What exactly is a dungeon core?

[Greywander]

Dungeon cores seem to be unique to dungeon sims games and their novelized form of dungeon core fiction. They're not the same as a powerful entity who just happens to own a dungeon. A dungeon core is a sentient dungeon, with the core acting as its brain. The dungeon core can be a creature, but they usually can't leave the dungeon, and their power is typically limited to within the dungeon's territory.

I understand what a dungeon core is as a concept, what I want to know is what the purpose of a dungeon core is within a given setting? Why would adventurers/civilization seek to destroy them? Why wouldn't they seek to destroy them? What are a dungeon's goals? How are they made? What do they do?

Now, there's a very obvious meta-answer in that it works a lot better narratively to give a sentient structure a load-bearing macguffin rather than forcing the dungeon's enemies to fully dismantle the dungeon with a demolition crew. It's the same reason the first Death Star had that vulnerable exhaust port. There's also a meta-answer in that it allows for the reconstruction of a lot of classic dungeon tropes that otherwise don't make sense when you think about them too hard. The reason the dungeon never runs out of treasure is because the dungeon core makes new treasure to attract adventurers. It never runs out of monsters (and those monsters are always combatants without families) because they're constructs also created by the dungeon to protect itself. Usually, killing adventurers grants power to the dungeon, allowing it to grow bigger, spawn stronger monsters, and create better loot. In extreme cases, this can even lead to an MMO style dungeon based economy, where repeated runs of a major dungeon are a cornerstone of the local economy.

But I'm looking more for the in-universe answer. One such answer I've seen, which seems to be a stock answer to this question, is some kind of mana management. A dungeon either whips up stagnant mana to prevent magical dead zones from forming (especially important if mana is also vital for life), or it calms down turbulent mana to prevent wild magical surges or non-dungeon monsters from appearing and running amok. I find this answer to be rather bland.

My preference is for something where the dungeon acts as an actual dungeon; a prison meant to contain something. For example, perhaps there exists evil that can be defeated but not destroyed. After defeating such an evil, a ritual can be performed that gathers up that evil and contains it inside a dungeon core, and so long as the core remains intact that evil cannot escape back into the world. The core draws power from the evil contained within, which is what allows it to spawn monsters, e.g. a lich imprisoned inside a dungeon core might create an undead-themed dungeon. To take this a step further, perhaps each instance of evil is but one shard of an evil god's essence, and if every single dungeon were somehow destroyed, it would herald the resurrection of that evil god. The dungeons themselves can be morally good or evil, so despite the fact that it will release the evil sealed within it might be worse letting that particular dungeon do as it pleases.

Another idea I had involves a primordial god who was imprisoned by his fellows in a dungeon dimension. In his search for a way to escape, he puts curses on adventurers that drains their life energy and causes them to turn into a dungeon when they weaken. Other adventurers then raid those dungeons and get cursed from the loot, continuing the cycle. Those adventurers who learn the truth about the curse will then look for a way to break it before they succumb, and the imprisoned god in turn watches their efforts to see if he can learn any trick that might help him escape. This one is setting specific, and this is a mile-high view of the overarching plot I've been working on, so there's a lot of detail that I've left out for the sake of brevity.

Another option is for a defeated entity to revert to a dungeon core while it gathers power to reassert a corporeal form. This is one I've actually seen used in some dungeon sims. My main issue is that there isn't much room for diplomatic nuance; someone killed you and won't be happy to see you return, but a different faction might support you and actively work towards empowering your return to physical form. Even if you're, say, a celestial, rather than a demon king, your allies and enemies will generally be set in stone already. This works better for an evil dungeon, where diplomacy was never an option, whereas part of what makes a good dungeon interesting is navigating the competing interests of adventurers and kings and churches without them thinking you're a threat to them. If you want a story where there's actually a debate if people should try to destroy a dungeon core, this isn't the best option to pick.

I feel like there's a balance to be struck between viewing a dungeon as enough of a threat that the local government would consider destroying it, while also viewing it as either something helpful to have around or something vital to the natural order so that they'd be reluctant to destroy it unless necessary. Also, for a dungeon to have a defined purpose to work towards, which will likely tie in to its origins. Characters without goals don't make for good protagonists.

What are some other reasons dungeon cores might exist? I'm particularly interested in ideas that involve the dungeon actually imprisoning something (you know, what dungeons are supposed to do), but I'm sure there's also a lot of other interesting origins and goals for dungeons. As an aside, I'm curious if anyone has ever featured a dungeon core in their TTRPG campaign? Like I said, they're almost exclusive to dungeon sims, but there isn't any reason they couldn't work to implement in a more traditional D&D type game.

[thethird]

I have done two things that are comparable to dungeon cores in TTRPG before in my homebrew setting:

Dragons grow big, like very big, in my setting. Part of their diet is based on metal, they turn this metal into draconic versions of the metal (in the scales, claws, fangs, etc) and helps them grow. The more refined (pure) metals taste better and are more nutritious, thus dragons being sentient and intelligent prefer that. But a dragon on the wild, or someone whose hoard has been taken, can start eating ground looking for metallic traces on it. That kind of dragon will grow in size way faster while having it's intelligence devolve into a more bestial take. Those dragons normally end eating animals, monsters, or adventurers with their rocks (they are fond of mines). These eaten creatures are regurgitated by the dragon transformed into dragon-y versions of them and inhabit the dragon's insides. The dragon becomes the dungeon. The dragon's heart becomes the dungeon core. Dragon's materials are very valuable, and "dungeons" are a good source of it. Also dragon's are very good at refining those materials, smelting technology isn't advanced enough to replicate dragon enhanced metals, even if it should theoretically be possible.

You can put souls (or bits and pieces of souls) into objects. Liches' philacteries are the most clear example of this but it's also a way to get intelligent items. Many liches will get bits and pieces of their victims souls put into objects coaxed and controlled by the powerful soul energy of their philactery. Useful items to control are things like gates, traps, sensors, and other more random stuff. All these things are supeditated to the lich's philactery. If you take out the lich you can get partially animated objects, which might have their uses, and most people capable of replicating them are working soul magic (the path to lichhood) which is frowned upon. If you are willing to convive with the lich's philactery then you have a very nice and convenient lair.

[aimlessPolymath]

Disclaimer: This is actually inspired by something I read once on a D&D blog, waaay before I ran across dungeon cores in fiction. I actually like it a bit better than the MMO-like dungeons I see in fiction today.

I somewhat like the idea of a dungeon core as living architecture- not necessarily something that spawns monsters and treasure, but a sort of living building that 'grows' wider and deeper over time- somewhat analogous to an enormous subsentient burrowing mimic. The 'core' in this analogy is the living heart of the dungeon. They reproduce via spores, which are spread via visitors entering and leaving, and must take root in magically active places.

Why do monsters come and inhabit the dungeon? Maybe there's some perks to living there- in order to protect itself, the dungeon's aura sustains monsters, letting them nest there indefinitely without needing to find food or water. The dungeon controls creatures indirectly, controlling populations in aggregate through a gentle touch- a clan of rust monsters might find themselves gradually migrating, moved around to to avoid conflict with a hobgoblin clan. Nearby monsters might find themselves drawn to the dungeon if it's too empty, but if nobody comes in to visit, some monsters might find themselves pushed out entirely, forcing them to travel (and spread the dungeon spores as they do).

As for treasure, that's a way to handle things the dungeon finds indigestible- gold and gems are minted out of the earth (for the sake of argument, let's say this fantasy earth has a greater abundance of these things in its crust). Magic items are created to deal with indigestible magic- a dungeon that intersects a leyline (as most would once they grow big enough) condenses excess magic to create enchanted items, with aesthetics informed based on the dungeon's ancestry, just like the dungeon's apparent 'architecture'.

One consequence of this model is that dungeons tend not to refresh themselves, other than to create magic items (and this, itself, can be twiddled if desired, just by adjusting the rate of influx from the environment). Gold and gems, for example, are only mined where the dungeon is actively growing, and monsters only arrive if there's an entry point for them to access.

This means that dungeons aren't really a renewable resource- once a dungeon gets cleaned out, there's not much point in coming back for the next few decades. Adventurers returning to a previously cleared dungeon might find that most of the loot on higher floors is in the form of minor magic items. Dungeons with their entrances in cities might not be able to repopulate with monsters at all, meaning that the economy around them is based more around walking through the floors and picking up items that are 'ready to harvest'. (Presumably, the longer an item is left to absorb power from the dungeon, the stronger it gets- and floors near to the surface might accumulate power slower)

if this 'farming' model is undesirable, one twist would be to say that dungeons require monsters within to survive and continue to produce loot- if an organization wants to keep their dungeon productive, they're going to need to send in adventurers who are able to fight nonlethally, or import dangerous beasts to restock any monsters that were accidentally killed.

[Rynjin]

I feel like Mark of the Fool did it best, of the fics I've read with dungeon cores as a mechanic. While the exact reasons are still a mystery, it's actually a part of the plot for once so answers should be forthcoming by the end of the story.

What we know so far is more than most fics bother to figure out.

1.) The dungeon cores are deliberately designed by a creature known as "The Ravener" who spawns them once every 100 years or so to terrorize a specific country.
--1a.) At the same time, the country's patron god bestows five Heroes with supernatural powers to combat the Ravener.

2.) The Ravener may be some sort of magical AI equivalent. Some snippets of chapters have been from its perspective, and it appears to have deliberately built-in restrictions it must abide by until certain conditions are met, at which point it can escalate in certain ways. It cannot escalate without meeting these specific metrics or conditions, and its responses to certain criteria appear to be exact. If Scenario A, unleash Monster B kind of stuff.

3.) Dungeon cores can be analyzed, broken down, and repurposed.
--3a.) They can also be controlled by certain people who meet specific criteria (magic-users who worship the aforementioned patron god)
--3b.) The essence they're made of is an extremely efficient catalyst and accelerant, and can explode with equivalent to nuclear force (though with no fallout).

The prevailing theory currently, based on most evidence, is that the Ravener is a construct created by the patron god in order to deliberately terrorize the populace at regular intervals for unknown purposes. Dungeon cores therefore serve as a sort of "access terminal" for godly power, being usable to terraform regions and create life from nothing.

[Yanagi]

Dungeon cores seem to be unique to dungeon sims games and their novelized form of dungeon core fiction. They're not the same as a powerful entity who just happens to own a dungeon. A dungeon core is a sentient dungeon, with the core acting as its brain. The dungeon core can be a creature, but they usually can't leave the dungeon, and their power is typically limited to within the dungeon's territory.

I understand what a dungeon core is as a concept, what I want to know is what the purpose of a dungeon core is within a given setting? Why would adventurers/civilization seek to destroy them? Why wouldn't they seek to destroy them? What are a dungeon's goals? How are they made? What do they do?

Now, there's a very obvious meta-answer in that it works a lot better narratively to give a sentient structure a load-bearing macguffin rather than forcing the dungeon's enemies to fully dismantle the dungeon with a demolition crew. It's the same reason the first Death Star had that vulnerable exhaust port. There's also a meta-answer in that it allows for the reconstruction of a lot of classic dungeon tropes that otherwise don't make sense when you think about them too hard. The reason the dungeon never runs out of treasure is because the dungeon core makes new treasure to attract adventurers. It never runs out of monsters (and those monsters are always combatants without families) because they're constructs also created by the dungeon to protect itself. Usually, killing adventurers grants power to the dungeon, allowing it to grow bigger, spawn stronger monsters, and create better loot. In extreme cases, this can even lead to an MMO style dungeon based economy, where repeated runs of a major dungeon are a cornerstone of the local economy.

But I'm looking more for the in-universe answer. One such answer I've seen, which seems to be a stock answer to this question, is some kind of mana management. A dungeon either whips up stagnant mana to prevent magical dead zones from forming (especially important if mana is also vital for life), or it calms down turbulent mana to prevent wild magical surges or non-dungeon monsters from appearing and running amok. I find this answer to be rather bland.

My preference is for something where the dungeon acts as an actual dungeon; a prison meant to contain something. For example, perhaps there exists evil that can be defeated but not destroyed. After defeating such an evil, a ritual can be performed that gathers up that evil and contains it inside a dungeon core, and so long as the core remains intact that evil cannot escape back into the world. The core draws power from the evil contained within, which is what allows it to spawn monsters, e.g. a lich imprisoned inside a dungeon core might create an undead-themed dungeon. To take this a step further, perhaps each instance of evil is but one shard of an evil god's essence, and if every single dungeon were somehow destroyed, it would herald the resurrection of that evil god. The dungeons themselves can be morally good or evil, so despite the fact that it will release the evil sealed within it might be worse letting that particular dungeon do as it pleases.

Another idea I had involves a primordial god who was imprisoned by his fellows in a dungeon dimension. In his search for a way to escape, he puts curses on adventurers that drains their life energy and causes them to turn into a dungeon when they weaken. Other adventurers then raid those dungeons and get cursed from the loot, continuing the cycle. Those adventurers who learn the truth about the curse will then look for a way to break it before they succumb, and the imprisoned god in turn watches their efforts to see if he can learn any trick that might help him escape. This one is setting specific, and this is a mile-high view of the overarching plot I've been working on, so there's a lot of detail that I've left out for the sake of brevity.

Another option is for a defeated entity to revert to a dungeon core while it gathers power to reassert a corporeal form. This is one I've actually seen used in some dungeon sims. My main issue is that there isn't much room for diplomatic nuance; someone killed you and won't be happy to see you return, but a different faction might support you and actively work towards empowering your return to physical form. Even if you're, say, a celestial, rather than a demon king, your allies and enemies will generally be set in stone already. This works better for an evil dungeon, where diplomacy was never an option, whereas part of what makes a good dungeon interesting is navigating the competing interests of adventurers and kings and churches without them thinking you're a threat to them. If you want a story where there's actually a debate if people should try to destroy a dungeon core, this isn't the best option to pick.

I feel like there's a balance to be struck between viewing a dungeon as enough of a threat that the local government would consider destroying it, while also viewing it as either something helpful to have around or something vital to the natural order so that they'd be reluctant to destroy it unless necessary. Also, for a dungeon to have a defined purpose to work towards, which will likely tie in to its origins. Characters without goals don't make for good protagonists.

What are some other reasons dungeon cores might exist? I'm particularly interested in ideas that involve the dungeon actually imprisoning something (you know, what dungeons are supposed to do), but I'm sure there's also a lot of other interesting origins and goals for dungeons. As an aside, I'm curious if anyone has ever featured a dungeon core in their TTRPG campaign? Like I said, they're almost exclusive to dungeon sims, but there isn't any reason they couldn't work to implement in a more traditional D&D type game.

Okay...admittedly running at this with no experience in the genre, so YMMV.

Two ideas off the top of my head.

One...a dungeon core is a kind of algorithmic construct: it has instructions to do a thing--guard something specific--but has the ability to both appropriate available resources and alter itself to hone that ability. Much like a neural net, it has been trained to perform it's task, but then as it takes in new information, it begins to form it's own heuristics and base assumptions. This results in quirks and misfires that are within acceptable tolerances, as long as the central command has been fulfilled. But then you have dungeon cores that continue to exist in a state of failure--the central thing to be hidden or contained has already been busted out--but continue to do their task because they have no option to not do it. Like a golem or an AI chatbot, the core can be very "smart" at it's task but have no greater comprehension...but also it can accept a faulty premise early on and everything it plans thereafter bends around that premise.

Everything about dungeons that players interact with can then be seen as downstream of the management/economy by the dungeon core: the structure is a kind of enclosed logic system in which the core attempts to create the "best" containment system, including stress testing it's system with adventurers.

Two...and the one I like rather better...is that a dungeon core is like a mononoke, a bunch of twisted-up intent that takes on a life of it's own. At some point, there was a person with an intent to conceal or imprison something, and a thing/person that was locked away: the dungeon core is the byproduct of those intertwined drives--the desire to contain and hide and the desire to reveal and escape. The dungeon is the physical manifestation of the struggle between these concepts--it ages, it grow in size and complexity, as those two stresses stayed bottled longer and longer.

There's no specific intent, no strategic thinking: the spiritual tension grinds and the result is this Kafkaesque manifold that both invites and rejects exploration. The thing that starts this process isn't even precious or special in some objective way: a child buries a broken item to hide it from their parents, a hasty grave is dug after an impulsive killing fueled by shame. And from this the dungeon grows.

Everything about dungeons the players interact with is the side effect of the spirit of the place: there is a dungeon because here is where you conceal and imprison. The structure is a taproot spreading from that seed of intent: over time, the "dungeon" has been formed as beings have been drawn to the location and done what comes naturally...dig deeper, set up more traps, hide more precious things...but even the nature and supernature bends around this premise; caves form and refuse to obey geological rules of erosion and watershed, animals go into the dark and acclimate, settle, make lairs counter to all their instincts to roam and hunt.

[Greywander]

I have done two things that are comparable to dungeon cores in TTRPG before in my homebrew setting:
[...]
The dragon becomes the dungeon. The dragon's heart becomes the dungeon core.

This is such an interesting and organic (literally) way of having sentient dungeons as a natural feature of your world. It wouldn't be the first time I'd played a game where one of the levels was inside a dragon.

Really gives new meaning to the phrase, "Dungeons and Dragons".

Many liches will get bits and pieces of their victims souls put into objects coaxed and controlled by the powerful soul energy of their philactery. Useful items to control are things like gates, traps, sensors, and other more random stuff.

I'm not sure if that would count as a dungeon core, but it gets weirdly close. It kind of feels like it ends up in the gray area in between, but instead of starting with a dungeon core and tweaking it to be less core-like, you're starting from the other side and taking something that definitely isn't a dungeon core and giving it dungeon core-like properties. Regardless, there's definitely interesting potential there, though these types of dungeons would be limited to liches. A lich core, if you will. That said, there's no reason these would need to be the only dungeon-like structures in the world.

I somewhat like the idea of a dungeon core as living architecture-

I might be getting a little hung up on the "dungeons are supposed to be prisons" thing. If you expand the concept to include any kind of living structure, there's a lot of potential to do things outside the scope of a typical dungeon core story. For example, one idea I had for a video game was that the player controls an AI that is running what is basically a city inside a nuclear/apocalypse shelter after it has been sealed, and the AI goes rogue. There's be lots of different scenarios to play, including ones where you save humans rather than kill them, but the most basic one is to kill every human living in the city before they destroy the computers that the AI is running on. Those computers are basically your dungeon core, but the premise is very different from what you'd expect. It wouldn't really be a dungeon sim.

This could also make a leap to the related genre of "ship core". After all, isn't a space ship just a mobile dungeon IN SPACE? By broadening our thinking we can bring in related ideas we might not normally consider. At some point, I do feel like it's good to draw up limitations so that you're not all over the place (i.e. you need a specific definition of what a dungeon is and how it works, or else anything could be a dungeon, and then it's no longer meaningful to call something a dungeon), but you can put those limitations in slightly different places from where they were before in order to end up with a fresh take on the idea. I'm interested in video game development, and I've done some brainstorming on using this technique to develop video games; reducing an existing genre to its most foundational elements, tinkering with those elements, and then rebuilding the genre as something new. For example, a platformer where you can't jump. You might think "it could never work", but when you put such a strict limitation on yourself you tend to get creative with what you have left.

Okay...admittedly running at this with no experience in the genre, so YMMV.
[...]
the dungeon core is the byproduct of those intertwined drives--the desire to contain and hide and the desire to reveal and escape. The dungeon is the physical manifestation of the struggle between these concepts--it ages, it grow in size and complexity, as those two stresses stayed bottled longer and longer.

I think you nailed it, actually. I find this model for dungeons rather fascinating. If I understand right, you're basically saying that there are two personalities of sorts: the warden and the prisoner. The warden wants to keep the dungeon hidden and to kill any intruders with traps and monsters. The prisoner wants to entice rescuers to come with loot. The thing is, these aren't separate entities, but rather the dungeon core holds both conflicting perspectives at the same time.

Building on this model, we could probably sort dungeons into four typical extremes. Isolationist dungeons are all warden, no prisoner. They don't want to be found, they have terrible loot, and they tend to be very lethal. They're best left alone. Friendly dungeons are all prisoner, no warden. They tend to be rather harmless and love having company, offering some decent loot, though they may have a tendency to keep growing without stopping. Apathetic dungeons are no warden, no prisoner. They're not dangerous, but don't offer much of value. They just kind of exist. Sociopath dungeons are all warden, all prisoner. They aggressively lure in adventurers only to slaughter them mercilessly. They are an active threat that must be destroyed.

Another thing I like about this is that the dungeon core could be literally anything. It's not always going to be some glowing crystal, nor will it always be some kind of creature. Any item that is hidden away but simultaneously wants to be found has the potential to develop into a dungeon core. This could make dungeon removal more interesting, as it won't always be obvious what the core is, let alone where it might be hidden.

It does require a bit of explanation as to why dungeons aren't just everywhere, but I think that can be explained by the strength of the intent. A child who breaks his father's tools while playing with them won't have as strong an intent to hide the broken tools as a royal retainer who dies trying to prevent a national treasure from falling into the hands of a foreign power. The latter will also have a strong desire for the national treasure to be found by his own nation, and thus should create a conflicting desire both to hide and to be found. Likely the intent also takes time to coalesce into a dungeon core, so if the object in question is reclaimed quickly enough, the intent won't have time to settle on the core and gain sentience.

[aimlessPolymath]

I might be getting a little hung up on the "dungeons are supposed to be prisons" thing. If you expand the concept to include any kind of living structure, there's a lot of potential to do things outside the scope of a typical dungeon core story. For example, one idea I had for a video game was that the player controls an AI that is running what is basically a city inside a nuclear/apocalypse shelter after it has been sealed, and the AI goes rogue. There's be lots of different scenarios to play, including ones where you save humans rather than kill them, but the most basic one is to kill every human living in the city before they destroy the computers that the AI is running on. Those computers are basically your dungeon core, but the premise is very different from what you'd expect. It wouldn't really be a dungeon sim.

Of note is that the model of indirect control adds a lot of narrative potential to the dungeon's interactions with monsters- rather than being primarily improving through internal abilities and 'levelling up', a sentient dungeon that can't spawn its own monsters might have to to put work into attracting them, manage dungeon ecology as tribes or species grow over time, and limit the damage invaders can deal to the ecology within. Rooms for wounded monsters to recover? Escape hatches so that wounded monsters can retreat, rather than fighting to the death?

Closer to to a city sim/colony builder game, if you will.

[Tvtyrant]

My major setting, The Old War, is based on the idea of dungeons as containment. I essentially borrowed Mako from FF7, where there is magic life energy that seeps up from the ground that drives civilization. However it is a magical mutagen and causes all life that comes into contact with it to mutate rapidly, so cities both have to be over a Mako Well to survive and are also at tremendous threat of monster attacks and mutagen spills. To counter this they inevitably build mega-dungeons under the city to reduce the number of monsters that come up into the city, and power the traps and wards with some of the energy from the Mako.

The species that live on the world are all the result of different creatures mutating in the past, and spelunking old dungeons where the well died or where the city was destroyed over an active well is the way that adventurers make their money. As new wells open there are Mako rushes to come in and claim the land, clear the early monsters and make new containment dungeons.

[sktarq]

I have little but some experience with the genre so YMMV

But the one time I was asked to help support this for someone else's world what we ended up working out was that dungeons are are the material plane spawn of an extraplanar creature of godlike potential (who can create/destroy worlds) that is trapped/bonded to the material plane. The spawn shouldn't be tied to the material plane but are due to the binding of their parent and have been twisted by that binding during their formation on a metaphysical level.
Its spores pass through the ethereal plane and settle somewhere and start to try to create their own little world. They only really have power within themselves though but have some degree of senses beyond that and eventual light influence (grow vegetation, wil-o-wisp like lights, dreams in certain people etc).
Over time as they grow can start to not just copy monsters they sense but make up their own (a major source of weirder aberrations, undead, and magical creatures that would otherwise not evolve)...Also why their magic items/treasure mostly were stylistic copies of what was around them.

the "core" is a key organ that is basically where the immaterial and material versions of the creature link. (oh as an aside due to this extraplanar aspect teleportation, scrying, plane shift, passwall etc often didn't work) Destroy that and the material/extramaterial creature breaks apart and disincorporates.

We also had a couple varieties of them sketched out that basically was a
"normal style dungeon", burrowing through rock etc
"more weird" with tesseracts,more immaterial aspects, thought puzzles, etc of the more Cthulu/metaphysical/extraplanar aspect...
"other more weird" that were for solid cloud walls, underwater dungeons made of an air maze etc (basically all the wall art sprites from Unlimited Adventure Game from back in 1993)
I want to say we toyed with making these different genders of the spawn but all twisted by being linked to a plane they never should have been but the details have faded and there may have been more.

EDIT. I am now sad I never got to play in that world he built...wonderful idea wall...flubber level.

[Tectorman]

One thing I liked about the 13th Age was its explanation for its never-ending supply of dungeons. Essentially, the world has three realms: the world where everyone lives, an overworld (basically, heaven/the Astral), and the underworld. And wherever (and it can be anywhere) the underworld bleeds into the material world is where you have a dungeon. Essentially, all of the dungeons that just appear wherever you want without any other explanation is just "planar bleedthrough" from this "Plane of Dungeons".

So switching this to this "dungeon cores" thing could just be a matter of making them an inherent mechanism for how dungeons spontaneously appear (and what has to be dealt with to make them go away). Admittedly, this only trades one question for another ("What's the deal with these dungeon cores?" for "Why is there a Dungeon Plane?"), and I wouldn't really be able to expand on that (I don't recall if 13th Age ever gave an in-universe explanation).

[CanisLunis]

a small handful of dungeons scattered throughout my campaign setting are legitimately just epic-level mimics, who have taken on the form of dungeons and cave systems and other similar things to lure adventurers to their deaths. my players have discovered one such dungeon and have actually encountered its "heart", the brain of the mimic who (by way of it being epic level) is currently immune to virtually any damage the party might deal, but they've made plans to return at a later level to deal with it.

[Alcore]

War for the Overworld game me some experience with the genre. The dungeon core is basically a nexus for eldritch energies that serves philactery for an entity. The entity can have more than one (the primary one in located in what appears to be its own personal demiplane) with only one truly required to keep the entity grounded in the mortal plane.

From around that nexus the dungeon forms and the great many monsters that come with it. The gold is mined from the surrounding rock.

From game lore they are sent by a god of evil for... reasons. To conquor? Been too long since I played...

[Tristan Alkai]

Actually, the “hide something with the intent that it will only be found by the right people” is a dungeon origin that I have not seen before; I might want to use that idea later. A lich phylactery is only the start of the possibilities (the rules are pretty explicit that the lich respawns next to its phylactery, so it needs a way back out if it wants to get anything done, and it’s difficult to provide that way out without also providing a way in).

–

Another story I found that answered this question in an interesting way was “Dragonheart Core”: the starting point is a fairly old (and powerful) dragon that is losing a fight (to a mage of some sort) and about to die, and the dragon generates a dungeon core as a method to survive the encounter in some form and hopefully get revenge eventually.

Let’s see if this link works: Dragonheart Core | Royal Road

(edit) … apparently not. Oh well.

There are a few other potential directions to take this basic idea, starting with allowing other types of creature to pull it off. Some variety of “reincarnated as a dungeon core” is one of the stereotypes of the genre, but there is room for variation in who they were beforehand and how the selection process works.

–

One story I did for NaNoWriMo:

Our protagonist finds a spellbook in his family’s attic, tests a few of the spells inside, and his success builds his confidence until he decides to try the one labeled “Ritual of Immortality.”

When he wakes up, he quickly finds that he turned himself into a dungeon core. Obviously, this was not the kind of immortality he had in mind (too late to back out now, unfortunately). Shortly after that, a quick look outside identifies something that looks suspiciously like a planetary ring, therefore confirming that he’s no longer on Earth.

After a rant about his stupidity for getting himself into this predicament, and thinking back to the mess he left with that ritual (which he is no longer in any sort of position to clean up), he thinks over other stories of this kind that he’s read, pulls up his status screen, and starts building.

On the plus side, his wish does seem to have been granted, in a “cruel and creative genie” kind of way.

More than that is probably a spoiler.

I’m not sure I ever established the full rules on where dungeon cores in that setting come from, but at least this one puts a new spin on the “reincarnated as a dungeon core” cliché. “Dragonheart Core” above is the only other one I can bring to mind where it was self-inflicted like this (getting turned into a core by a different mage is also an origin I’ve seen a few times).

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In another setting I’m currently working on (admittedly based more on Magic: The Gathering than D&D, but hopefully interesting enough to include here), dungeons are a sort of cosmic leftover—sections that didn’t quite make it into the main world and now remain adjacent, rather than fully integrated.

The creation story goes that the original state of the world was numerous parallel layers of reality, and both creatures and objects could slide between the layers without warning (legend has it that a few managed to figure out how to trigger the slide at will and control the direction they went, but they were the exception). Conditions on the adjacent layer wouldn't necessarily be particularly similar to conditions on the departure layer, and very few species can survive this sort of treatment; the main ones that managed it were certain types of elemental. Plants, being rooted in place, suffered this much less frequently, but even large sections of terrain shifting around wasn’t unknown.

Related to this, building any sort of civilization is difficult when unattended possessions and tools may or may not still be there when the owner returns to pick them up, and the tool may or may not reappear (depending on what happens to it on the adjacent layer, or who finds it), and if it does it may or may not be close to the spot it disappeared from.

This state of affairs persisted until a wandering planeswalker (for those familiar with D&D but not Magic: The Gathering, I will oversimplify this as an epic level caster, usually a sorcerer and usually specialized in one of the five “colors” of magic) passed that way and started poking around. This planeswalker ultimately tried to modify the plane (both M:TG and D&D use that term, but most M:TG planes are effectively “material planes” in D&D terms) into somewhere that life closer to what the planeswalker was familiar with could arise and establish itself, accomplished by pulling all the various layers into a single world (so the accidental slides would be replaced with more “conventional” journeys over land or water—done on purpose and a lot easier to find one’s way back).

The integration was less than fully successful; one of the debates in-universe is whether something about the excluded sections prevented the spell from working correctly, or whether the spell itself was designed in advance to exclude layers based on certain criteria. Either way, there were at least three distinct failure modes, which led to, respectively, the planet’s moons, the numerous underground caverns, and dungeons (which are located “next to” the “main” world that the plane was organized into, rather than inside it, and accessed through portals). What is known for sure is that all three kinds of space received further tweaks.

One key detail is the world’s sun: it was created as part of the reorganization, and the layers that existed before involved either some other method of making light for plants to use or some method for plants to grow with mana instead of light. After the reorganization, both options remain available to both dungeons and caverns; dungeons tend to favor glowing ceilings, while caverns could go either way (it varies from one cavern to another).

All three kinds of space have air, flora, and wildlife, and are habitable for humanoids that know what they're doing and have the right gear (much like more mundane varieties of wilderness). The caverns have a lot more local plant and animal life than the standard D&D Underdark, and are ironically much less hostile to explorers (also less dark).

Without going into too much detail, in order to grow dungeons in this setting need to present adventurers with challenges and see those challenges passed—there is no reward for killing adventurers in this system. As a result, they are generally friendly, and adventurer deaths in the dungeon are unusual; the general view of dungeons is positive (there is rarely much reason to try to smash the core, and most settlements are built near the entrance to a dungeon).

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For the imprisonment angle:

The specific dungeon that the protagonists (planeswalker original characters) mostly interact with was originally a human, until he got caught with certain books, accused of trying to become a lich (he denies it, and claims that a more detailed investigation than what actually happened would have proven his lack of anything else of the long list of things that would be needed to attempt such a project, proving his innocence), put on trial, convicted, and imprisoned as a dungeon core, newly created for the purpose. The magistrates that oversaw his trial will readily admit that he still has tools for causing problems even in that state, but that’s still a lot better than a lich (or even a displaced angry ghost) going who-knows-where and doing who-knows-what.

I haven’t decided whether this sort of imprisonment is the norm in this setting, but the genre stereotype that dungeons can live indefinitely is in effect here (and the generally positive view of dungeons means his new core isn’t likely to get smashed anytime soon, and he’s not likely to get out without that happening even if he was about to turn himself into a lich), so imprisonment is a potential way of dealing with such things.

Another important aspect of the decision is that, since dungeons in this setting only grow when adventurers do stuff (kill monsters, complete quests, explore new areas, etc.), a smart dungeon core needs to be fairly accommodating in order to actually grow (which provides a “community service” aspect to the sentence that the stereotypical “grow from eating adventurers” dungeon wouldn’t have, and possibly some degree of mandatory rehabilitation). They also made sure to place their new "dungeon" prison in the middle of nowhere (no adventurer traffic means he can’t grow his dungeon and become a problem) and in the middle of a dense forest (associated with green mana, which is an “enemy color” to the blue and black he had studied before).

As for our reluctant new dungeon administrator, his original human body was rather unimpressive—sickly and doomed to die young (tentatively some strain of cystic fibrosis, but I haven’t fully settled that detail). His studies of magic focused on blue mana (associated with, among other things, shapeshifting) and later black mana (associated with, among other things, undead) with the specific intention of fixing the problem. As a dungeon, he now has an indefinite lifespan to look forward to, and he quickly determined that his goal was technically met (in a “creative and cruel genie” kind of way, and at a much steeper cost than he had in mind).

Hm; now that I have the two stories together like this, I seem to be using that idea a lot. I might want to think that over a bit more.

[paddyfool]

I had an idea of them being the offspring of divine beings, with the world they were in being at least partly created for these offspring to learn and practice creation, with adventurers providing entertainment. And although their origins would be kept a secret from mortals, damaging or destroying them would be firmly prohibited and tend to invite divine retribution. After a period of time, a dungeon core would vanish and a new little godling would ascend; however due to some ongoing conflict with eldritch entities, the gods aren’t as overpopulated as you might think from this.