The Serpent-Men are the spawn of Sligguth, son of the Elder God Set, created about 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 Pre-Cataclysmic Age, first by early Man (using the sentence Ka Nama Kaa Lajerama, discovered by a Pict shaman, the Snake People automatically reverting to 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 Indian mythology; possibly the source of inspiration for the serpent men. There, Kull met their version of the conflict between Serpent men and "true men"; a version where "True men" are a bunch of kill-crazy imperialists impossible to live with. Indeed, it seems also in "The shadow kingdom" as if "true men" were the aggressors, the elder races only defending themselves. Probably Kull did abandon his vow about a final solution of the serpent man question after that. Then, 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, into the Modern Age.
The Serpent-Men appear in the Hyborian Age as schemers who can disguise themselves as normal men through magic. 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 their illusions can become quite impressive and detailed.
Their true appearance is revealed when their hear the sentence "Ka Nama Kaa Lajerama", a sentence only a human mouth can pronounce. When a human hears it, it sounds as if the person has already heard it before, even if it's the first time learning of it.