Underground City
Suhyun had consumed a ton of creative works back on Earth. Among them, he vividly remembered stories where the outside world became uninhabitable due to radiation or an ice age, forcing people to retreat underground. And with those tropes came all sorts of settings for what was needed to sustain life below ground.
An underground city. On Earth, no massive residential underground metropolis had ever been built due to economic reasons and a lack of necessity. But in Bostain, the reasons to build one were overflowing.
Underground spaces were warm to begin with, safe from monster attacks, and connecting them via tunnels to external territories solved transportation issues. Even though it was an underground city carved into the mountains, it sat higher than the plains outside the range, so air circulation was manageable to a degree. Install railroads, and mobility problems were solved too.
Plus, defending narrow, elongated tunnels was far easier and more convenient than manning city walls, keeping monster raids at bay. There were issues like structural support and reinforcement, ventilation, and lighting, but Suhyun was confident he could handle them with magic.
Using the stones unearthed while expanding the underground city for construction materials, and mixing in bio-metal transmuted from monster corpses hauled from the surface, he could create sturdy buildings. Ventilation could be achieved by linking tunnels to the outside through the mountains and drilling vertical shafts. For lighting, he could magically draw in the light reflected off the mountain snow.
Inspired, Suhyun whipped up blueprints on the computer in seconds.
Artificial Semi-Biological Giant Construct Using Bio-Metal
Habitually titling it like a thesis, Suhyun poured out his ideas. First up: the heart of the underground city.
He designed a pseudo-living entity that drew mana from the ley lines connected to the Magic Realm, generating heat while devouring rocks and monster corpses fed by humans to produce bio-metal and gradually grow in size. Around this heart, Suhyun sketched the underground city humans would build.
A multi-dome structure using granite and bio-metal to withstand underground pressure. Twin mage towers harnessing heat from the heart and invading cold from outside. A magic TBM for tunneling to connect the underground city to the world beyond the mountains, complete with railroads. A bio-metal smelter processing monster corpses. A massive refinery using the heart's heat to smelt ores unearthed during excavation. A stone processing plant melting down fed ores into usable construction stone. Multi-refractive cables channeling external light via cold manipulation. Smart farms growing plants with that captured light and underground water sources.
It would hunt monsters emerging from the northern Magic Realm, transmute useless parts into bio-metal, melt excavated minerals into stone, produce materials to build its own body, extend roots outward to form a self-sustaining transportation network, and coexist symbiotically with Bostain's residents.
Suhyun wanted to start building the underground city right away, but he lacked materials. Even if the heart grew gradually using bio-metal, he'd need enough to fill a small house. Plus, the design required numerous key components from monsters with special abilities: at minimum, ones that spat fire hot enough to melt metal, sandworm stomachs that digested rock, and monsters with robust skeletons and shells.
In the old days, he'd just hunt them solo, store the parts in his inventory, and call it done. But now Nautilus existed, and he couldn't leave her.
With Suhyun as the only one who could pilot Nautilus, he couldn't separate from her. So, to explain where he wanted to go and why, he summoned all of Nautilus's executives.
On the bridge, after hearing Suhyun's plan, Nautilus's leaders were floored.
"Is this even possible?"
Creating a living furnace from metal, then using its output to develop the underground? Even for a princess who'd seen and experienced much, the scale was incomprehensible. Isolde, listening beside him, thought it sounded like the ravings of a madman.
But she couldn't outright deny it—Suhyun had already built Nautilus.
Before he'd done it, if he'd claimed he would, Isolde would've dismissed it as mad ravings. A city-sized mobile structure? How could it move, and where would the energy come from? She'd have laughed it off. But he'd made it real.
Impossible by common sense, yet the man who'd achieved it spoke with undeniable credibility.
"Even if possible, it'll cost a fortune. And compared to building elsewhere, it's less practical, harder, and time-consuming. Why bother in Bostain territory specifically?"
Aurelia's question drew a betrayed, shocked look from Eira. The princess suggesting Suhyun abandon Bostain for something better, ignoring their friendship—it stung. But Eira couldn't refute it.
No matter how little she knew of politics and economics, even she understood installing such a marvel in Bostain was a loss. She knew best that he had no obligation to spend vast sums and time on an artifact just because she'd asked.
But Suhyun's answer was simple.
"Because I want to, and I can."
Aurelia burst out laughing.
"Hahaha! The craziest thing I've heard. Don't worry about political or procedural hurdles—I'll handle them."
With Suhyun—the only one above her on Nautilus—and Aurelia on board, Daiyen had no choice but to agree.
"Sigh. I'll gather funds as best I can and find ways to make it viable."
Typically, underlings cleaned up after their superiors' bold moves. Only then could Eira, who'd stayed silent thinking it was purely for her sake, finally speak.
"Th-thank you so much, you know."
Leaving Eira's tearful gratitude behind, with the next direction set, Nautilus set off. Food prepped on Nautilus flowed to Bostain, while Bostain loaded timber, herbs, and monster byproducts aboard.
As Nautilus opened and locals flooded in for cheap food and goods—their first taste—Eira returned to the lord's castle.
"Father."
"You're back?"
Seeing her father penning letters to central nobles as always, Eira broached carefully.
"I will never give up on Bostain, you see."
"The longer you cling, the more it'll hurt you."
"Lord Suhyun gave me a solution, sir."
"You believe him?"
"He's far greater than you imagine, Father."
Her father looked up from his letter, studying her face. No longer the lost expression from their last parting, but resolute. He found it adorably precious, yet worry flooded him.
Like parents who know a child will fall but can't stop them from walking, the lord could only support Eira.
"I just want you happy, wherever, whenever."
"It will be, sir."
Even with one effective solution, no reason existed not to pursue others. Suhyun gathered warriors recommended by Eira.
Having fought monsters right beside the Magic Realm, most were Squire-level with mana accumulated in their bodies. But they'd never learned proper martial arts—time had been too scarce. What they knew came purely from battlefield improvisation: brutal, risky styles aimed at killing before being killed.
Suhyun had come to teach them how to protect themselves and slay foes.
"Is it really okay for us to be here?"
Bostain saw relentless monster descents—three waves a day was light; up to fifteen or constant combat otherwise. Seeing most wall defenders gathered here, one voiced concern. Suhyun replied.
"Don't worry. Our forces are manning the walls."
"No, even the strong falter against Magic Realm beasts—they're no joke."
Heukgu, beside Suhyun, chuckled at the worry.
No matter how dangerous the Magic Realm, could it top Suhyun's training hell?
"Then spar with me."
A veteran Bostain warrior stepped up first. The typical northern two-handed axeman eyed Heukgu.
"Ortan of Bostain."
"Heukgu of Nautilus's Rehabilitation Squad."
Words done, Ortan's axe crashed down on Heukgu's shoulder. Dodging reflexively, Heukgu tried deflecting the follow-up with his forearm.
But the moment skin met axe, Ortan's helmeted headbutted him.
"Kuh!"
Gasping at the unforeseen strike, Heukgu reset his stance, glaring.
"Blocking that? Impressive!"
"Thanks."
He wanted to curse a blue streak, but Suhyun watched, so Heukgu held it in. His trembling arm confirmed it: martial arts. The lingering shock despite the impact suggested a concussion-inducing head clash.
Unlike cautious Heukgu, wary of variables, Ortan charged like a beast.
In the squad, dual-wielding screamed easy prey; axes weren't favored either. Weight-forward for power, but too predictable.
To martial artists dodging blades paper-thin without fear, axes hit hard but telegraphed. Dual-wielding squared the difficulty, not doubled it. Single-sword left half-strength; blocking and loading weight got trickier.
But twin axes? Heukgu dodged the storm and headbutts in panic.
Axes were just chops—no overlaps, simple, so squared difficulty stayed manageable. Magic Realm mana-boosted power was no slouch, and unpredictable battlefield arts flustered Heukgu.
Seeing Ortan's kicked snow morph into icy spikes, Heukgu smashed them with a fist.
Endless axe rain seemed simplistic, but Ortan filled gaps with stamina and Magic Realm experience, earning Heukgu's admiration.
As Suhyun said, hyper-aggressive—one mistake fatal. But Ortan's age meant mastery.
Each chop dictated escape routes, pinning him. Feeling the "masters' fights are number games" truth in his bones, Heukgu bulled inward desperately.
"Didn't think a monster would close inside?!"
Folding his chopping arm for an elbow to the back, knee rising.
But Ortan didn't know Heukgu's specialty: joint locks.
Qin Na Shu () - Snake Foot Coiling Body ()
Caught in what jiu-jitsu called a heel hook, Ortan panicked. Joints on an armed foe?! He swung his axe—any angle worked—but agony tore his knee ligaments instantly.
Heukgu withdrew, chaining seamlessly.
Qin Na Shu () - Twin Snake Arm Break ()
Fluidly shifting leg to arm, Heukgu wrenched an armbar, snapping ligaments. Ortan surrendered.
"Kuh, no joke."
Limping with wrecked knee and arm, Ortan drew near; Suhyun healed him promptly.
"I yield his strength, but those joints are human-only, right?"
Suhyun grinned like he'd waited for it.
"Then let's have the others show."
Different warriors sparred Hwanggu and Cheonggu. Both defensive styles, but distinct: Hwanggu hardened his body to tank all; Cheonggu parried minimally, exploiting gaps surgically.
Blocking full-power axe swings and counter-punching for Hwanggu; piercing sword-shield gaps in one stroke for Cheonggu. The warriors had to concede.
From there, it was smooth sailing. Suhyun formed their dantians with Mind-Body Unity, teaching prepared arts.
Cold Ice Bodyguard Technique ()
Suhyun deemed their offense sufficient. What they needed: self-protection. The Cold Ice Bodyguard Technique stored ambient cold in the dantian, using it to shield the body.
"Good."
Ortan, the most senior and advanced, formed his dantian and wielded the technique instantly post-learning. Designed for mass adoption, it wasn't complex; lifelong cold acclimation made it intuitive.
Circulating, inner power flowed subsurface, absorbing cold. Easy accumulation was nice, but the armored solidity felt better.
Under attack, subsurface power flash-froze into hard ice. Boosting total inner power via cold absorption plus protection would strengthen Bostain warriors faster, safer.
Ortan's sole worry—young warriors dying easy—eased; grinning, he gripped his axe.
Master new arts quickest via strong foes; aid youths safely by hunting beasts. With many sharing his mindset, they'd hustle daily for satisfying fights.