I really like the concept of the Mythic Underworld (a place inherently hostile to intruders), but I also really value a place having somewhat coherent and predictable rules so players can make estimations about what lies ahead and how to tackle that.
Anyone here have experience with running a Mythic Underworld? I’d love to hear how you went about it.
Running one - not yet, as I am currently working on writing up the dungeon itself hah. But it very much will be a type of Mythic Underworld sort of deal.
I often see the “Mythic Underworld” being positioned as somehow a diametric opposite of a “natural” dungeon or a coherent dungeon, nevermind that literally no dungeon in any game of D&D is really ever coherent or sensible when compared to any structure that exists in real life. I have been in literal dungeons meant to hold and torture prisoners, in catacombs and underground cities and trust me the only RPG thing that I’ve ever seen even remotely reference those is, funnily enough, Veins of the Earth which is pure mythic underworld nonsense hah.
Anyway, my point is - if you want rules and predictability, then just put those into the Mythic Underworld. In OD&D the default per the text is that all doors in the underworld need to be forced open by the players, close back themselves unless pinned in some way, and yet those rules do not apply to the monsters that live in the dungeon (and if a monster joins the players, it loses that ability).
That is a trait of the Mythic Underworld, and it is completely coherent and predictable - the dungeon acts against the player characters and their allies, and does not act against its own inhabitants, and can switch someone from one category to the other based on their actions.
The main point of using this kind of dungeon design is that it allows you to approach it as a pure game space - the dungeon does not need to exist for any logical reason within the setting’s fiction, does not need to exist for broader thematic context in what you’re trying to or not trying to convey in your game. The dungeon’s function begins and ends with being a place to facilitate play.
As long as you do not break the assumptions of this play by changing things up on the players too much, your mythic underworld play space will have coherency and be something that players (not the characters, the players!) can learn how to navigate as they continue throughout the campaign.
Now obviously it is easier said than done, to just say “yeah make this a good space for play that lets players learn it through interaction” but it is good sometimes to actually keep an eye on the goal you have with why you create a specific piece of game design. For good examples outside of TTRPGs I would say check out some of the traditional roguelikes - NetHack, Dungeon Crawl Stone Soup and Brogue. Those video games are simplified versions (not simple by themselves, but definitely simpler than the ability of a referee to adjudicate literally any input by the players) of the very same Mythic Underworld dungeons of a lot of early D&D play. And looking at the much more stripped down and limited model can help us see what work and why, or at least it has for me.
Because similarly to what I said above, a roguelike dungeon is purely a place of play. There are often no stories, no themes, no big logical reasoning as to why they exist, they simply are there to provide interesting gameplay, and I think that can be absolutely a nice and good goal to have with a TTRPG game too.
The “checklist” in Philotomy’s Musings (Mythic Underworld information begins on page 22) is pretty useful:
Some common characteristics and philosophies for a mythic underworld or megadungeon (keep these in mind when creating your dungeon):
Note how #2 specifies that it does follow Rules, but they are not necessarily the rules that one might expect to see followed elsewhere or on the “Surface.” For me this is the one of the creative spaces I like to exploit when designing certain Hazards and Challenges. In a way, the Mythic Underworld can function a bit like a puzzle: You have to figure out it’s Rules and then you can even use them against it!
Consistency is still possible within the context once those “Rules” are established and the Players begin to learn how they work, but you aren’t necessarily burdened by having to make sure these match up with reality as we know it. A room with Plants that seem to grow without Sunlight is strange and mysterious but this establishes a fact about the Dungeon.
I generally like to start off gradually in terms of these Rules though: Closer to the Surface, things do tend to operate under more normal expectations, but the deeper you go, the stranger things become. You might find a Mouse that Talks on Level 2, or a Waterfall that flows upwards on Level 3 to gradually start hinting that the “Rules” are starting to break or bend into something new.
I don’t know if this will help you or not, because it’s not for D&D or anything like it, but I wrote up the Underworld section for the upcoming Dreamland game from Exalted Funeral.
It was mostly based on the descriptions of places from H.P. Lovecraft’s stories–the Vale of Pnath, Peaks of Thok, gugs, ghouls, ghasts and all that goodness, because Lovecraft is one of the big influences on the game (less than Lord Dunsany, but still).
For my parts of Dreamland’s worldbuilding, I was generally going for the symbolic and metaphorical. I researched different cultures’ Underworld lore, and tried to wrap the creative chaos into a lawful summary (with room for many exceptions). Here’s my intro to the section (unrevised):
For every world, there is an underworld.
Wildalone - Krassi Zourkova
The underworld cannot be mapped, but it can be reached—easily, in many cases.
The underworld cannot be destroyed, but it can be sealed—for a time, at least.
The underworld cannot be tamed, but it can be exploited—by the brave, maybe.
The Underworld is the first dream born of primate humans and fledgling gods. They buried their creation beneath a thin, porous patina of culture, propriety and civilization, that which we call “aboveground.”
Here in underworld churns the craven needs, feral desires and instinctual fears of humanity, including the strongest and oldest emotion of mankind, fear of the unknown.
Known or rumored entrances to this landscape without sun, moon or stars are in the Enchanted Wood, Sarkomand, Mount Ngranek, the Village of the Little Abyss, The Dark Highways and sailing the Twilight Sea. But surely others exist (ask a city’s Rat Catcher), or will exist if a Gravedigger pushes his spade just a little deeper. If ghouls can use underworld passages to reach Waking World cemeteries, dreamers can as well. Other physical portals to the Waking World—in any time period—could be found in the below, too. Furthermore, in dream, descent to earth’s inner core could bizarrely lead to outer space and other worlds.
Dream Masters running underworld adventures should strive to play up apprehension and threat. That noise in the distance? Was it clawed steps, the growl of a multi-gutted beast, the slink of a serpentine blade surreptitiously drawn from a scabbard, forked tongues clicking commands between one hunter and another or just a rock knocking into the abyss? Think of everything here as being swaddled in a vague fog—objects either coalesce and form when they are paid attention to or they were always there, submerged below the surface of perception and never noticed.
Many human cultures throughout history mythologized an underworld, often a waiting room for the dead until oblivion. Even more than aboveground, symbolism and suggestion impress deeper in the underworld. Dream Masters should consider hinting at the crude, primal urges of people through gross semblances—for example, the vagina dentata of a gug’s mouth, the sexual cannibalism of ghasts or the filthy contamination and conversion into a ghoul.
Although the underworld is inhospitable, chthonic beasts are not necessarily innately hostile the way abhumans or Nightmares are. If dreamers can speak the language of ghouls or gugs, they can Persuade or Haggle with them. The underworld may be the gods’ trash bin of discarded attempts at life that mushroomed within their own, separate universe. Like dreamland atop, the underworld is infinite.
The secret of the underworld is this: as inimical as the lightless domain is, the alternative—a blank void—is worse. Perhaps gods and humanity dreamt the underworld into existence first to make sound and fury against the significant nothing before refining the din into the more melodic upper dominion. Or perhaps this is what happens to paradise after the inhabitants leave for better real estate.
I wrote up a few random encounters for the Underworld, and one I liked is a table to meet mythic heroes on their quest: Orpheus & Eurydice, Enkidu, Ninshubur, Lemminkäinen’s mother, Phra Malai, Heracles, etc.
I suggest that for the encounter, the tables are turned: the PCs are actually NPCs to that mythic figure, part of that grand story, and the GM-controlled person might be expecting advice (or a magic item!) from the player characters.
In the Underworld, up is down, down is up, but: “Smokey, this is not Nam, this is bowling, there are rules.”
This is the same sort of tension that I feel any “magic system” runs into in games. You definitely want there to be something that is mysterious and unpredictable, but our corner of the hobby is all about player agency, so you can’t have too many elements that are completely un-grokkable.
I like to think of the mythic underworld as layers, and kind of conceptualize it how BECMI does their trinary “civilized/borderland/wilds” dichotomy - the further away from capital-L law you get, the weirder things can become. You might have stuff in a dungeon that establishes bubbles to give the players respite (like altars in a roguelike), but the further down you go, the less players can be assured of things working in any semblance of their reality.
Something Ktrey mentions in other spaces that’s tangential to this post relates to tiebreaks.
TL;DR is that Players win ties when the party above ground and during the day, and that monsters win ties when the party is below ground or during the night.
Stuff like this is pretty easy to think of and implement at the table, and more importantly it’s pretty close to the bone in regards to how players actually experience it. What I mean is, in contrast to a headier, more complicated rule that the players might not even understand or interact with, it’s a simple (if usually not that important) change that communicates the stakes in a satisfying way. Putting these sorts of “rules” into a dungeon space has essentially no downside, by my understanding. Here are a few I thought of while reading what everyone has said above:
Combining these sorts of, for lack of a better term, “gimmicks” can help create a cohesive identity for a place in play that I don’t think any amount of “There are frescoes on the wall which depict the saga of Maximum Prose-Hard, Chief Archivist of the…” in a text can ever hope to accomplish.
In regards to the broader discussion, I have a bone to pick with whoever decided to present the “Naturalist vs. Mythic Underworld” sort of thing as a strict dichotomy; my relatively juvenile understanding of what folks have done with modules is that it’s very rarely so cut and dry. Maybe I’ll make a blogpost one of these days about it.
Yeah, the tie-break thing is a nice way to convey that these spaces operate under different rules quite literally, but also helps stress that Dungeons are inimical to intruders 
I also have things Round away from the Players’ Favor down there, and in the Players’ Favor above ground as well should that situation come up.
Thanks for all the great replies.
I’ll gave to let it ruminate a bit, but I do feel a lot more like it is something I could attempt.
I am now considering adding a ‘weird’ rule each level (or expanding an established rule), and maybe give certain areas their own additional rule.
E.g. what I might do for my prison themed dungeon:
Level 1: Don’t run near plants: Vegetation seems to constantly get in your way causing you to trip or have your gear/bag get caught in vines and branches.
Level 2: As above + closed portals, latches and containers are always stuck.
Level 3: As above, but these portals etc. are always closed when you encounter them, even if they’re ones you’ve left open.
Etc.
With area specific stuff layered on top. E.g. when speaking in the Arena everything you say is as loud as shouting.
Hopefully that way weirdness inceases incrementally enough that the players will catch on to the rules as they explore without being overwhelmed by them.
I did a few rooms in an old project that related to noise; figured I’d drop them here in case they sparked any ideas:
The walls here change from bare stone to a stone that echoes; any speech within the lair is amplified and echoes through the pores of the stone.
…
The room is shaped like an amphitheater. In the center, a living statue performs an intricate dance. Dozens of other statues in the room are all watching intently. Crushed glass is scattered along the floor. If the party speaks or makes a loud noise, the echoing walls of the area amplify the noise and deafen the party for 1d6 turns.