What is a Dungeon Core

Quite Simply

a dungeon core is a philosopher's stone. It also has elements of a nuclear reactor and a computer, but it is still the zenith of alchemical perfection and godcraft. As a philosopher's stone, the core is a catalyst of transmutation, converting matter and energy (mana) with equal ease, changing matter on a fundamental level, converting the elemental aspects of mana, and even turning matter into mana, and mana into matter. Sentient attempts at creating transmutation at this scale are simply impossible, and the handful of successful philosopher's stones created are not equal to the capabilities of a single mediocre dungeon core, combined.

On the nuclear reactor level, a dungeon core can use its abilities to convert matter and energy back and forth, which means that even low-density matter can be quickly converted into mana. Mostly this is how it powers the creation of its dungeon ecospace and creates its internal layout, which in turn is patterned in such a fashion as to naturally accumulate mana and condense it. Accumulated and condensed mana can be used to create things within the core's sphere of influence without requiring the conversion of matter into mana.

And in the matter of the computer, a core retains the fundamental pattern of everything that it has absorbed within its sphere of influence. Any items lost in a dungeon become part of its materials dossier, the same going for monsters, animals, sentient beings, and the rest. A creature that dies in a dungeon and isn't recovered becomes part of its portfolio, not the specific individual but their species and their likeness. More advanced cores can start advancing and making changes to these patterns, to improve on them, or make them more fitting to its own personal preferences.

Who Knows This?

No one, this is meta-level knowledge, unless somehow the Numinous Road snagged a nuclear engineer, a medieval alchemy enthusiast, or someone else fluent in matter/energy transmutations.

Why Is This Not Known?

Previous civilizations in the DungeonVerse did reach levels of magical sophistication so that they did learn and understand the fundamental nature of dungeons and dungeon cores. As sentient beings generally have the same socio-economic interests, this previous level of civilization made major inroads in dungeon studies, and found ways to capture dungeon cores and control them, allowing them to power a high magic level civilization on par with the typical crystal spires and togas magitek.

Then they screwed up.

Unlike modern power sources, utilizing a dungeon core as a power plant doesn't have the same ecological trade-offs. There are no smokestacks, no transportation system to keep them fed with coal, and no transmission lines, the system is magical and invisible. As demand increased, the captured cores were placed under greater and greater mana draws. This caused stress fractures in the cores and started causing fluctuations in the flow of natural mana through the DungeonVerse plane. Reflections and emanations of the DungeonVerse started collapsing, entire dimensions dwindled as their stars and mana were devoured to power magical showers, self-filling and heating coffee cups, and endless entertainment venues.

The cores started failing. The tertiary and subsidiary cores first. These suffered non-catastrophic failures, the cores venting their mana load and just burning out. They were small, and their containment deep in their dungeon spaces limited the damage to a minimum. As these failed, secondary cores started overheating, and the thaumaturgists worked on cycling them in and out of use, creating rolling mana blackouts across the magical empire. The catastrophic failure of secondary cores was noteworthy, but as secondary cores were not near the major cities, this information was easily concealed.

Wars began, rebellions and riots broke out, and there were widespread problems across the magitek civilization. Mana enrichment for farming was failing, so there were famines and food shortages. There were strange outbreaks of unknown diseases, the undead appeared at random intervals, and there were incursions of terrifying bugs, superpredators, and other horrific events that were coming to light. Municipal utilities and services started failing. The cadres of unseen servants vanished, the trash pits quit eating trash, water supplies quit, and all of the magical supercilious hypertech failed.

Then the bad shit started happening.

The Imperial Capital was lost in a thaumaturgical cataclysm of explosions, mana storms, and monstrous incursions.

The largest and wealthiest city of the civilization died a horrific death.

The Terrasque made its first appearance

Entire species vanished in a series of racial pogroms, exterminatus magics, or were consumed by cosmic horror level monsters.

It was the end of the world, and this calamity echoed across all of existence, and even reverberated through the membranes to make entire other omniverses shudder with how close everything came to collapsing and all of reality vanishing back into the primordial formless chaos.

But there is good news, this all happened twelve thousand years ago.

The end of the High Magic era was twelve millennia ago, and as such, very little remains of that time period. The most prominent examples are the wastelands and dead cities that have survived. Things like books, written lore, and the rest of that have long since vanished. The races that populated the High Era are also gone because it wasn't elves, humans, dwarves, and orcs. These races have all risen after the fall of that time. To delve into the secrets of High Thaumaturgy is the task of trying to penetrate into a time period predating the modern races, modern magic, and even the modern dungeons.