The Difference in Diffusion Speed
"You're back, sir."
The first thing I saw when I stepped back into the house was Jang Yeongsu of Cheongseol Guild, wearing an apron.
And that wasn't all. In the living room, Yoon Jiseok was playing with Seojun.
"Where is everyone? It's just you two with Seojun?"
I asked Jang Yeongsu.
"Something's come up in Busan, sir. Not only Hunter Choi Taemin, but Seo Jinwoo and Guildmaster Kang Iseol have all rushed to Busan as well. And this—Hunter Choi Taemin asked me to pass it along to you."
Jang Yeongsu handed me the note my brother-in-law had left behind. It said that he had headed to Busan ahead of me and to contact him the moment I confirmed its contents.
I read it and immediately called my brother-in-law. Before the ring had sounded twice, that familiar voice came through the receiver.
"Brother-in-law? You're back in Korea?"
"Just got home. Where are you right now?"
At my question, he gave me a quick rundown of the situation. Team Leader Park Jaehyun of the Hunter Management Department and Seoyeon had both already set out for Busan, and thankfully, he added, they hadn't arrived yet.
I told him to hold off on entering the Busan area and wait for me, then hung up. I understood why he'd moved first given the urgency, but now that I was back, there was no reason for any of them to be heading toward an S-rank gate on their own.
"Oh, right. Where's Han Chaeyun?"
I asked Jang Yeongsu, who had been staring blankly at me while I was on the phone. He'd never met Han Chaeyun face to face, of course, but as the one managing the day-to-day affairs of Hyeolcheon Guild alongside Kang Iseol, I figured he'd at least know her whereabouts.
"Ah, Dr. Han? She should be nearby right now. Shall I call her over?"
"Nearby?"
I gave him an incredulous look, and the answer that came back was another surprise. In the few days I'd been in China, Hyeolcheon Guild had gone ahead and purchased nearly the entire apartment building where my sister's old home was located.
Everyone had paid fair market prices, apparently, and in Han Chaeyun's case, she was apparently making rushed use of one of the vacant units they'd bought.
It was unexpected, but not incomprehensible. I wasn't the only one who'd heard that a thousand Blood Heavenly Cultists had crossed over, so they'd prepared for it in their own way.
"No, I'll go see her myself."
I told Jang Yeongsu. Samsik was already on his way with Yuk Jurim and the Blood Shadow Squad anyway, so I had a little breathing room.
"Oh? When did you get back?"
Han Chaeyun was staying one floor directly below our apartment. Despite only a few days having passed, the unit she occupied was already a complete disaster—as chaotic as the interior of that abandoned building in Gyeonggi Province we used to work in.
"This is the formation we talked about before, all written up."
I handed her the book as I spoke. I had put it together during the idle stretches of my time in China, organizing the formations my master had left behind. The reason I was giving it to Han Chaeyun now was simple: if she studied these formations, she could open the door that led to wherever my sister might be.
Finding my sister was naturally my highest priority, and the only reason I was handing over this information only now was that organizing the formations from memory had inevitably taken time.
"Oh! Let me see!"
Han Chaeyun's eyes lit up as she took the book, and she immediately buried herself in its contents. Exactly ten minutes later, she closed it slowly and looked up at me.
"This is way more complicated than I expected?"
"...What's that supposed to mean? You said yourself that this alone would be enough to reproduce it."
"Reproducing it, sure, that's possible. The problem is that this isn't Murim—and the destination isn't Earth, either."
I couldn't help but frown at her utterly unbothered expression. It was too late for excuses like that when everything she'd just said was perfectly obvious.
"I knew that much already, obviously."
"Of course you did. But even taking into account how ignorant I am about formations, I thought I'd get a feel for it once I looked. This one's... a bit much. It's going to take longer than I thought. I'll say it flat out—the person who created this formation was a genius."
I nodded slowly at her words. I had no disagreement about my master being a genius, either.
"So, how long?"
"Charging in blind won't get us anywhere—I'd move faster if I understood formations properly first. Could you... teach me about them?"
I let out a sigh at the sight of her staring straight at me as she asked. Teaching her the basics of formations wasn't difficult—but it wasn't something she could learn in a short time either.
And with the situation demanding I head down to Busan immediately, I couldn't afford to spend that time on her alone.
Ah, Wi Cheonghan. That guy exists, doesn't he.
But in that moment, the perfect person to teach her came to mind. I didn't know how accomplished he actually was, but he hailed from one of the most renowned formation families in existence—at the very least, he could drill the basic concepts into her.
"I'll send someone who can teach you."
"...Well, it's a little disappointing it won't be you, Jaeseong, but okay."
Leaving Han Chaeyun to her strange mutterings, I let my eyes drift over the papers plastered across one wall of her room.
"Is this the Busan gate data?"
"Ah, yes, that's right. It's my analysis of the material the Hunter Management Bureau sent over just now. Jaeseong, do you remember what I told you before? How the returnees' points of origin fall into four broad categories?"
"...Murim, Terra, the Yoma World, and Arcana, was it?"
Han Chaeyun smiled wryly at my answer and nodded.
"Looking at the hunters active right now, returnees from Terra are overwhelmingly the most common, with Arcana and Murim roughly tied. But if you count pure return experience alone, the Yoma World might actually have the most returnees of all."
"Really? I've never laid eyes on a single one."
That was my honest reply. The returnees I'd encountered most often were, without question, from Murim, and I suspected Seo Jinwoo—and the old Jin Dohyuk—were from Terra. On the other hand, I'd never seen a returnee from Arcana like my sister, and the very notion of a Yoma World returnee was foreign to me.
"Naturally. Most Arcana returnees are scattered across the Western world, and as for the Yoma World ones—most of them just live ordinary lives."
"...Ordinary lives? Returnees?"
"Yes. You see, most people transferred to the Yoma World didn't survive long after the transfer. So even if they're technically returnees, they're hardly any different from ordinary civilians."
A question suddenly struck me at her words.
If they'd died almost immediately after being dropped there and come right back, it made perfect sense that they'd have no special abilities.
"But if they all died and returned that fast, how does anyone know the place is the Yoma World? Information about the environment itself would have been hopelessly scarce."
"That's what the gates are for, obviously."
Han Chaeyun adjusted her glasses as she said it.
"You remember when I told you that gate interiors were either points of contact with other worlds, or copies of them? Didn't it ever strike you as strange that you've never once seen a gate with a Murim-like setting?"
When I nodded mid-thought, she pressed on with her explanation, and I found myself nodding again. If gates really were copies of other worlds, or connections to some place at some point in time, then gates set in Murim should have appeared at some point too.
But every gate I'd entered had been packed wall to wall with monsters, with no connection to Murim whatsoever.
"That's only natural. Terra has accounted for an overwhelming share of the gates that have appeared so far."
"...Why?"
"It's simple. Including Earth, five worlds are blending together—and they don't blend evenly, all at once."
I looked at her quietly, and she gave a short sigh before starting to explain with an analogy.
"Put simply—if you imagine Earth as a bowl of water, then drop in ink, paint, oil, and honey, each spreads through the water at a different rate. It's what fluid dynamics calls a difference in diffusion speed, but let's not get into that. The point is, ink spreads through water in an instant, while honey just sits there. In this analogy, ink would be Terra and oil would be Murim."
I nodded as if I roughly understood. What mattered, after all, was simply that each world mixed into Earth at a different rate—and that a gate set in Murim might yet appear someday.
"So you're telling me this now because the gate that opened in Busan is special."
"Exactly. According to my analysis, the wavelength of the Busan gate is highly likely to be from the Yoma World."
"...And Japan barged in without knowing that?"
Han Chaeyun slowly shook her head.
"I doubt it. If anything, they probably went in knowing full well. Japan is currently the nation on Earth with the deepest ties to the Yoma World."
I nodded again. I didn't know much about the current state of affairs in Japan, but recalling the Japanese awakeners we'd encountered in the North not long ago, it made a rough kind of sense.
"So be careful. A Yoma World gate might as well be home turf for the Japanese awakeners."
"Thanks to you, I've just heard a very interesting story."
I answered her words with a smile. She'd given me knowledge I'd had no access to before, and it would undoubtedly prove useful in what was to come.
"I'll see you again soon."
With that, I rose from my seat. The Yoma World was certainly unknown territory for me—but that didn't mean I was worried. Geographic advantage was meaningless in the face of an overwhelming gap in power, after all.
When I came back upstairs, I found Samsik, Yuk Jurim, and the Blood Shadow Squad waiting in the living room.
"Half of the Blood Shadow Squad stays here and guards the house. The other half, along with Yuk Jurim and Samsik, comes with me to Busan."
"As you command!"
Hah. This is one godawful situation.
At that very moment, inside the S-rank gate in Busan.
Dobuchi Hikio, who had crossed into Korea alongside Sato Kenji under special orders, was letting out a hollow laugh as he surveyed the landscape inside the gate.
Dobuchi's orders had been simple: stay at Sato Kenji's side and find out how the man had suddenly come into his newfound power.
Even so, the few days he'd spent with Sato Kenji had taught Dobuchi quite a lot. He'd been given that mission in the first place precisely because of the peculiarity of his ability.
Dobuchi possessed the power to quantify and read other people's abilities and conditions, as though they were a character's stat screen in a video game.
And the status window of Sato Kenji as it appeared through Dobuchi's eyes was, frankly, the very definition of grotesque.
Strength, magic power, agility—every single stat is still climbing. But the more they rise, the more his sanity seems to be slipping away.
Even without any special ability, anyone could see that Sato Kenji's condition was deteriorating into something inhuman. His speech had grown more slurred by the day, and his behavior more simplistic and violent.
The problem was that Dobuchi still hadn't pinpointed where Sato Kenji had gotten his power—but he'd judged that confirming that much wasn't his job anyway.
Maybe it would have been different in the past, but these days, it was impossible to exit a gate once it had been entered without clearing it.
Since his ability was largely useless in combat anyway, Dobuchi had been planning to use that as his excuse to slip out of the gate at the entrance.
"N-no can do. If you're in, you're in—till the end... together."
But with those words, Sato Kenji had flung Dobuchi bodily into the gate. And so Dobuchi, who had been trying to bow out gracefully, found himself forcibly dragged into the S-rank gate.
And what greeted Dobuchi—who had tumbled through first by sheer accident—was an endless horde of monsters surging up along the coastline.
"...Kuso. No wonder I never wanted to leave the house."