Sounds cool. although I do wonder how a civilization living under a desert of eternal night manages to feed their millions of slaves. Fishing is the only thing that springs to mind, or possibly some artifact class food generating items.
So if we were to play the evil campaign, we would be starting as tagged human minions? Are humans the only slaves or are there other minion races like the classic werewolves, or merely other D&D humanoids. Working your way up to vampirehood is an obvious path, but what other options exist? Other forms of undeath?
Hmmm. One of my all time favorite PrCs back in 3e was a necromancy inclined bodyguard from the excellent Hollowfaust book for scarred lands. Could I take a stab at adapting it for 3.5? ... On second thought that book is in storage, damm. I'll have to see what I have available.