Long ago, before human race there were vampires. They were peaceful race, at least before hildens atacked them. In the war against hildens very principles of nature were destroyed, and vampires tried to repair damage by building Pillars of Nosgoth (Nosgoth is land were everything happens). Every Pillar held power over one principle of nature (Pillar of Death, Conflict, States, Energy, Time, Dimension, Nature, Mind and finaly Balance). Every Pillar had it's own Priest guarding it. Power of the Pillar was power of the Priest - they were linked. But destroying nature wasn't only thing that hildens did - they also casted curse over vampires, making them blood-addicts. Many of them died of their own hand - pain of that curse was unbearable... But some of vampires addapted, they became predators.
It was then when human race appeared - ideal source of food for remaining vampires. But as humans were smarter than animals another crusade had begun. Massive ammount of vampires were slaughtered.
Noone knows when or how but contol over Pillars came to the hands of the humans, and although defeated once hildens decided to use that. Some of hildens came to humans and humans believed them to be gods, they worshiped them. And here begins real story...
Kain was nobleman, traveling through forest. Soon he saw an inn and decided to drink something. He didn't even know what was waiting for him outside... As soon as he got out bandits atacked him, stabbing him. He died cursing and killing them.
Kain woke at some sort of Hell, land of the dead. First person he saw was necromancer that offered him a chance for revenge, and Kain accepted it... He accepted it a bit too hastly, as he was revived as vampire. But he didn't care, all he wanted is to kill his own killers... He killed them, but then he saw another problem - as vampire he wasn't welcome to human cities anymore, and as he was immortal he didn't know what to do with eternity he had.
While walking near Pillars he saw Ariel, spirit of last Guardian of Ballance Pillar... Ariel sent him on the quest - Kain was supposed to kill all Pillar Guardians because Guardian's madness (I don't want to explain even their madness 'cause I'm trying to shorten the story... Yeah, this is short version XD ) corrupted the Pillars which corrupted very life in Nosgoth. Kain slaughtered them all, except one - new Balance Guardian. Killing him would be simple... But there was one little problem. Kain found out that he himself was that guardian. So, there was choice - to sacrifice himself, or to refuse sacrifice and to claim power of Pillars for himself.
Ofcourse, Kain refused sacrifice... What happened next? Story is too long so simply...
http://en.wikipedia.org/wiki/Legacy_of_Kain