Inner Sea - Campaign Diary

Inner Sea - Campaign Diary

Feb 20, 2023    

Inner Sea

An underwater homebrew campaign for D&D 5e

Back in August 2020, I finished running Tomb of Annihilation. It was a great experience. Polling our gaming group for what comes next, the winning pitch was for an underwater 5e campaign. We ran for 43 sessions, the game eventually coming to an end due to Zoom fatigue and work time conflicts. We called the game ‘Inner Sea’ and ran it on Foundry VTT in an entirely isometric perspective.

Tomb of Annihilation taught me that I have much more fun adapting a well-written campaign, patching any plot holes, and adding content where fun. I used content and ideas liberally from Dragon of Icespire Peak, dmsguild darling Call from the Deep,, and Pathfinder third-party setting Cerulean Seas

Given that my game was so derivative, I haven’t posted about it before. D&D is a game experienced with friends and not-retold as a story. In my view, D&D campaign retellings have a ‘you had to be there’ quality. However, the #OpenDND debacle has left me realizing just how important the non-WOTC sources were for me in making this setting. Others willing to share and even post helped me greatly. Before the campaign passes too far from memory, I’m posting bits of it here in digestible pieces, with links to art where possible.

Making your own immersive and high-theme campaign doesn’t have to be a grand act of Tolkein-quality world-building. Hopefully, this example proves it. Let’s start with the original Pitch:

The Pitch

On other worlds, the mortals look to the stars in wonder and terror. Here they look to the depths of the sea.

Long ago, the world was flooded. The Surface is an uncivilized archipelago amid stormy seas. Below the waves is the Inner Sea, home to both water and air-breathing civilizations. Deeper still is the Dark Abyss, where no surface light may penetrate and few surface dwellers venture. Brave captains pilot submersible craft between settlements and cities in search of trade, adventure, and the mysteries of a world surrounded by water.

The map is broken into vertical layers, with strong currents, coral, and volcanic rock creating natural boundaries and passages between areas of the depths. The characters are both water and air-breathing adventurers who grew up on the Surface or Inner Sea. Non-magical means of breathing and underwater travel are common, the game takes place 90% underwater.

Pick Inner Sea if: you want to explore a blank canvas setting with more homemade content. Pick this if you think taking your submarine on a shortcut through bio-luminescent razor reefs to deliver something to the last Atlantean city before an unknowable horror consumes it sounds fun. Pick if you want a break from familiar Tolkien fantasy and aesthetic with predictable monster stat blocks.

Challenge for the GM: Don’t get lost in world-building, make it approachable

Inner Sea Map
A three layer map was a key part of this underwater campaign

How the campagin played

  • Chapter 1 - read here
  • Chapter 2 - read here
  • Chapter 3 - read here
  • Chapter 4 - read here
  • Chapter 5 - A future diary entry to be posted here.
  • Post-mortem - A future diary entry to be posted here.

Setting Categories

Magic: High

Civilization: Uncivilized surface; Kingdoms with medium cities; scattered sunken a natively aquatic unaligned settlements

Government: Dynastic Monarchies and large refugee populations dating from whatever event flooded the world

Gods: clerics worship nameless concepts of the sea

Technology: Late medieval? Advanced? what’s cooler?

Character Races

We want the world to be more than a re-skinned Forgotten Realms. Pick something that is not in Tolkien and let’s craft together how it fits into the world. D&D Beyond can do full homebrew races, so no worries there.


Turtles - the aquatic reptile variant of Tortles. air-breathing

Merfolk - upper body is humanoid, the lower body is piscinoid (monster entry), amphibious

Atlantean - Water adapted humanoids, last survivors of an ancient and powerful sunken civilization (Triton), air-breathing

Selkie - Aquatic fey changelings , amphibious

Locathah - nomadic piscinoid merchants (monster entry), amphibious

Cecaelia - upper body is humanoid, the lower body is octopoid (pathfinder), water breathing

Karkinoi - crustaceans that dwell near thermal vents (tribality), amphibious

Wereshark - nomadic half-sharks, refugees who used lycanthropy to survive the great flood (tribality), water breathing

Cnidarian - pronounced Ni-dar-i-an, bioluminescent Dark Abyss dwellers (tribality), amphibious

Classes: All classes

Backgrounds: All

Goals: up to the players, starting goals decided in Session 0


Call from the Deep, Out of the Abyss, Ghosts of Saltmarsh, Cerulean Seas