The mythological blemmyae have their origin in Pliny the Elder's depiction of the historical blemmyae, that's for sure (they tied their capes to the sides of the head, so if you saw them from certain distance, you'd think that they were headless). But the "mythological species" took its own way into history and separated itself from the actual blemmyae that, as you say, ruled a long-lasting empire. You can find the MythBlems in the Prester John's Kingdom, for example, a fictional nation located somewhere in the eastern lands of Asia.
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