Now when going to the volcano on Conan Exiles and seeing the Serpent Men - my thoughts were -
Kull and Valusia.
I'll explain why -
Kull's time happened beforr the time of Conan, long before and Kull is from Atlantis however the focus is more on "the serpent men." I'll take this from the wiki about them - In Robert E. Howard's King Kull stories, the serpent people worship a god known as the Great Serpent. Later writers would identify the Great Serpent with the Great Old One Yig and with the Stygian serpent god Set from Howard's Conan stories.
But after the descrution of Valusia they went to Yoth, a cavern K'n-yan in North America(Also known as the Pictish Isles of the Kull stories), Tsathoggua idols brought their downfall again by Yig as they abandoned him, thought who still followed Yig were forced to go to Mount Voormithadreth in its caverns.
And in Shadows in the Skull by L. Sprague de Camp and Lin Carter, they are mentioned on the Conan stoties for the first time, so tis possible Funcom added more to the Serpent Men by having them in this unknown land they added to Hyboria.