Detail from King Conan #4 (1980), Marvel, art by John Buscema
Today we head back to the Blue Book, where the illustrated "Sample Cross Section of Levels" features a "Great Stone Skull" on "Stone Mountain", known colloquially as "Skull Mountain". A "Great Stone Skull" also appears in the Conan story “Shadows in the Skull” by L. Sprague de Camp and Lin Carter. This story is most easily found in the book Conan of Aquilonia (1977), a compilation of four Conan stories, but was first published as the featured story in the February 1975 issue of Fantastic magazine. This places the story’s first publication before that of the Holmes Basic Set in mid-1977.
This late story (completely non-Howard) recounts King Conan's search for the sorcerer Thoth-Amon in a region far to the south of Zembabwei. Traveling south with son Conn on flying dragons, they spot a strange rock formation:
"Following his son's indication, Conan peered through the haze and saw a curious thing. This was a mountain of chalk white stone, the lower slope of which had been rudely carved into the shape of an immense grinning death's head.
Conan's barbarian heritage of superstition rose within him, bringing a gasp of awe to his lips and a prickling of premonition to his skin. The Great Stone Skull, whereof Rimush [royal soothsayer of Zembabwei] had spoken!
Conan's blazing blue eyes stabbed through the murk. Ahead a flat, barren strip of dead earth stretched to the foot of the cliff. There, the black arch of a portal yawned. Its lintel was carved like the fanged upper jaw of a skull. From the upper works peered two round ports, like the eye sockets of a skull. It was an eerie thing to see" (pg 145-146).
The Great Stone Skull of the Blue Book is an entrance to a dungeon leading to an underground city ("The Domed City"). Likewise, the Great Stone Skull of the Conan story turns out to be the entrance to a dwelling of "prehuman serpent-folk", former rulers of the world, and the last allies of Thoth-Amon.
This story was later illustrated in King Conan #4 (Marvel, Dec 1980; see detail above). While it remains to be shown conclusively whether the details of the Sample Cross Section illustration originate with Dr. Holmes or someone else at TSR (e.g., the artist, probably DCSIII), there is a feature similar to "The Pit" in Holmes' later novel Maze of Peril: an open hole leading straight down into the Underworld. Furthermore, Holmes was a fan of Conan: he wrote in his 1980 Psychology Today article that his "players have wandered ... through worlds created by ... Robert E. Howard".
Update: In 2014, we learned that the Skull Mountain drawing is by Tom Wham, and is not present in the Holmes Manuscript. Thus, it would seem that any influence de Camp's "Great Stone Skull" on this drawing would have come solely through Wham and not Holmes.