Volux Skraven:
Being high enough level as a lich or evolved undead you naturally attract a certain amount of HD worthy of undead to follow your commands and lead, mindless or not.
I mentioned this pteviously and was told no, silly really as people (and undead) would want to be known to be working with you or feel you are inspirational. If you end up being unworthy, they'll leave naturally.
Talpac now has his apprentices though.