One thing I'd suggest would be to look through the theology of existing and past religions. When I first started thinking about this project, I noticed that many of those I looked up had at least one aspect that was echoed by the last invasion (if not the previous). I don't remember the specifics off the top of my head, but it's things like the Cult who preached the divine power of blood (the Light used it as a weapon), the druids who talked about some tree acting as the seal against evil (the NPCs spoke about a seal keeping the dark forces out that was wearing down and then breaking), and stuff like that. A number of faiths had "predicted" little bits of lore and could have easily said, during the invasion, "Ha! Told you so! We are the real deal around here!", but none of them actually did.
