X-COM meets Diablo 1?
Works pretty well with the Horadrim backstory (prequel material?). Your scouting camps (radars) will detect smaller incursions first, Fallen, Undead, the bat thingies. You'll send (far-portal?) teams to engage them in dungeons, caves, plains. As the game progresses, you recruit and gear up, maybe research old artifacts (primitive form of identify?). Eventually the larger demons do terror missions (where you'll find pretty nasty stuff, maybe unique enemies/mini-bosses). Capturing demons could reveal weak points (allowing weapons to be blessed against them?). For the end game you have to assemble a super-team to take down a Prime Evil and imprison it.