The ancient Serpent Empire was based on sorcery and alchemy, but collapsed with the rise of the dinosaurs about 225 million years ago during the Triassic era.
The Serpent Men, one of the few surviving "elder things", infiltrated human society and ruled from behind the scenes for a time, but were again discovered, defeated, and cast out in a secret war.
Soon, they added the front of a Snake Cult religion, which gained power and influence within Valusia while the Serpent Men used their abilities of disguise to murder or replace each reigning monarch.
Their power is eventually broken by King Kull, formerly an Atlantean barbarian who had recently conquered Valusia, and the Pict Brule the Spear-Slayer, whose society was aware of the Serpent Men's infiltration.
[1] After the destruction of Valusia, the Serpent Men escaped to Yoth, a cavern beneath K'n-yan in North America (ironically, the Pictish Isles of the Kull stories).
[1] These creatures, unlike their kin and predecessors, have the bodies of giant serpents and the heads of human beings with smaller snakes for hair like Medusa.
Lovecraft's 1936 story "The Haunter of the Dark" explicitly mentions the "serpent men of Valusia" as being one-time possessors of the Shining Trapezohedron.
These are in fact Serpent Women, who magically show an alluring female appearance to King Conan and plan on sacrificing him while Thoth-Amon watches from hiding.
Their deception is discovered by Conan's teenage son, Conn, who sees at first what appears to be a beautiful girl; but her reflection in a burnished shield reveals the woman's true identity by showing him a serpent-like head.
Others become hosts for the spirits of long-extinct original Serpent Men, who transform their bodies into duplicates of their own, complete with their power to take the form of any human.
Once in the sewers, Spider-Man was duped by the Devil-Slayer imposter where they encountered the Defenders members Doctor Strange, Clea, Gargoyle, Hulk, Namor, and Valkyrie.
After destroying the statue, Spider-Man freed the Defenders (which had the true Devil-Slayer among them) as Doctor Strange banished the Serpent Men to Limbo.
A Serpent Man appears in "The Deathless Snake", the last story in Edward M. Erdelac's Rainbringer: Zora Neale Hurston Against The Lovecraftian Mythos.