DoI have to invent spells based on skills or are there some preset spells to chose from with description and everything?
You can make your own, but you should also be given a handful of random pre-made spells for you, based on the skills of your character. I'm not entirely sure how it works because the first few characters didn't go through that process, but I'm guessing there's a button somewhere that says "give me spells now, please"?
