Building a New Campaign, Part 3: The Drowned Labyrinth

Part 1, Part 2

Art from Moria: Through the Doors of Durin by Free League

This is the third part of a new series in which I put together my next project: an open-world sandbox campaign using Old-School Essentials. If Part 1 was a campaign overview and Part 2 was an introduction to the town of Greyhold, today’s article is an introduction to the megadungeon itself, along with some supplementary rules for making the dungeon feel distinctive. What you won’t get today is keyed room descriptions and a map—that will come later, much of it on my Patreon.

Future players: light spoilers ahead, although it’s nothing you wouldn’t learn in play. And as always, everything here is a working draft and may change as I write and playtest.

Overview

Ten miles west of Greyhold, a maze of sea caves leads into a sprawling network of flooded halls and corridors. Few who dwell there or plunder its treasures know the truth: that it is an ancient prison of a dreaming god. While monsters, scavengers, raiders, and cultists all vie for dominance, the labyrinth quietly rearranges its halls and corridors. The longer you stay, the less you feel you can trust your maps and memories.

My primary influence for this megadungeon is ‘the House’ in Susanna Clarke’s Piranesi, although I am also taking inspiration from the Cove in Darkest Dungeon, François Baranger’s illustrations for The Call of Cthulhu, the ancestral tombs along Azura’s Coast in Morrowind, and, of course, Undermountain. There’s probably a bit of The Last of Us, Moria, and Stranger Things in there, too.

Entrances

There are several entrances to the Labyrinth, some hidden, some widely known.

• The ruined watchtower overlooking Greyhold marks the entrance to a great drop going down into the upper levels. This is probably the safest access point for novice adventurers and the most well-used, but also the most likely site of competition from rival parties.
• A causeway in the cliffs west of Greyhold offers a convenient access point but is fiercely guarded by a clan of harpies. They lair in elevated sea caves, some of which offer additional access routes.
• A sinkhole in the burial mounds north of Greyhold leads down to the Worm Roots.
• An underground river further along the coast leads to the port of Underharbour.

There are numerous other connecting tunnels from the Underworld of the continent, although many are undiscovered and unmapped.

Structure

The Drowned Labyrinth is made up of several distinct zones linked together. This is a working list of zones; I fully expect it to evolve as I write more of the project.

In addition, there are many connections to both the overworld and the underworld, including stairs, tunnels, portals, and so on. As a living dungeon, it’s easy to add or rework levels between the party’s expeditions.

Factions

These, too, will be explored further in a later article. For now:

1. The Cult of the Prisoner. Exiles and madmen who want to wake the dreaming god and don’t care what happens to Greyhold in the process.
2. Rival adventurers. Other treasure-hunters of varying experience levels. Some are friendly; most are competition.
3. Smugglers of Underharbour. Crooks and ne’er-do-wells who run contraband through the back doors of the Labyrinth. They know secret routes and safe rooms but are not to be trusted.
4. The Drowned Ones. Watery undead who haunt flooded corridors and guard forgotten shrines.
5. The Hivemind. A growing network of spores and fungi that quietly colonizes forgotten corners of the Labyrinth.

Beyond these factions, the Labyrinth crawls with all sorts of denizens: scrags, fishfolk, fungal horrors, barnacled undead, and many more.

Exploring the Labyrinth

For the most part, the Drowned Labyrinth can be explored like any other dungeon, using the normal procedures in Old-School Essentials. These rules are entirely optional, and as yet unplaytested. Feedback welcome. Hopefully they will help to simulate its flooded, shifting, maddening nature.

1. Shifting layout

The Labyrinth is a living dungeon. Some parts of it are shifting and may change between expeditions. The deepest parts are wildly unstable and may change even partway through an expedition.

• Each zone is either Stable, Shifting, or Unstable.
• After each session, for every Shifting or Unstable zone the party entered, roll 1d6. On a 1, that zone has shifted in some way (see below).
• During expeditions (Unstable zones only): every 3 turns spent in an Unstable zone, roll 1d6. On a 1, that area shifts.

Shift results
When a shift is triggered, roll 1d6 (or just choose one):

1. A known corridor collapses or is blocked (debris, fungus, flooding).
2. A door becomes stuck that was previously open.
3. A door merges with the wall, becoming a secret door.
4. A new crack or tunnel opens into a neighbouring zone.
5. Two familiar corridors now join in an unexpected loop.
6. A new trap appears, or an old trap resets itself.

2. Water and drowning

Old-School Essentials already has rules for swimming and drowning, but if you want to lean harder into the flooded nature of the Labyrinth, the procedures below add another layer of challenge.

Swimming and movement

• Characters are wading in 3′ of water (half movement, cannot run) and swimming in 4½′ or deeper. (Small characters like halflings are wading in 2′ and swimming in 3′ or deeper.)
• Everyone can swim unless they’re heavily encumbered or in heavy armour.
• Surface swim speed is ½ normal exploration movement.
• Characters in chain or heavier armour with more than light encumbrance essentially sink if they end up in deep water.

Difficult swimming

• If you’re in rough water, fighting while swimming, or carrying a heavy load, make a Strength check each round.
• On a failure, you go under and start using the breath rules.

Holding your breath

• A character can hold their breath for a number of rounds equal to their CON score if they keep calm.
• If they’re exerting themselves (fighting, panicking, forced swimming), they can only hold it for rounds equal to half CON (round up).

Drowning

Once those rounds are up, a character starts to drown. Each round, they must save vs Death:

• On a success, they cling on for one more round.
• On a failure, they drop to 0 hp and are drowning.

The drowning character eventually dies after a number of rounds equal to half their CON (round up) unless brought to air and stabilized by healing or first aid.

Exhaustion

Any character who reaches the ‘save vs Death each round’ stage but survives is exhausted: they can’t run, and they suffer a −2 penalty on attack rolls and saving throws until they get a full night’s rest somewhere safe.

An exhausted character who is forced to keep fighting or fleeing without rest is a walking liability. Monsters gain +2 to hit them.

3. Dreams and sanity

The longer you dwell in the Labyrinth, the more it seeps into your soul. Memories become foggy, time starts to blur, and eventually you begin to doubt your own senses.

• At the end of any expedition where a character has spent more than 12 turns (2 hours) in the Labyrinth, they save vs Spells.
• When the party first enters a Deep zone during a session, each character saves vs Spells.
• Certain encounters, artefacts, or direct manifestations of the Prisoner can force an immediate save vs Spells.

Progression

• On the first failed save: gain a short-term affliction.
• If you fail a save while suffering a short-term affliction, it becomes long-term.
• If you fail again while suffering a long-term affliction, it becomes indefinite.

Short-term (shaken)

• Lasts until you get a full night’s rest in Greyhold (or equivalent safety).
• You’re jittery, haunted by echoes and glimpses of things that may not have happened.
• While shaken, you suffer are less attentive to your surroundings. Reduce any ‘listen at doors’ or similar x-in-6 chances by 1-in-6 (for example, 2-in-6 becomes 1-in-6).

Long-term (worn down)

• Lasts until you spend about a week resting in Greyhold, or receive help at the Lady’s Chapel (for example, remove curse, cure disease, or similar cleric magic, at the referee’s discretion).
• In addition to the short-term effects above, you endure poor sleep, recurring nightmares, and intrusive memories of the Labyrinth.
• While worn down, you must save vs Spells after a night’s rest to gain the normal benefits of rest.

Indefinite (scarred)

• Persists until cured by significant magic or a specific quest.
• In addition to the long-term effects above, memory and perception become unreliable.
• While scarred, you suffer a −1 penalty to saving throws against magic, and hirelings and retainers can sense something is wrong with you: you suffer −1 on reaction rolls with them when planning expeditions into the Labyrinth. They must also check morale the first time you ask them to go back down.

That’s enough of the Labyrinth for now. In the next post, I’ll zoom back out to the wilderness around Greyhold: the woods and hills, the local threats, and the kinds of journeys I expect parties to make between expeditions.

As always, I’d love to hear what excites you about this, what’s missing, and what you’d most like to see next. More dungeon content? Wilderness procedures? More on factions and rival adventuring parties? Let me know in the comments below.

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