The Cthulhu Mythos in D&D in the 1970s

Cover by Bruce Pennington for 1975 Panther paperback reprint of Tales of Cthulhu Mythos (1969), including Lovecraft's "Call of Cthulhu" and Mythos stories by Howard, Smith and others

A chronology of early references to the Cthulhu Mythos in Dungeons & Dragons:

Feb 1975
Greyhawk --- Supplement I by Gygax and Kuntz
Description of the Gate spell: "Employment of this spell opens a cosmic portal and allows an ultra-powerful being (such as Odin, Crom, Set, Cthulhu, the Shining One, a demi-god, or whatever) to come to this plane" (pg 28)

July 1975
Letter from Gary Gygax, Alarums & Excursions #2, an APAzine edited by Lee Gold

"...in Greyhawk we do not have existing religions included, for this is a touchy area. We have such groups as "The Church of the Latter Day Great Old Ones," "Church of Crom, Scientist," "Brethren of St. Cuthbert of the Cudgle", and so on. Gods sometimes intervene"
Mike Mornard (aka Gronan) also mentioned the first two of these in a post on ODD74.

July 1976
God, Demi-gods & Heroes --- Supplement IV by Kuntz and Ward
No Cthulhu mythos per se, but does include Robert E. Howard's Hyborea (i.e., Conan), on pages 45-52 including numerous deities, monsters and magic items. Some of note: Tsathoggus [sic, for Tsathoggua], a Clark Ashton Smith creation also used by Lovecraft and appearing only in Conan the Buccaneer by De Camp/Carter (pg 46). A note at the bottom of page 46 includes "The Old Ones" among other deities mentioned by Howard, likely due to a reference to the "Nameless Old Ones" in the Phoenix on the Sword. The Kraken "comes to us fully described in A. Meritt's Dwellers in the Mirage" (pg 48).

See also this article: Conan and Cthulhu Mythos

Dec 1976
"Fantasy/Swords & Sorcery Recommended Reading" by Gary Gygax, The Dragon #4
Gygax includes H.P. Lovecraft, Algneron Blackwood, Robert E. Howard and A. Merritt in this list, which can be seen here. With some revisions and additions, this list would later appear as Appendix N in the Dungeon Masters Guide (1979, see below). Lovecraft was also included the new Fantasy Gaming Hall of Fame in The Dragon #7 (June '77, pg 4). 

Mar 1977
The Adventure of the Lost City: Part Two, Boinger & Zereth campaign story, by J. Eric Holmes, in the APAzine Alarums & Excursions, issue #19. 
The party finds excerpts from the Necronomicon and spies the Spawn of Cthulhu: "In the center of the tower, rising up from the sea caves beneath, momentarily visible above the horizon before it fell back into the depths, the polyploid tentacled monstrosity wavered in the air, and its eerie screeching reached them across the waves. 'The Spawn of Cthulhu!'"

July 1977
Dungeons & Dragons Basic Set edited by J. Eric Holmes
"When characters swear they call on the wrath of their appropriate deities, be it Zeus, Crom, Cthulhu or whatever" (pg 39). 

Aug 1977
Kuntz and Holmes meet at Gen Con and discuss "ideas for a revision of the old "Gods Demigods & Heroes" D&D supplement ... a brief summary of the gods presented in the works of H.P. Lovecraft". This led to the article below.

Feb 1978
“Lovecraftian Mythos in D&D” by Holmes and Kuntz, The Dragon #12
"J. Eric Holmes (known for his work with Basic Dungeons and Dragons) with additions by my humble self bring you parts of the Cthulhu mythos. They are laid out to be compatible with Dungeons and Dragons Supplement IV: Gods, Demi-Gods & Heroes" (i.e., for OD&D).
Includes Azathoth, Cthulhu, Hastur, Nyarlathotep, Shub-Niggurath, The Necronomicon, The Elder Sign, Cthuga, Ithaqua, Yig, Yog Sothoth, Byakhee, The Deep Ones, The Great Race, The Old Ones, The Mi-Go and The Shaggoths [sic].

May 1978
"The Cthulhu Mythos Revisited" by Gerald Guinn, The Dragon #14
A letter from a reader criticizing some of the interpretations of the Mythos in the Feb article.

July 1978
"A Rebuttal to the Cthulhu Mythos Revisted" by J. Eric Holmes, The Dragon #16.
Holmes' response to Guinn, mentions that sources for the article included "The Cthulhu Mythos: A Glossary" by Francis Laney, in Beyond the Wall of Sleep (1943), and "H.P. Lovecraft: The Gods" by Lin Carter, in The Shuttered Room and Other Pieces (1959).

 

Summer 1979
AD&D Dungeon Masters Guide
Appendix N on page 224 is a revision of Gygax's 1976 recommended reading list (see above). The names of Lovecraft, Howard and Merritt are retained. No specific works are specified for Lovecraft, but Howard is tagged with the Conan Series, and Merritt is tagged with three works, incluidng the Dwellers in the Mirage. Algernon Blackwood is the only author from the earlier list that is dropped. August Derleth is added without specifying any particular works. Notably, Clark Ashton Smith is still not included.

Nov 1979
Trollshead, a Boinger and Zereth short story, by J. Eric Holmes, The Dragon # 31
At one point, Zereth swears, "Oh, Hastur, Cthulhu and Azathoth!", to which Boinger responds with a Sign of Protection (pg 42).

April 1980
Giants in the Earth column, by Lawrence Schick and Tom Moldvay, The Dragon #36
Includes an entry for "H.P. Lovecraft's Richard Upton Pickman (King of the Ghouls)", a character from the story Pickman's Model, including his seven magical paintings: The Lesson, Ghoul Feeding, Subway Accident, Welcome to Innsmouth, The Silver Key and The Colour Out of Space. Illustration of Pickman by Jeff Dee. The next issue of The Dragon included Lovecraft's Abdul Alhazred in list of candidates for future write-up, but this entry never appeared.

Aug 1980
Cthulhu Mythos in Deities & Demigods, AD&D hardcover rulesbook, pages 44-48. 
This is a revision by Jim Ward of the material from Holmes & Kuntz article.

1981
Call of Cthulhu RPG, Chaosium
The is a non-D&D game, but is included here for reference. Reviewed by Dave Cook in Dragon magazine in 1982.

X2 Castle Amber, Expert D&D module by Tom Moldvay
Clark Ashton Smith finally gains some belated appreciation in this module which includes an Averoigne setting based on Smith's stories set in the region of the same name.

This is just a start; I haven't done a comprehensive search of periodicals of the time period, so there are probably other references in Alarums & Excursions, The Dragon, White Dwarf, The Dungeoneer, Judges Guild Journal etc. If you come across them, let me know & I'll update this list.

One thing that I noted is that until Deities & Demigods in 1980, not one of these articles has a single illustration of any of the eldritch horrors that are mentioned! Which is why I included a 1975 paperback cover at the top of the post as a representation of what was available in bookstores around the time the chronology begins.

Updated 1/22 to include more Dragon Magazine references; thanks to jbeltman for valuable assistance in finding these.

Updated 6/16/20 to include a July 1975 reference by Gygax in A&E (see above)