This is essentially a pretty clone of Minesweeper by Rivas Games - but it's really nicely done all the same! Find the trolls (the mines) spread across the land by clicking and placing down markers.
Unlike the original game you also have a limited set of fireballs which you can blast trolls away with, thus revealing a little more of the map - so it's handy to save these for those situations where it comes down to a 50/50 guess.