Based on the essay “The Hyborian Age” and the research done by Dale Rippke The Mystery of Pre-Human Stygia who I think has a fairly convincing argument based on various bits and pieces left by Robert E. Howard the Stygian people of the Hyborian age came from the the area of what would at this point in time (game time) be part of Khitai. They were disposed by those they had enslaved and had traveled for a long period of time conquering and absorbing small tribes along the way until they fall upon the land which is now known as Stygia. The race there has become weak and decedent and was ill-prepared for such and onslaught of desperate savage people who had no where else to go. Some of their nobility instead betrayed them and sided with these new people thus why the nobility of the Stygian people tend to look lighter, taller and thinner than the common Stygian even to this day. They are still very much of mixed heritage. The former inhabitants of Stygia fled and founded the empire of Acheron and were eventually crushed by the invading bands of savage barbarian Hyborian peoples.