I'm trying to come up with a magic system for a "Cyberpunk and Sorcery" game I'm looking to run; my goal is something about halfway between "normal RPG magic" like Zird has in the examples throughout FATE Core and "you would have to be desperate or an idiot to use this" like the system toolkit's Void Calling.
Extra: Evocation magic. The world is full of Powers that you can call upon for aid, if you know how. Sometimes, though, they have more of an effect than you were expecting. Or desiring.
Extra Skill: Evocation. Anyone may take this skill (or possibly even use it at mediocre level if you didn't take it; I'm not sure if I want this or not.) You use it in the usual way, to Overcome or Create an Advantage. Maybe Attack, probably not Defend. If you fail your roll, you can choose to not get what you wanted, or you can choose Encroachment. If you choose Encroachment, you add shifts to make your roll a tie or success (you can't succeed with style), but you take Encroachment damage equal to the number of shifts you needed.
Encroachment consequences: You can reduce Encroachment damage by taking consequences, the same way you reduce physical or mental damage. If you have Evocation at Superb (+5) you have an additional minor consequence box which can be used only for Encroachment, same as for Physique or Will. Encroachment consequences should be named to represent the Powers gaining influence over you; it's not actually "possession", but your will isn't quite your own while the consequence lasts.
Notably, you don't normally have any Encroachment stress boxes. If you want, you can buy some with refresh. Spend 1 refresh to get your first box, and 2 refresh to get your second box. You don't get bonus boxes for +3 or +4 in Evocation unless you've bought boxes with refresh.
My goal is a magic system that represents a viable option without unduly limiting those who choose not to take it, and which is dangerous to the unskilled and untrained without being so obviously a bad idea that heroic characters can't use it. There's a hint (I hope!) of "do not call up that which you cannot put down" in the mechanics, with more to be added with fluff. How does it look?