Diving Deep Into The Temple of the Forgotten Depths
9 months ago
– Tue, Nov 25, 2025 at 09:58:09 AM
Our very first One Room One Shot
Temple of the Forgotten Depths was the first adventure ever written for One Room One Shots. Drawing from ancient Greek mythology (we told you this would come up again) is a story set in an otherworldly space, filled with ancient monsters and primordial gods. The temple itself is a vast domed structure with an upper balcony, long believed to be lost beneath the ocean. If Tomb of Grief is our purest “single room,” then Temple of the Forgotten Depths is the adventure that most fully embodies the idea of a single, contained location.
The premise is intentionally straightforward: an ancient artefact said to imbue the wielder with control over the ocean itself has been discovered, and adventurers are sent to the temple to retrieve it. Players begin after swimming down to the ocean floor, where the temple now rests - the reason for its hidden location until now. Whether driven by gold, academic curiosity or the pull of powerful magic, players expecting a simple treasure hunt quickly find themselves facing corruption, moral choices and the tragic history of a once-majestic sanctuary.
The artefact in question is known as the Jewel of the Ocean, or the Heart of Ceto. Ceto, a primordial sea deity associated with monsters and the many dangers of the deep, was once worshipped here through offerings, rituals - and sacrifices - meant to secure protection and safe passage across her domain. However, the temple has been abandoned for so long that the power of Ceto has waned. The surrounding waters have grown perilous, and without the protective presence once radiating from the Heart of Ceto, a creeping corruption has begun to spread and the ancient children of the goddess now seek to claim the artefact for their own designs.
The temple is built of living coral, enchanted to keep the water out and the interior breathable. Coral bleaching - when vibrant coral turns pale and brittle as its symbiotic algae die - became the inspiration for the temple’s corruption. Once colourful walls are now bone white, cold and calcified. The creatures within bear similar marks; with some attacks leaving permanently bleached scars. These visual marks create lasting reminders of the temple’s corruption, giving players memorable role-play moments without requiring long-term status tracking.
Dangers of the Deep
Exploring the temple is fraught with danger. Drowned Ones - water spirits bound to the Deep One - rise from the seabed to claim the Heart of Ceto before others can reach it. Enchanted traps await discovery, shrines hide secrets that only reveal themselves through intuition and clever problem-solving and any who delve too deeply risk provoking the temple’s last remaining guardian: the Coral Abomination. This hulking elemental can become ally or adversary depending on player choices, offering a flexible narrative pivot point in the adventure. The story culminates in a dramatic showdown as the temple floods and the corrupted Hydra of the Deep - a monstrous denizen of the deep - rises to seize the Heart for itself.
Should the players manage to claim the Jewel of the Ocean, they are faced with a difficult decision. The default adventure hook is that the party is initially hired by the mage Grimcowl to retrieve the artefact, and crossing him is unwise - especially as he represents their only means of returning to the surface.
Yet two other factions reveal themselves throughout the adventure. Within the temple, players may meet the Jengu, a beautiful divine water spirit desperate to heal the temple using the Heart of Ceto, and lurking in the deep is one of Ceto’s own children, the Deep One, who seeks the artefact to strengthen its power in the abyss. The Deep One’s agents already haunt the temple and it will not hesitate to sway or tempt the party.
Join a Cult, Fight Undead Pirates & Sail the Sea
In this Expanding the Adventure chapter, players are able explore a small chain of islands to uncover the temple’s history and location. In this nautical campaign, they'll face off against undead pirates, infiltrate a cult tied to the Deep One and discover oceanic horrors shaped by the spreading corruption. Once they’ve gathered enough knowledge and supplies, they must set sail across the perilous ocean where X marks the spot for the sunken temple - turning the original one-shot into a dramatic final dungeon with a multi-phase boss encounter and protective enchantments that can prove deadly if unprepared.
This short campaign is structured into three clear chapters - the islands, the voyage and the temple. We wanted the journey to feel truly epic while still offering a predictable number of sessions, making it easy for groups to plan and adjust as needed.
New Lore, New Monsters, New Mysteries
Finally, we delve deeper into the temple’s history and our interpretation of Ceto, her children and her followers. This adds new monsters, encounters and oceanic culture elements that any DM can incorporate into their own worlds, complete with new elemental beings and powerful hydromancers.
As we continue polishing Temple of the Forgotten Depths and expanding its world, we’d love to hear from you:
What are you most excited to discover during your voyage across the ocean?
Forgotten sea gods, undead pirates, the Cult of the Deep?
Or perhaps you have other ideas you'd like to see!
Tell us in the comments - we can’t wait to hear where your curiosity pulls you beneath the waves.