystal pieces for fine-tuning Lumen stones, and a full set of tools from the tech crew. Renji and Volg headed for a workbench in the corner of the hangar.
"What I need to do is make the frequency modulation pattern manually fine-tunable."
Renji began a process of trial and error, desperately recalling the resonance theory he had memorized in Japanese science class. He disassembled the Sonar Lance and incorporated the crystal pieces into the frequency modulator's circuit. Beside him, Volg, covered in grease, handed him tools and sometimes forcefully pried open the casing.
"No, this needs finer adjustment here. I have to change the circuit's resistance value."
"Kid, this screw's too damn tight. Hey, hold that part down."
Commander Mira, hearing the commotion, came to inspect. She looked down for a while at the parts scattered on the workbench and at Renji and Volg gripping tools with serious expressions. Then, she wrote something in her notebook and silently showed it to them.
"Don't be reckless. But if you're going to do it, make it succeed."
That was all. But Renji could understand how much trust and expectation were packed into those brief words.
As they were absorbed in their work, faint footsteps sounded behind them.
"...Renji."
He turned around. Kirsche stood at the entrance. Her waist-length silver hair was tied into a single ponytail, and she wore her navy combat uniform. Even through her clothes, he could tell her left side was still bandaged. She was supposed to be resting in the infirmary.
Her large violet eyes gazed at the modified Sonar on the workbench, then looked at Renji's face.
"...I heard you'd started something. And I thought I needed to talk to you properly, too."
She slowly approached and looked down at his hands.
"Before, there was a time my song didn't work. It was a battle you witnessed, too. At that time, I felt the Kegare God's miasma layer was much thicker than before. It was as if it had learned from our attacks and was strengthening its defenses."
"The miasma layer was blocking your song. But Kirsche, this time it'll be fine. With this modified Sonar, we can break through the miasma wall before your song even needs to reach it. The theory is perfect. I'll definitely make it succeed."
Kirsche quietly stared at the modified Sonar on the table. Then, she gently reached out and took Renji's hand, soiled with tool grime. She held it tenderly with both hands, as if enveloping his wounded fist. The same gesture she had made in his room that night.
"I believe in you."
Her vo
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