Megadungeon Idea: Vertical Highway/Abyss


A common setting for D&D adventures is the dungeon: an enclosed, usually underground space, divided into rooms and hallways and so on, filled with monsters (some sentient and some not; not all of them necessarily hostile), traps, and treasure, providing a certain amount of constraint and direction for the group’s exploration.

A megadungeon is a dungeon of considerable size, such that it can take many sessions (or even an entire campaign) to explore. Megadungeons (or at least the good ones?) tend to be characterized by multiple sections or layers, complex (nonlinear) connections between those sections, multiple factions of sentient residents that hold each other in check, and secrets that can be discovered through clever or lucky play.

Anyway, here’s a smattering of ideas that grew in my head and could be assembled into what I hope is a novel megadungeon. I don’t have the time to devote to it right now (there are other projects I’m spending my free time on), but I did want to post it here – first, to prevent it from fading away on the back burner, and second, to present it in case anyone is inspired to develop it further (if so, let me know! I’ll be interested in seeing what you came up with).


The core conceit is a megadungeon in which every level (perhaps not every sublevel, but at any rate a clear majority of the areas) are all connected by one large open space. Easy to find, easy to access; of obvious utility.

There are plenty of forms this could take, including an expanded version of the Circle of Doom, but the one that appeals to me is an extinct volcanic cone. Okay, there’s some sort of structure – a school, a fort, a monastery, or the like – built on top of an extinct volcano, one with a (perhaps unnaturally) large open tube. The structure has basement complexes that open out into the tube (not always on purpose?), perhaps including some – smoky forges, etc. – that specifically take advantage of the space.

The problem with this design is twofold. First, one of the tropes of dungeon design is increasing danger with increasing depth, and players will lose some of this implicit metric that they would otherwise factor into their plans. And second, the volcanic shaft threatens to become a cheap shortcut. There’s not much point in building a dungeon that is mega if you give the players a free pass to bypass as much of it as they want, any time they want.

My solution is to make the central artery difficult to use. It’s not just a clean tube of basalt or obsidian; no, it’s a stinking, shrieking abyss. It’s vast, echoing, dark, with a hot wind blowing up from below. The walls are sharp and crumbling in some places, encrusted with crystal or slick with condensation and slime in others. It’s infested with an entire ecosystem of critters and monsters, from weird subterranean flora and the swarms of insects that live in and off of it, to the flocks of bats that eat them, to the increasingly large and dire beasts that eat them (and wouldn’t mind spicing up their diets by adding human explorers).

Because we’re responsible DMs, we throw in some warnings. The structure on top is specifically built in such a way as to plug the hole – it’s actually a fortress designed to defend the surface world against the horrors of the depths. It’s in ruins now (what isn’t, these days?) but writings can be found referring to how deadly “the abyss” is, or graffiti might provide a limited but ominous bestiary. The upshot is that successfully navigating the central artery is one of the more dangerous challenges the site has to offer, and that the players can realize this quickly and plan accordingly.

That’s all you need for a solid adventure site, but let’s spice it up a little! Any selection of these could make for an even more memorable play experience:

  • Floating structures: “islands” suspended in the air by magic or fantastic architecture. In a huge dark space, these would essentially be secrets, with hints about their existence and location scattered around in the rest of the dungeon. They might be prisons still containing dangerous creatures, high-security treasure vaults, labs separated from the rest of the complex for safety reasons, or the private fortified residences of powerful loners.
  • A magma lake at the bottom: it is, after all, a volcano. Maybe the party has a limited time (measured in weeks, sure, but limited) to get into the complex, find a target MacGuffin or secret, and get back out. Or maybe it’s just that the lower you go, the hotter and more poisonous the air is.
  • A sea of bones: volcano or not, it’s long dead. Also long-dead are the millions of ancient bones that can be found in a grotesque and painful mound at the bottom of the shaft, ranging in size from tiny bat and rat bones to mammoth skeletons that could be built into human-scale structures. Also possibly not dead: the horrible things that live in the sea of bones. How did all this stuff get down here anyway?
  • Failed experiments: the monastic order that once guarded the volcano’s mouth was fanatically devoted to self-improvement. Unfortunately, some of their experiments in body- or mind-warping magic went hideously wrong, eventually leading to the collapse of the order and the abandonment of the site. Both the results, or their descendants, and the secrets that spawned them remain in the complex’ depths.

About Confanity

I love the written word more than anything else I've had the chance to work with. I'm back in the States from Japan for grad school, but still studying Japanese with the hope of becoming a translator -- or writer, or even teacher -- as long as it's something language-related.
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3 Responses to Megadungeon Idea: Vertical Highway/Abyss

  1. Marlin-sama says:

    Possibly a bit difficult for the average campaign, imagine if this volcano was not open air, but an unimaginably deep crater lake with the network of tunnels you described accessible via air pockets. The issue in navigating the main shaft comes from the fact that unless you’ve got an entire party of merfolk (an interesting solution considering their limited abilities out of the water) you have to use some smart planning, implement the use of high-level technology (or magic) or as you suggested, get lucky. My imagination is always stirred when I start considering the unique possibilities a water dungeon presents. My world of Skyroad possesses just such a flooded labyrinth (the Void Sea)

    • Confanity says:

      I’m sorry for not making this clear in the original post — you say it might be “a bit difficult,” but my intent was for this to be a feature added onto an otherwise ordinary megadungeon. So the party can go in at the top, investigate the surface ruins, and proceed downward and laterally as normal through corridors, burrows, stairs, secret doors etc. The volcanic tube itself would be an additional space within it that acts as a landmark of sorts, a defining trait/feature, and (for parties bold, powerful, and lucky enough) an added space for exploration and shortcut to the deeper levels.

      Nice segue, by the way. 8^P

      • Marlin-sama says:

        Well, the emphasis is on POSSIBLY at bit difficult. Depending on the frequency of magic items in the setting, a bottle of air for each party member could remedy the most obvious hindrance to a flooded labyrinth.

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