Fleshing out an idea.

[Callan S.]

Perhaps the villains have their own moral conudrum as well? Corruption or lose troops? Choose! Perhaps players could actually help invent details to make it even more of a hard choice?

[Corey_at_Keene]

You guys have definitely given me lots to think about.  As always here are some thoughts and ideas floating around in this brain of mine.

At some point before the Crusade a collection of knights made a pact with the fallen angel Azazel (a fallen nephillim in hebrew lore who taught man the working of weapons and armor), and part of the benefit of this pact was to help these knights in their upcoming quest.  This angered god and he cursed these particular knights with leprosy.  After the success of the crusade, the knights formed a hospital to take care of lepers..partly in an attempt to atone for their sins and partly to hide their own growing curse.  They named themselves the Order of St Lazarus.

The pc knights are probably for the most part the children of these knights, or possibly just new recruits. 

In this model, the players completely control their own access to the powers granted by Azazel, not the GM, however doing so still invokes the anger of God, leading to corruption and over time to the effects of leprosy.

As for who the antagonists are, there really are countless options.  The conquered heathens who lived in this land before the crusade, other demons and fallen angels, archaic babylonian deities all but forgotten,  scorcerers and satanists..and the list goes on.