A dim evening.Near the old warehouse of Eredor Territory, where the Alchemy Club used to be located, stood an abandoned school building.As Ciel and I waited by its entrance, a small light in the darkness slowly approached.After a moment, a female student appeared, illuminated by a flashlight.A girl with a large backpack on her back and a camera for recording hanging from her neck — it was Chloe from the Newspaper and Broadcasting Club.She was currently working part-time as a reporter, but her actual major was in the Dilettante Academy’s Archaeology Department.She would be of some help in today’s ruin exploration.Pushing through the grass with a tense expression, she finally spotted us and sighed in relief."Why did you pick such a creepy place for a meeting this late? They say ghosts appear here. Haven’t you heard the rumors?"Chloe said that while shining her flashlight over the building.The structure was completely broken, with exposed steel bars inside.The windows had long since shattered, and cobwebs and overgrown weeds were everywhere.Even in daylight it would have looked bleak, but the darkness of the hour made the place all the more eerie.The site we were to explore today was one of the Academy’s Seven Great Mysteries — the origin of the “Abandoned School Building Ghost Story” in Eredor’s old school.Strictly speaking, it was the ruin — or dungeon — located beneath it.To reassure the nervous Chloe, I greeted her casually."I’m glad you found the way. I’ve been reading your recent articles. Sales numbers doing any better?""Thanks to the information you gave me — about the grading ratio being adjusted to five percent minimum — I did make a bit of profit."Chloe nodded with a reluctant expression.These days, her stock within the Newspaper and Broadcasting Club was rising fast.After her article about the Territorial War exploded in popularity and her piece on the practical exam showed a sympathetic tone toward Verdandi, she’d become a hot topic in more ways than one.And thanks to the information I’d tipped her off about, circulation numbers went up, earning her a nice bonus.Even the amount of sponsorship money coming into the Union Daily had increased considerably."Before coming here, you also wrote an article about the Seven Great Mysteries.""Yes. Like you said, the reaction was surprisingly good. I thought those old tales were too outdated to trend again.""There’s a reason things that sell well do so. Nothing becomes a bestseller by accident."Mysterious phenomena occurring within the Union Academy and the ancient ruins said to exist somewhere — those were more than enough to pique the curiosity of students who were emotional and full of wonder.And if you numbered and systematized them, the effect multiplied.Just like the Seven Wonders of the World, or the Seven Millennium Puzzles.Of course, such things were only fleeting trends — once a mystery was fully revealed, attention would drop sharply…'Still, every year graduates leave the Union and new blood fills the gap.'Each time, these Seven Mysteries would be brought up again and capture everyone’s interest anew."If you write an article about tonight’s event too, the response will be good.""Yes, I already announced it as part of a series."In that article, Chloe had vividly introduced the Union’s Seven Great Mysteries, ending with a declaration that she herself would venture into the [Abandoned School Building Ghost Story] to uncover the truth.Regardless of her photography skills, her ❀ Nоvеlігht ❀ (Don’t copy, read here) writing had a distinct feel to it — enough that many students found the article engaging.Survey results showed that most readers were waiting for her follow-up after the actual exploration.Chloe looked at the abandoned school and said, "But does this place really have a path leading underground?""It does.""According to the records of the previous seniors’ investigations, they didn’t find anything like that."In fact, the Seven Great Mysteries of the Union had been continuously investigated for years by the Archaeology Department.Because of that, the material was considered completely worn out — within the department, no one treated it as anything more than an old topic.'They think the place has already been completely figured out.'That was why Chloe had opposed turning it into an article in the first place."Structurally, it’s impossible for a large ruin to exist underground in the Union. Below the surface are the sewer systems, and under that is the Underground — the massive underworld city.""True enough. Half true, at least. Anyway, let’s go."Instead of explaining further, I stepped into the building.When I took the lead, Ciel silently followed behind, and Chloe — glancing nervously around — hurried after us, clearly uneasy."Explain more! And if we’re exploring, why not do it on a weekend? Why on a weekday night? Don’t you have lectures tomorrow?""I already got the professor’s permission."Tomorrow morning’s class was Zimmer’s Basic Swordsmanship lecture.Since he and I were working together, he excused me as a kind of field study."What? Then what about you?"Startled, Chloe turned to Ciel, who replied calmly, "I used my leave days."At that, Chloe clutched her head in despair."You should’ve told me in advance! I have archaeology class tomorrow morning!""That’s unfortunate.""…….""But don’t worry. If you submit this exploration report to your professor, attendance points will mean nothing. Isn’t that right?"Since Dilettante was an adventurers’ academy, its curriculum was very liberal.Attendance contributed little to overall grades — exploration and research reports carried far more weight."Fine, fine! I’ll go! But if this turns out to be nothing, you’ll regret it!"***We advanced deep inside the abandoned building, relying on the light of a mana flashlight.Rustle—The sounds of insects and tiny creatures scurrying about.Probably small animals like snakes, rats, or cats that had made this place their home.Even the Dark Rats I’d exterminated before had been found near here.Crackle—Each time a sound echoed, Chloe flinched and started complaining loudly."Ah, damn it! That rat scared me!""……."Grumbling all the way, she followed us to the center of the abandoned school — what must once have been the main hall of this large structure.Step—I stopped and looked up.Through the gaping hole in the ceiling, I could see the vivid night sky.The central hall of the four-story ruin looked as though a beam of light had pierced straight through it — a massive hole running from the first floor all the way to the top.Through that opening, starlight and moonlight poured down, spreading across the floor.It had a mysterious, almost stage-like atmosphere, as if under a spotlight.Chloe looked up at the layers of collapsed ceiling."There are records saying the seniors also explored this area. It was said there were clear signs of artificial destruction. Maybe if we search carefully, we’ll find some clues—""Ciel, break this floor.""Yes."As I stepped aside from the moonlight, Ciel silently approached and struck the spot with her fist.Bang—!The shock shook the ground, sending up clouds of white dust."Aaah! You can’t just smash things! You said we’re looking for ruins!"Her near-screaming protest was understandable.In archaeology, site preservation was basic — and here we were breaking it apart bare-handed.When the dust settled, the marks of destruction came into view.It looked as though a meteor had struck — the floor beneath us was utterly shattered.I knew Ciel was strong, but I hadn’t expected this level.I felt a twinge of fear looking at the crater."…That felt like you put emotion into it.""Just your imagination, probably."Ciel blew the dust off her hand and stood quietly beside me.Chloe came running, shouting, "You can’t just destroy it like that! There might’ve been clues left we couldn’t see—""Quiet. Look down."I cut her off and pointed to the ground.Beneath the thick concrete Ciel had crushed was a dark steel door.It was a dungeon entrance made of [Eternal Cold-Iron].True to its reputation for extreme hardness, even Ciel’s blow — which had shattered concrete like tofu — hadn’t left a single scratch on the metal."What, there was something like this under the floor…?"Chloe gaped in astonishment, quickly taking out her camera to capture the scene.Click—She photographed the broken floor, the exposed dark metal door, and the two of us standing above it.Flashes went off several times as she recorded the moment.Ignoring her, I brushed away bits of concrete with my boot, then knelt down and wiped the dust from the surface.As the dust cleared, an engraved grid pattern appeared on the door.Three squares across, three down — nine boxes forming a larger square, with numbers carved into some of them.'A Sudoku puzzle.'A simple number puzzle where each row and column must contain the numbers 1 through 9 once, and each 3×3 box must also include all digits without repetition.Normally, it wasn’t difficult.'But the peculiarity here is that only ten numbers are filled in.'Which meant there wasn’t just one solution — countless possibilities existed.And since this was a mana-based Sudoku, each number had to be entered using magic power. Too many attempts would cause mana depletion and unconsciousness.Chloe frowned as she looked at the puzzle."This’ll… take some time to solve.""Oh? You know what this is?""Of course! I told you, I’m in the Archaeology Department.""How long would it take to solve?""Hmm, if it follows normal Sudoku rules, maybe about a week? Faster if we bring students with plenty of mana. We’re good at this stuff. Judging by the craftsmanship, I’d guess it was made about four hundred years ago, during Emperor Cornelio’s era—""That’s enough. I don’t need a lecture."I cut her off before she could go on."Tch, then we’re heading back, right? We can’t solve it now anyway."Chloe spoke hopefully, but I shook my head firmly and checked my watch."No."Then I looked up at the bright full moon hanging in the sky.Tick—I calculated the precise moment when the moonlight would pass directly over the steel door.Hum—I stretched out my hand and infused mana into the door.The blue light pooled on the surface instead of being absorbed — Eternal Cold-Iron rejected my mana.Yet the shimmering mana reacted with the carved grid and numbers, revealing several hidden digits.Flash—"Oh! The numbers…!"New ones glowed on the puzzle, illuminated in blue.Now the Sudoku grid contained seventeen starting numbers — the minimum required for a unique solution.I withdrew my hand from the door."Now you can solve it quickly, right, Chloe?"That was why I’d come here on a weekday night instead of a weekend.To solve this puzzle swiftly, it needed the moonlight imbued with the full moon’s mana.Before the Sports Festival, I needed to obtain the blueprints for the Carbonated Water Generator and begin production. Tonight was the only window."Y-yes! Leave it to me!"Setting down her large backpack, Chloe put on her glasses and focused silently on the puzzle.Before long—"I-I did it!"Flash—With Chloe’s exclamation, the steel door flared with light and began to tremble.Rumble—The ground shook again as the door slowly opened.Watching the sight from a short distance away, I smiled faintly.'…The blueprints for the carbonated water generator should be down in this dungeon.'And if an artifact or trait suited to me awaited as a clear reward, that would be perfect."Let’s go in."

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