In my setting, a magic cast is able to alter the fate of a given person.
In this context, fate is not "set in stone" from the moment you are born, but rather given by the endless chain of causes and consequences given by your actions and everyone's else. So, someone "destiny" is like a tapestry given by a number of threads. One's choices may switch some threads or change some colors, but will never modify the whole picture.
Now, fate-magic users are able to make major changes to one's "destiny". From simple games of sheer luck (a dice rolling on a side instead of another, a coin flip) to actual skills (a person faring well in a sword fight, even if he never trained with a sword before) to, potentially, life altering changes (a person being born in a family different than its own, or never being born at all).
Those changes can be seen as "weaving" the tapestry in the example in a different way; e.g. shifting all the threads until the target person becomes a "good swordsman", as if he trained for his whole life. The intended effect may be temporary or permanent.
Now, as you well imagine, I do need some interesting drawbacks to this magic.
I've got some safeguards in place, but they are not enough against a form of magic that can potentially rewrite history:
- The usual rule of "the bigger the change, the more effort the magic requires" still apply, but it's not enough imho.
- One limit is given by the ability of the user to see the future and the past (divination is a thing in this world). For example, let's say you are tasked with saving the king. You foresee that he will die from an heart attack, due being overweight, on the following day. To "fix" that, you should change a considerable size of his past in order to make him eat healtier foods. But that's months of history to rewrite and it might be unfeasible. If only you had foreseen that sooner ...
- Another limit is given, in-world, by society. Strong marks of binding are placed on fate-mages in order to keep them tame. A mark of binding is a spell that keeps you from doing certain things, e.g. disobeying to your superior. Fate mages are kept on close watch by an international council. The warlords and the leaders of this world are interested in keeping the mages binded, since a unbinded mage could potentially destroy them out of spite. This is similar to what happens in DragonAge: Origins.
While both 2 and 3 are fine limits to the fate-mage powers, I'm not satisfied. A very talented fate mage could, for whatever reason, escape the marking process and just start wreaking havoc. While this is a nice concept per se, I'd like some plausible drawback other than "getting tired".