You should probably read Robert E. Howards "The Shadow Kingdom", widely held as the first swords and sorcery story from 1929. It deals will Kull the King uprooting hidden Serpent Men withing his palace, and their ability to assume the facial form of others.
It was also published the same year as Lovecraft's Dunwich and features many references to his cannon of work, almost to the point of tying the two worlds together. Kull's world predates Hyboria in that timeline as well.
It is in print in both the Del Ray trade paperbacks; The Best of Robert E. Howard vol 1 and Kull: Exile of Atlantis. The best of also features the story "The Black Stone", which is a Mythos tale that is the first appearance, amongst others, of the book Nameless Cult (Unaussprechlichen Kulten). Awesome stuff really.