A Serpent Man.
The Serpent Men were created untold aeons ago by the Great Serpent, later known to be Yig. The seat of the First Empire of the serpent people, during the Paleozoic era, was Valusia. 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 originally ruled over humans in Valusia but were defeated and almost wiped out in humanity's battle for survival against the Elder Things that predated them. Over time, humans dominated Valusia and the Serpent Men became a legend.
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. However, they later repeated this tactic but added the front of a Snake Cult religion, which gained power and influence within Valusia while they also used their abilities of disguise to murder and 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.