The story is very very complicated with a *beep* of twists and turns and time travelling and people killing themselves in the past thus becoming...
In any case, if anyone actually described the backstory it would be very long and will probably not be able to answer all your questions to your satisfaction so I would suggest that you play the games.
Now since Blood Omen 1 can be a bitch to get running you can start with the Soul Reaver games and then do Blood Omen 2. This way you will understand the backstory, but if you do play through all the four previous titles I guarantee you you will want to play over defiance again because the story will make sense to you but you will still have questions that you didn't have before that will now be more clear to you.
The story is the main reason this franchise has survived as long as it has.
You should relish in it.
Actually, I played a couple more of the series six years ago and got a hold of the back story. was checking out my old posts in IMDb and found this lol