I learned how to get 1, 2, or 3 stars. I learned you get more stars at higher difficulty level. The one thing I couldn't find (and I looked hard) is what those stars are used for. What good are campaign stars? I've been playing 3 days now 😁
Each difficulty for campaign has rewards for getting stars. For example if you 3 star every level and get the 252 total stars available for the difficulty, you'll get either a void or a sacred shard (depending on the difficulty)