Need a quick underwater dungeon? You can follow this five-step random generator to create an entirely random dungeon shell you can fill in and personalize until your heart's content!
The entries are deliberately somewhat vague to allow more GMs to use them, as they’re open to incorporating varieties of monsters with varieties of challenges on most outcomes.
While these reference monsters from D&D, the results can work at most fantasy RPG systems beyond D&D.
1. Where is the Dungeon?
The following table determines where the dungeon is within an underwater environment:
Underwater Dungeon Location Table
| d10 | Location |
|---|---|
| 1 | In a largely vacant abyssal plain |
| 2 | Below a landmass in a series of underwater caves decorated with glowing fungus |
| 3 | Within a massive growth of seaweed covering the entrances and masking it |
| 4 | Built centered in a coral reef, grown over by the vibrant plants and creatures |
| 5 | Surrounded by wrecked ships from a massive naval battle |
| 6 | Stuck within an underwater chasm, startling between the walls |
| 7 | Nestled around thermal vents releasing toxic and intensely hot bursts of gas |
| 8 | Hidden within a sunken city |
| 9 | Deep within a blue hole flourishing with rare sea life |
| 10 | Protected by a city of underwater creature’s complicit in the dark dungeon’s dealings |
2. What is the Dungeon’s Purpose?
The following table determines the builder’s purpose for this dungeon:
Underwater Dungeon Purpose Table
| d10 | Purpose |
|---|---|
| 1 | The dungeon offered refuge from surface air-breathers |
| 2 | The dungeon was built to plunder riches the waters offered |
| 3 | The dungeon acted as a conduit to create horrific elemental disasters on the shores |
| 4 | The dungeon provided ample access to marine life for dark experiments |
| 5 | The dungeon was a surface-dwellers outpost used in a war against an undersea threat |
| 6 | The dungeon is a seal keeping an ancient Kraken asleep |
| 7 | The dungeon kept discreet black-market deals safe from prying eyes |
| 8 | The dungeon’s nature provided the creator’s shelter from above-water disasters |
| 9 | The dungeon was originally an outpost for excavators pursuing a mysterious artifact |
| 10 | The dungeon gave a cult worshiping a sea monster a close location to their master |
3. What is the Dungeon’s Structure?
The following table determines a general structure you can use to build your dungeon:
Underwater Dungeon Structure Table
| d8 | Structure |
|---|---|
| 1 | A wrecked ship turned habitable lair through clever reconstruction |
| 2 | A series of metal vessels chained to the sea floor, each its own room |
| 3 | A retrofitted behemoth whale’s skeleton turned stone fortress |
| 4 | A sunken ruined castle having been teleported here from above ground centuries ago |
| 5 | An inverted pyramid temple with its entrance on its wide flat top supported by stone pillars |
| 6 | An underwater marketplace held to the floor by enchanted seagrass and seaweed |
| 7 | A massive coral plant and rock formation carved into a multi-room structure |
| 8 | A sparkling crystalline palace standing out eerily from the surrounding waters |
4. Who Currently Resides in the Dungeon?
The following table determines what kinds of creatures reside in the dungeon:
Underwater Dungeon Creatures Table
| d20 | Creatures |
|---|---|
| 1 | Merfolk/merrow outcasts seeking vengeance on their once allies who shunned them |
| 2 | Humanoid pirates led by a rogue druid who provides water breathing to the crew daily |
| 3 | Crab-folk mercenaries enjoying the spoils from their last raid |
| 4 | Kuo-tao cultists owned by a hyper-intelligent sea-monster |
| 5 | Giant aquatic beasts that patrol the dungeon as their hunting grounds |
| 6 | Water elementals free from their summoner, yet still patrolling their dungeon |
| 7 | Demons trapped, tethered by a water-breathing spell that fades over distance |
| 8 | Devils who bargained with the sea denizens for water breathing in exchange for magic |
| 9 | Undead spectral and skeletal sailors, called to the dungeon by a necromantic spell |
| 10 | A single incredibly clever giant octopus who uses traps to keep enemies out |
| 11 | Sirens, who take their true form when unobserved within the dungeon walls |
| 12 | Swarms of mystical jellyfish that shock and cause hallucinations on touch |
| 13 | Weresharks shunned to the dungeon who’ve adapted to their roles as villains |
| 14 | Deep-dwelling dwarves, adapted for underwater life with bioluminescent eyes |
| 15 | Triton cultists attempting to control a storm giant to drown a city in the name of their god |
| 16 | Sahuagin warmongers preparing for their next attack on an enemy city |
| 17 | A dragon turtle who claims ownership of the dungeon and its defending merfolk |
| 18 | Fey spirits drawing people below to meet them before trapping and drowning them for fun |
| 19 | Constructs from another world sunk to the dungeon on a mission to find their way home |
| 20 | A prophecy coven of sea hags who terrorize creatures both below and above the waters |
5. What Rooms Are in the Dungeon?
Consider either rolling as many times as you’d like ahead of time to plot out the dungeon, or rolling as the players progress through where the new doors may lead. There are two options: an ordinary rooms table, for when you want the players to explore some mundane options, and extraordinary rooms for when you want to spice things up a bit.
The following tables can be rolled on to determine what rooms the players encounter as they progress:
Underwater Dungeon Ordinary Rooms Table
| d10 | Ordinary Room |
|---|---|
| 1 | Freezing kitchen adorned with magical barricades to maintain the foods temperature |
| 2 | Storage room stashing basic cleaning goods and maintenance equipment |
| 3 | Infirmary with chains in the center of the room to hold patients still above to be worked on |
| 4 | Greenhouse containing flourishing undersea vegetation on the walls and in floor planters |
| 5 | Vault protecting secret marine treasures plundered from sunken ships |
| 6 | Dining hall with ventilation on the sides to release the remains to the scavengers outside |
| 7 | Prison cells, some with air pockets, used to keep prisoners alive below the waves |
| 8 | War Room with multiple massive maps of the sea and levels marked for notable locations |
| 9 | Bunks for underwater creatures ranging from smaller than humanoid to large |
| 10 | Stables for quick underwater creatures like giant seahorses and undersea serpents |
Underwater Dungeon Extraordinary Rooms Table
| d6 | Extraordinary Room |
|---|---|
| 1 | Fish hatchery, with thousands of tiny eggs covering the walls and a central feeding cage |
| 2 | Arcane laboratory with glass chambers containing various colored fluids, one broken |
| 3 | Ritual chamber funneled down to a deep central pit that routinely releases bursts of gas |
| 4 | Temple to a undersea deity whose central altar whose lid is a swirling portal |
| 5 | Beast trapping chamber lined with gear and tools for domesticating giant sea monsters |
| 6 | Puzzle room where the waters rise and lower to release waves of monsters |
