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To Build an Undersea Hex

by Senarch from Threshold Magazine issue 32

Image: Triton knights
http://pandius.com/Tritons.png
Caption: ‘Triton knights guarding their castle', original digital artwork by Senarch https://www.deviantart.com/senarch/gallery

This article offers a set of random tables and process for creating an Undersea hex, for sandbox and hex crawl design. It is inspired by the excellent OSE supplements Filling in the Blanks by Todd Leback¹ and Aquatic Adventures by Kepr and Lothamer², which are both recommended for adding more depth to your sunlit waters. We play BECMI/OSE at my table, in line with the Gazetteers, so while this is system neutral, it errs in that direction.

One of the aspects of the game I personally enjoy as a DM is improvising from random tables and using random tables to spice up my games. It keeps me on my toes, forces imagination, and can be a lot of fun. Similarly, when creating a living world for the stories to take place in, with hex crawls and emergent storylines, I find my imagination is aided by random tables.

Note that this is tailored to Undersea and might need some modification to work with the other seas of Mystara. PC3 The Sea People uses 8-mile hexes, which is big. Undersea is a shallow, busy, thriving place, and I am working here on the assumption that underwater population density will be roughly analogous to overland (except in the Abyss, which should be dark, cold, and scary, not to mention the pressure).

I provide a hex template at the end of the article, as well as an example hex “Old Mariner’s Bluff.” I picked a 6-mile hex, but it should be relatively easy to adjust to a 3-, 5- or 8-mile hex.

1. Name your area and note the scale you are using and the area location

2) Roll or choose a dominant environment and depth for your UNDERSEA hex

Bear in mind that Undersea is very shallow, mostly less than 200 feet or 60m. Sunlight goes to 656 ft, so we can assume that as we get further south and closer to the Terraces, the depth drops, and some deeper blue areas of the PC3 map will be between 200 and 656 ft.

Each habitat tends to have its own flora and fauna, though some are richer than others.

d12		Description	Depth
1	Coastal shallows	Limited depth where terrain affects the waves	<100'
2	Coral reefs and atolls	Living coral formations (rich habitat)	<230'
3	Sand plains	Mostly flat sandy seabed	<656'
4	Rock plains	Mostly flat rocky seabed	<656'
5	Sea grass	Underwater meadows (rich habitat)	3–190'
6	Kelp forest (L, M, H)	Underwater forests of giant kelp (rich habitat)	49–131'
7	Submerged broken lands	Rock formations and caves	<656'
8	Submerged mountains	Mountains reaching towards the surface	<656'
9	Submerged volcanoes	Warmth and hydrothermal vents (rich habitat)	<656'
10	Terraces and Twilight	Underwater terraced cliffs, half light	<1000'
11	Trench	Deep depressions in the ocean floor	>10000'
12	Abyss	Utter darkness, cold and high pressure	>30000'

3) Roll or choose the main local population and population density

Population density refers to the number of intelligent creatures in the hex and will affect settlements and lairs. 1 – Abandoned, 2 – Wild, 3 – Settled, 4 – Urban.

Optional: roll or choose secondary populations (or roll for each settlement in stage 4).

Whatever the population or number of populations, give each 1d4 competing factions (e.g., ruling house, nobles, merchants, crime syndicates, hunters, army, and so on) to add flavour and sources of drama and conflict. Note that if the area is coastal, then surface-dwelling populations along the shore will most likely be a given.

2d6		Notes
2	Pirates and surface dwellers	Surface settlement, sailing through, depth explorers
3	Sea giants	Twilight / Depth dwellers >500'
4	Sea elves	Live in hidden coral reef and atoll cities
5	Shark-kin	Semi-nomadic (see nomad settlement table)
6	Merrow	Nomadic (see nomad settlement table)
7	Triton	Live in coral castles and cities
8	Kna	Semi-nomadic (see nomad settlement table)
9	Nixies	Live hidden in coral reefs and crevasses
10	Kopru	Prefer volcanic areas
11	Devilfish	Depth dwellers >700'
12	Other or rare	e.g., sahuagin if used

4) Choose or roll for settlements, lairs, and other features

Each hex has several features depending on population density.

Abandoned: 1d3-1 settlements, 1d4+2 monster lairs, 1d8 features

Wild: 1d3 settlements, 1d4+2 monster lairs, 1d8 features

Settled: 1d6 settlements, 1d6 monster lairs, 1d6 features

Urban: 1d8 settlements, 1d4 monster lairs, 1d6 features

Population Density	Settlements	Monster lairs	Features
Abandoned	1d3-1	1d4+2	1d8
Wild	1d3	1d4 +2	1d8
Settled	1d6	1d6	1d6
Urban	1d8	1d4	1d6

Roll or choose the settlement and features (I provide examples but leave the detail of the features to your imagination). Lairs will usually be part of a feature.

Some populations are sedentary, others nomadic. I have marked the kna as semi-nomadic (they return to the Deepwater Pool), and the shark-kin also (since they usually have an island which they return to, to choose their leaders). If a nomadic population is migrating or travelling through a hex, choose an entry and an exit point.

Sedentary Settlement table

Nomadic Settlement table

2d4
2	Ruin / abandoned
3	Dwelling
4	Hamlet
5	Village
6	Castle
7	Town
8	City
2d4
2	Abandoned camp
3	Supplies cache
4	Refuge
5	Migrating/travelling through
6	Camp
7	Market camp
8	Tribal gathering

Image: Kna
http://pandius.com/Kna011.png
Caption: ‘What yuu say bout our wares, hoooman? - Original digital artwork by Senarch https://www.deviantart.com/senarch/gallery

Features Table

d20
1	No feature	The sea is empty
2–5	Resource	Something of value
6–8	Structure	A lone structure of some kind
9–11	Hazard	A danger
12–14	Terrain	A geological or environmental feature
15–17	Ruin	Ruins of various size
18–19	Sign	The signs of a possible encounter in the region
20	Magic	A magical hazard or feature

All the tables below could themselves have sub-tables (e.g., Which a supplement like “Filling in the blanks” mentioned earlier, or to an extent the 5e DMG, provide). I leave those details to your imagination though. Here are a few ocean-related feature examples though, often where one feature implies or ties together others (examples mostly extrapolated from the Blue planet television series).

• Deep-sea vents releasing black clouds of mineral-rich smoke and drawing in an abundance of sulphur-based life-forms and giant carnivorous tubeworms.

• Bioluminescent plants and animals that light up the Abyss. Deep sea creatures use bioluminescence for defence, communication, camouflage, finding food or a mate. Ability to see bioluminescence or not being luminescent may have an impact on surprise in encounters (predators look up at the shadow cast by moving prey, some creatures use luminescence to confuse this. Yet being non-luminescent could have advantages as well).

• Gigantic kelp forests that sway and move with the current, making it difficult to navigate and creating a hypnotic effect on swimmers, or an underwater forest of seaweed and kelp that has grown so thick that it creates a dense, tangled maze for swimmers, within which lair many creatures.

• A deep-sea lake that is filled with a dense, viscous substance, such as a unique form of algae or bacteria, perhaps hiding slimes and oozes.

• A massive underwater sinkhole that creates a whirlpool and pulls objects and creatures into its depths, perhaps into a cave or canyon system.

• A layer of underwater fog created by a unique mix of temperature and salinity, making it almost impossible to navigate.

• Coastal upwelling zones: areas of nutrient-rich water that can support large populations of fish and other marine life, or underwater canyons that create upwelling currents that bring nutrients to the surface, similarly supporting a diverse array of life. Nutrients bring many fish and predators (including birds and other flyers) for what can seem like huge feasts.

• Algae blooms: These large accumulations of algae can create "dead zones" where oxygen levels are depleted, and where many species of marine life cannot survive.

Resources Table

Useful stuff for hunting, foraging, and mining.

Ò
1	Metal nodules
2	Oyster field
3	Seahorse feeding ground
4	Fish and sea life (domesticated)
5–6	Fish and sea life (wild)
7	Mineral (natural)
8	Mineral (mined)
9	Vegetable (farmed)
10	Vegetable (wild)

Structures Table

Inhabited structures or signs of inhabitation, past or present.

d8

Hazards Table

d8
1	Battle ground	Sacred, cursed, carrion eaters, undead
2	Geological	Quicksand, collapses, sinkholes, vents, whirlpools, gas, cold, hot
3	Flora	Poisonous, dangerous, carnivorous, magical
4	Monstrous	Large sea monster passing through
5	Weather	Surface weather events affect the depth (e.g., sinking ship, nutrient clouds moved by rough seas)
6	Fauna	Crazed, rabid, diseased, acting unusually
7	Magical/Unusual	Ambush, magic storm, disease, gas, general weirdness
8	Dead zone	The water here is unbreathable (except by magic)

Dangerous areas and their causes.

Terrain Table

Geological features.

d8
1–2	Caves or mines	From single chamber to large complex
3	Change in elevation	Seabed drops one or more depth levels
4–5	Change in dominant environment	Roll again on the environment table
6-7	Rock formations	Unusual formations, carved, magical
8	Change in the water	Temperature, salinity, breathability, visibility

Image and Caption: ‘Taymoran ruin - Done by Midjourney at Senarch's prompting'
http://pandius.com/sunken1.png

Ruins (roll 1–4 Taymoran, 5–6 roll on Population table to determine origin)

Will contain, roll 1d6: 1–3 lair, 4–6 dungeon (roll 1d10 for number of levels/difficulty).

Taymoran ruins are arranged around one or more central conical towers of varying sizes.

Shipwrecks: see 7) below.

d10
1	Single dwelling or structure	House, tower, building
2	Hamlet	1d6 houses or structures
3–4	Village	1d20 houses around 1d6 buildings
5–7	Shipwreck(s)	1 or 2/6 chance of d6 d10 d12 d20 (roll 1d4)
8	Castle (+1 dungeon level)	Single structure
9	Town (+2 dungeon levels)	1d100 houses and 1d20 large buildings
10	City (+3 dungeon levels)	3d100 houses and 2d20 large buildings

Note that most ruined houses and buildings will just be structural walls around empty space.

Signs

Roll on the encounter table you are using and foreshadow the possible encounter with signs of the creature or creatures.

Magic Table

Weird or dangerous magical effects.

d6
1	Space and time distortion	Things get odd around here
2	Magical fauna	Unusual or sentient magical animals
3	Magical flora	Unusual or sentient magical plants
4	Magical terrain, breathable water	The stone, sand, or water is magical
5	Magical structure or affected settlement	Village under a spell, magical statue…
6	Ley lines and strange fields	Ley lines, anti-magic, wild magic

5) Placing your settlements and features, adding lairs

Place your settlements and features as you wish on your hex map. Add lairs by attaching them to appropriate features.

Image and Caption: ‘Merrow refuge - Original digital artwork by Senarch'
http://pandius.com/MerrowRefuge.png

6) Using encounter tables for encounters and lairs

I tried to make these tables system- and edition-neutral. The problem with that is that they are a little limited and repetitive. I would recommend swapping out monsters depending on the edition you use to add variety or use monsters from supplements like Aquatic Adventures mentioned earlier, or from THRESHOLD magazine!

The tables on the Hex template are blank so that you can tailor your hex areas to your taste.

Encounter Tables

d20 Coastal		Coral	Plains	Seagrass
1	Dragon turtle	Seals	Killer whale	Were-shark
2	Giant crab	Eels	Whale	Eels
3–4	Seals	Were-seals	Merrow	Merrow
5–6	Dolphin	Dolphin	Aquarendi elf	Aquarendi elf
7–8	Aquarendi elf	Aquarendi elf	Fish	Fish
9–10	Jellyfish	Giant fish	Sea hag	Nixie
11–13	Main local pop.	Main local pop.	Main local pop.	Main local pop.
14–15	Shark	Nixie	Sea horse	Sea horse
16–17	Giant fish	Ooze/jelly/slime	Shark	Shark
18	Merrow	Triton	Triton	Triton
19	Sea serpent	Sea turtles	Kna	Kna
20	Killer whale	Giant octopus	Spider, aquatic	Sea serpent
d20 Kelp Forest		Broken lands	Mountains	Volcanoes
1	Dolphin	Giant octopus	Dragon, aquatic	Dragon, aquatic
2	Eels	Giant crab	Giant crab	Spider, aquatic
3–4	Merrow	Sea troll	Sea troll	Whale
5–6	Aquarendi elf	Ooze/jelly/slime	Giant shrimp	Flame salamander
7–8	Giant fish	Giant worms	Giant worms	Giant worms
9–10	Sea hag	Sea hag	Were-shark	Sea giant
11–13	Main local pop.	Main local pop.	Main local pop.	Main local pop.
14–15	Sea horse	Sharks	Kopru	Kopru
16–17	Sharks	Shark-kin	Were-shark	Devilfish
18	Triton	Undead	Sea hydra	Giant crab
19	Kna	Were-shark	Eye of the deep	Black ooze
20	Spider, aquatic	Spider, aquatic	Giant squid	Eye of the deep
d20 Terrace		Trench	Abyss	Ruins
1	Sea hydra	Sea serpent	Behemoth	Giant crab
2	Triton	Sea snakes	Ooze/pudding	Sea troll
3–4	Undead	Ghoul fish	Ghoul fish	Sharks
5–6	Giant octopus	Giant octopus	Jellyfish	Ghoul fish
7–8	Sharks	Ooze/jelly/slime	Eye of the deep	Skeletons
9–10	Shark-kin	Undead	Devilfish	Undead fish
11–13	Main local pop.	Main local pop.	Main local pop.	Undead
14–15	Sea giant	Sharks	Undead	Zombies
16–17	Giant fish	Sea hydra	Sea serpent	Devilfish
18	Jellyfish	Giant fish	Dragon turtle	Sea serpents
19	Manta rays	Manta rays	Whale	Giant octopus
20	Whale	Sea giants	Giant squid	Giant squid

7) Shipwrecks

Roll d6, on 1-2 the ship was a pirate ship. Then roll 3d20.

d20 Origin		Type	Encounter
1	Sind	Canoe	Giant crab
2	Ylaruam	Lifeboat	Sea troll
3–4	Alphatia	Warship, small	Shark
5–6	Thyatis	Warship, large	Spider, aquatic
7–8	Karameikos	Galley, large	Devilfish
9–10	Minrothad	Galley, small	Skeletons
11–13	Ierendi	Sail ship, small	Undead
14–15	Five Shires	Sail ship, large	Zombies
16–17	Darokin	Troop transport	Spider, aquatic
18	Northern Reaches	Longship	Sea serpent
19	Atruaghin	Raft	Giant octopus
20	Other	Sailing boat	Were-seals, were-sharks

Completing your area

Once you have set all features, settlements, and lairs, including population and monsters, you will have a pretty good idea of the feel of your hex, and probably the elements of a story to tie it together. The following text and tables are just idea seeds, to be used by the hurried DM if needed.

8) NPCs

Rather than provide an Undersea NPC table that would be far too generic to be much use or fun, here are a few suggested design principles.

When fleshing out your hex, focus on the following key roles:

Allies: These are usually urban centre leaders, patrons, henchpersons, guides, or information sources. This can include lair inhabitants.

Enemies: Either persons in inhabited areas, or intelligent monsters in lairs, either way they have an agenda potentially pitting them against the PCs.

Neutral: Any of the above, but PCs’ actions, or some kind of bargain or task, can flip these between ally and adversary. Many merchants and artisans might sit here (who are the top three of each, and what are they famous for?).

I personally like this simple shorthand for an NPC when brainstorming (not all are needed):

Name (Ally/Enemy/N) role, appearance, story, goal, strength, weakness, quirk.

Examples:

• N'thaka (N) Makai Kraken hunter, grizzled, hates krakens following tragedy, wants to kill krakens, high-level fighter, partial to rum and brawling, chews a strange root that makes him red-eyed but alert.

• The Oracle of the Abyss (A) 8'-bell sentient jellyfish, bioluminescent, ancient Abyss dweller, delivers accurate future possibilities through telepathic imagery, 120' poisonous tentacles / jellyfish swarm, riddles, collects luminescent items for his cave.

9) Local flavour

What are some elements of the hex that make it unique? Most of this will derive from the features, but you can layer in added detail. This could be many things, ranging from:

d10
1	Food or clothing customs.
2	Unusual or unique festivals, social or religious events.
3	Accents and body language quirks.
4	Social or religious rules and taboos.
5	Flora or fauna unique to the hex.
6	Currents, travel lanes and other water effects.
7	Named geological and terrain features (and whether sacred, forbidden, pilgrimage or leisure location and so on).
8	Architectural quirks.
9	Famed resources or crafts produced here.
10	Reputational and faction effects.

9) Adventure seeds

The “main quests” will most likely follow from the interplay of settlements, features, lairs and factions and NPCs, but here are a few quickfire ideas to get you started, or for “side quests”. These are pretty generic, but then I find that even an irritating dice roll result is often enough to trigger an idea…

d20
1	Oh look, a fetch and carry quest (message, goods or items, or person(s)).
2	Undersea pest removal, or maybe something is emerging from a ruin…
3	Open conflict is brewing between two factions.
4	Raiding, or protecting, a nomadic group passing through.
5	Pilgrimage to a holy place for an answer, blessing, or other boon.
6	Spying on one faction for another, running political interference, or thuggery.
7	Exploration and mapping a ruin or area for a patron or a domain.
8	Hunting expedition, open or specific to a monster-lair, for trophies.
9	Protecting settlements from raiders or attacking encroaching hostiles.
10	Treasure hunting in ruins or shipwrecks.
11	Dealing with bothersome surface dwellers (pirates, navy, merchants, polluters).
12	An embassy arrives, or the PCs are asked to serve there as.
13	A named monster must be eliminated, and its lair cleared.
14	A merchant has bought a disused coral castle but… Oh, you know the drill…
15	A daring rescue, kidnapping, or assassination must be attempted.
16	A magical item must be recovered or stolen.
17	A dangerous ruin must be cleared for x faction or patron.
18	A behemoth or other sea monstrosity has been sighted. Maybe it ate a village.
19	A panic involving were-folk occurs.
20	The Devilfish are up to no good. Again.

10) Conclusion

It would of course be possible to provide far more tables, and sub-tables for the tables, however I hope this is enough to kickstart your imagination when designing Undersea areas. Happy creating!

Undersea Hex Template

Image: Undersea hex Template and hex

http://pandius.com/UnderseaHexTemplate.jpg

http://pandius.com/UnderseaHexTemplate.docx

http://pandius.com/Hex.jpg

1https://www.thirdkingdomgames.com/product-page/filling-in-the-blanks-pdf

2https://knightowlpublishing.com/product/aquatic-adventures-collectors-set/