The Temple of Poseidon

"The Temple of Poseidon" by Paul Reiche III was published in Dragon #46.  In designing the adventure, Reiche drew upon the works of H.P. Lovecraft and Clark Ashton Smith to achieve a particular mood.

"The Temple of Poseidon" (16 pages) by Paul Reiche III was published in Dragon #46 (February, 1981).  Illustrations by Susan Collins.

Reiche and artist Erol Otus were high school friends.*  Together with Mathias Genser, the trio published OD&D supplements with illustrations by Otus, such as "Booty and the Beasts" and "The Necromican" (Fantasy Art Enterprises, 1979).

*see Grognardia interview, here; also Papers at the Strong, here

"The Temple of Poseidon"* is a well-designed, inventive, high-stakes adventure that gradually develops its Lovecraftian theme, then spirals downward into horror and madness.

*see my review on Dragonsfoot

There are similarities with module WG4 "The Forgotten Temple of Tharizdun".  Both begin as one kind of adventure, and suddenly develop into another, Weird Tales-inspired one:

Years ago this area was found to have incredibly high mana, the power on which all magic feeds.  To take advantage of this natural wonder, my ancestors built this temple on top of the mana source.  How were they to know that they were not the first?  How were they to know that beneath our stone corridors there resides another complex, built by darkly evil inhumans in millenia past?  The earthquakes must have awakened them, and now they rise again in their attempt to conquer the world for their evil lords!

excerpt from the journal of the arch-priest, from "The Temple of Poseidon"

However, whereas the black cyst in the Forgotten Temple of Tharizdun is regarded as anti-climactic by some, the secrets beneath the Temple of Poseidon are more like something out of Chaosium's "Call of Cthulhu" RPG.

In the middle of the chamber stands a shiny, black, four-sided pillar that reaches up into the darkness above you.  All four sides of the pillar are covered with grotesque runes and hieroglyphics.

Paul Reiche III, from "The Temple of Poseidon"

Reiche introduces several new monsters (the stats for which are embedded in the room descriptions, rather than collected together in a separate section at the end):

Cannibal Carnation, Carrion Crawler (Aquatic), Devil Wyrms, Efreet (Pasha), Living Statues, Myconymph, Screamers, Sand Squid aka "Subsilicate Cephalopod" (similar in some respects to the Groundsquid from Dragon #39), Spiders (Giant/Sea)

The climax involves a race against time, as the adventurers attempt to prevent the inhuman Priests of the King Ythog-Nithlei from releasing their alien King Ythog-Nithlei from his ancient captivity.

Other Settings:

Despite having been designed for use with AD&D, there's an undeniable BX* vibe to this adventure (Tom Moldvay was likewise heavily influenced by the Weird Tales triumvirate of Howard, Lovecraft and Smith).

*it would be very easy to run "The Temple of Poseidon" using BX and a copy of the 1e Monster Manual, as necessary details for adjudicating new monsters are provided in the text

"Poseidon" should be replaced with "Protius", based on the mythological sea-god "Proteus" (a better fit for the adventure, given his oracular attributes).  Protius "the Old Man of the Sea" is a Mystaran immortal.

Proposed location for "The Temple of Protius" on an island in the Kingdom of Ierendi in the BX "Known World" setting.

The "Temple of Protius" could be situated within the sea-cliffs of a large island close to Karameikos.  A suitable hook for the adventure might be that the PCs have been hired to determine the whereabouts of a Thyatian noble or Minrothad merchant.*

*Karmeikan/Thyatian nobles (room B), Minrothad merchants (rooms C/D)

The sea routes from Specularum to the islands to the south are indicated on this map from the 1983 Expert Set.  (The island in question was named Safari Island, sort of a Jurassic Park, in GAZ 4 "The Kingdom of Ierendi").