"The Descent into the Abyss" is a Cthulhu Mythos short story by Lin Carter and Clark Ashton Smith, albeit written after the latter's death. Carter wrote the tale based on a story prompt by Smith, similar to the "posthumous collaborations" of Derleth and Lovecraft.
"The Descent into the Abyss" was first published in the 1980 anthology Weird Tales #2 (not to be confused with the magazine it is named after). At the time, the story was credited to Smith, with a note saying "completed by Lin Carter".
This story functions as a prequel to Smith's "The Seven Geases", as it reveals the backstory of the mysterious Haon-Dor.
Synopsis[]
In ancient Hyperborea, at a time before the advent of humans, the antehuman sorcerer Haon-Dor goes through a crevice at the bottom of Mount Voormithadreth to find the way to the underground realm of grey-litten Y'qaa, which is rumoured to function as a dimensional nexus that connects several different worlds. Deep within the abyss, Haon-Dor hopes to locate the domain of Ubbo-Sathla, and retrieve the Elder Records: the stone tablets stolen from the Elder Gods.
Once inside the tunnels, Haon-Dor comes across Zushakon, one of the entities spawned by Ubbo-Sathla, and respectfully asks to be allowed passage while displaying the sigil known as the Elder Key. Later on, he enters a huge cavern, with alcoves gnawed into the stone wall. Inside each of these is a creature, which Haon-Dor recognizes as the "prototypes" of several races that serve the Great Old Ones. Quumyagga of the Shantaks, Nug of the Ghouls, etc., each being the first of their respective kinds to have been created. To each of them, Haon-Dor asks for passage and shows the sigil.
Leaving the Cavern of the Prototypes behind, Haon-Dor crosses a bridge over a colossal chasm and finds a luminous portal guarded by K'thugguol, the prototype of the Shoggoths. Unexpectedly, the Shoggoth allows Haon-Dor entrance even before the sorcerer has time to ask it. When he passes through the portal, Haon-Dor gazes into the pit and sees the primordial Ubbo-Sathla, as well as the Elder Records scattered around it. However, reading just one glyph of the tablets is enough to send the sorcerer running back the way he came from, terrified of the undisclosed cosmic truth he learned.
After this traumatic experience, Haon-Dor sets residence in the caves beneath Mount Voormithadreth and never returns to the surface.