You aren't convinced the Acheronians were a super race of sorcerers? Here are a couple of quotes:
"Well," said Orastes, "I am no Acheronian, but I am versed in some of the arts, and the control of certain spirits which have cloaked themselves in material substance. Perhaps I can aid you in this matter."
~The Hour of the Dragon
And we have loosed a demon upon the earth, a fiend inexplicable to common humanity. I have plumbed deep in evil, but there is a limit to which I, or any man of my race and age, can go. My ancestors were clean men, without any demoniacal taint; it is only I who have sunk into the pits, and I can sin only to the extent of my personal individuality. Out behind Xaltotun lie a thousand centuries of black magic and diabolism, an ancient tradition of evil. He is beyond our conception not only because he is a wizard himself, but also because he is the son of a race of wizards.
~The Hour of the Dragon
The Acheronians are described by Howard as a race of wizards. As for his appearance, you should read Dale Rippke's essay on the subject, where he links Xaltotun with others of the ancient race (including a Kull tale) and shows they all display those physical attributes.