In regards to Death, according to the monster encyclopedia in LoI, he is a soul-hunting being who is loyal to the owner of the crimson stone. However, it isn't explained why this is the case. This relationship between Drac and Death would explain how Drac becomes the "lord of all vampires." All he has to do is have Death collect the souls of defeated vampires, thus increasing his power is tremendously increased.
As for Olrox, I have no idea in regards to him. To me, he just seems to be another boss without any important part.