Have you played any of the Legacy of Kain games so far? Because, if you haven't, I'd be willing to bet that the story won't make a lick of sense to you. If you want to understand the plot more thoroughly, I would suggest playing at least Soul Reaver 2 before taking on Defiance.

But, if you don't really care about the plot, and just want to kick some unholy ass, then by all means, pick the game up.