I don't think we are ever told his name, but I think it is reasonable to conclude that he was a Numenorean and he may well have ruled in the South, at Umbar or any of those southern coastal ports.

Sauron would have chosen for his captain a powerful man, and the Numenoreans were the long lived, powerful, advanced men of that time and the Numenoreans that returned to Middle-earth did so after Numenor came to the aid of the Elves of Middle-earth at Gil-galad's request, we know that some turned bad not long after that and began setting up ports on the coast and demanding tribute etc. So the time he appeared, his race, and the place he most likely ruled all fit in with that assessment. I don't have the references on me, but I want to say UT and HoME XI-XII is where I would start, plus the Silmarillion