The Serpent Men are the spawn of Sligguth, son of the Demon God Set, created approximately one million years ago to serve Set on Earth. They are considered spawn or creations of Set himself on occasion.
Having been seemingly annihilated during the Thurian Age, first by early man (using the sentence Ka Nama Kaa Lajerama, discovered by a Pictish shaman, with the Serpent Men automatically reverting back into their true forms upon hearing it), then by the Atlantean warrior-king Kull of Valusia. However, "Delcardes' Cat" brought Kull in contact with, apparently, a community of Serpent Men living under a lake - like the Nagas, wise and often good serpent beings from Hindu mythology (possibly a source of inspiration for the Serpent Men). Here, Kull discovers their version of the conflict between Serpent Men and 'True Men'; a version where as 'True Men' are viewed as genocidal imperialists impossible to live with. Indeed, it seems also in "The Shadow Kingdom" as if 'True Men' were the aggressors, these elder races were only acting out in self-defense. Possibly, Kull did abandon his vow to eradicate the serpent men afterward. However, they were seemingly annihilated again during the Hyborian Age by King Conan of Aquilonia and his son Prince Conn, but the Serpent Men kept surviving and plotting, well into the Modern Age.
The Serpent Men appear in the Hyborian Age as schemers who can disguise themselves as normal men through sorcery. They often operate in the shadows, trying to get in contact with human rulers so they can kill them and take on their appearance. They are capable of performing minor magic, mostly illusory, and when they join their efforts these illusions can become quite impressive or detailed.
Their true appearance is revealed when they hear the sentence "Ka Nama Kaa Lajerama", a sentence only human lips can pronounce. When a human hears it, this phrase sounds as if they have already heard it before, even if it's the first time learning of such a spell.