So in Spirit Island there are decks of cards that you draw from, one that shows what areas the settlers are going to try to invade/build in and two that have new powers for your spirits as they grow more powerful. The interactions themselves are deterministic, but there's definitely an element of chance (if you get a difficult to manage sequence of invasions, or keep getting new powers that don't synergize with your spirit you could run into difficulties that aren't due to poor play) but it's not like "Well I used my 'massive wildfire' power to burn the settler city down, but only rolled a 2 so I didn't work.""