Sounds like it'll get boring fast. The initial mystery won't last and it it does, it means it's dragging on. After that, it'll just be a generic noble monster story.
I'm not quite this pessimistic, but I agree that it could have pacing/premise issues. Plus the usual issue of people either figuring everything out pretty fast ("Sunlight negatively affects me in some way, I'm a vampire!") or having really incoherent monsters ("Weak to water, frost breath, teleport between shadows, and super endurance. Uhhhh...?")
Any way to make a crusade game work? I don't mean a historic crusade game (there's no way to salvage that), I mean one set in a fantasy world. How do you get a mass army joining in an offensive campaign while keeping them the good guys?
Depends on your thresholds for "good guys." A
relatively safe assumption is that the bad guys are doing something that directly and severely impacts the good guys; you can try for the usual "they're slavers!" or "they have sex in ways we don't like!" type angles, but you tend to get into weird cost:benefit analyses and sovereignty issues pretty quick. Rotting the soil beneath the good guys' feet or opening the gates to oblivion basically cheats by effectively making the good guys' behavior in self defense/preservation, meaning all that sieging of cities and pillaging of crops and so on is totally something they have no choice but to do.
If you don't want to go quite so blatant, you can also try the long haul for cost:benefit issues, basically taking the "let's liberate them from their evil selves!" approach on a more rationally defensible line. If your kingdom/empire/city state/clan is awesome and wealthy and happy and content and do everything right, while the enemy is really just terrible at everything, not just in a moral sense but in legitimate efficiency issues, you can make a decent argument that murdering the shit out of them and then ruling them with an iron fist is literally better in the long run than just letting them mind their own business. You run into issues almost
immediately, of course, but keeping the thing's head above water is really all you can hope for if you're trying to be more realistic than "the world will end if we don't."
Final note: Mind the methods. "An offensive campaign" can mean anything from lightning strikes against forges so the enemy lacks the weapons to hurt you, to a grueling scorched earth extermination campaign wherein being buried alive is one of the nicer fates to be had. What the good guys do when they win, what they're willing to do to win, and what they consider winning to mean can go a long way towards redeeming an iffy cause or damning a good-looking one.
Eesh... Having the good guys be crusaders even in a fantasy context is iffy to begin with. And making the opposite side evil is just chock-a-block with unfortunate implications.
The only way I could see it not being uncomfortable would be to distance them from the actual crusaders while having the background of a massive crusade. That way you have your cake and eat it.
As for ideas in that direction... Perhaps have them be deserters trying to cross hundreds of miles of desert to return home, or religious pilgrims seeking absolution for their sins while trying to avoid being killed in the crossfire. Think outside the box.
Pfff, everything has unfortunate implications. The bigger issue for me is believability/interest; cardboard cutout evil goons tend to be way less interesting than nuanced individuals who happen to consider flaying prisoners to be a legitimate hobby.