From what I read, it was actually supposed to be its own game. It wasn't supposed to be part of the Legacy of Kain series. Legacy of Kain wasn't supposed to be a series at all. The original Blood Omen Legacy of Kain was a 2D game, kind of like Castlevania, but not exactly. Legacy of Kain Soul Reaver was originally supposed to be a game about a fallen angel, and the main character was supposed to be able to utilize the ability to shift in and out of the spirit world to re-establish himself. The game was supposed to be called Shifter, but Eidos didn't think the game could succeed on its own, so, they slapped the Legacy of Kain name on it, and sold it as part of a game series. At least, that's what I gathered, from what I read. I could be wrong.
I played and finished all the other games that came after: Soul Reaver 2, Blood Omen 2, Defiance. They were all crap. I even bought and played the original Blood Omen Legacy of Kain. I got as far as Vorador's mansion. I got tired of the hack and slash and the repetitive booby traps. I finally gave up on it. You'd be better off just watching the FMVs on YouTube, just so you could understand the story line.
Eidos should've left Shifter alone, rather than tainting it with that stupid Legacy of Kain BS. Soul Reaver "1" (Shifter) was one of the best games ever. They don't make them like that no more.