The campaign in Faith in Destiny involves the sudden appearance of the Nameless, and your efforts to thwart them. You play as a shaikan who has lost his memory, which means you start at level 5 and then have to work your way back to level 30. Sort of oddly, the whole memory loss thing is ignored after the introduction. I don't think you're supposed to be the same shaikan as from the earlier SpellForce 2 campaigns, but it's tough to tell since the dialogue is so poorly written.
There are only four maps in the campaign, and the whole thing only takes about 12 hours to complete. Annoyingly, the campaign comes to a sudden end right when you find out what the Nameless are up to -- as if Mind Over Matter is confident people will want to play a sequel. Peter Jackson can get away with this sort of thing, but Faith in Destiny has a lot more in common with Eragon than The Hobbit, and I wouldn't be surprised if it disappears into ignominy just as quickly.
Other than the new faction and the new campaign, Mind Over Matter left the SpellForce 2 engine basically intact. They tweaked some things here and there -- the skill trees are a little different, there's support for widescreen monitors, and you can zoom the view out farther now -- but otherwise Faith in Destiny looks and plays like a six-year-old game. If it was also priced like a six-year-old game, then that might be all right, but at $20 it costs about the same amount as all of the earlier SpellForce games combined, while offering only a shadow of the earlier content. That is, Faith in Destiny is no improvement at all.
Creating an expansion pack for a six-year-old game is one of the most curious things I've ever seen a developer do, and I've been covering games for a while now. I liked the earlier SpellForce games pretty well, but I don't think they're particularly well-known or have a rabid following, so Faith in Destiny was likely doomed from the start -- even if it has been a quality addition, which it isn't. If the idea of playing an RPG/RTS hybrid appeals to you, then I'd recommend the earlier SpellForce games and perhaps even Dragonshard before wasting any money on Faith in Destiny.