I really like the idea of the purge being a source of materials/loot you can’t get anywhere else. The stygian recipes and the material for making fire siege cauldrons is definitely in the right ball park. A unique resource to make unique armour and weapons would be cool, akin to khari steel.

I think the rewards should also help sustain the war effort, so obviously repair materials and arrows and gold, but also something to help handle the next purge better. Something like an exp potion for thralls would be good, that way your fighters can actually level up from all the fighting without having to follow you (which I find often results in their death). This would be easier to implement than a live exp system for thralls as some suggested too (though the live exp version is definitely more fun)