Blood Omen game is kinda like 2d Zelda, but very dark and gritty, it's my favorite.
Soul Reaver 1/2 are 3d platformers/action games with some simple puzzles and a normal/spectral world mechanic (you change planes and stuff you can do as a spectre you can't do in the normal world, like going through certain walls).
Defiance kept some of the platforming from previous games but it's more combat oriented, it has different skills and combos, kinda like a Devil May Cry game, it's the first game in the series in which you control both Kain and Raziel (Kain is the dude from blood omen, Raziel that edgy looking fellow from Soul Reaver)
Then there's blood omen 2 which I never played but I've heard it's pretty bad.
All of them are decent but none excel in their own genre (gameplay-wise), there are better platformers than Soul Reaver, better hack n slash games than Defiance.
If you're planning on playing them be warned, the main focus of these games IS the plot (at least in latter games) and they're full of cutscenes, in the case of Soul Reaver 2, unskippable.
Also there was a sequel planned but it was cancelled.