This article mostly pertains to fantasy writers, but if you can find some way to weave magic into a science fiction novel (by justifying the magic with science, or some other way), go for it! I’m going to go over some things to think about when you put magic into a story, so you can keep it believable (or at least, make it make sense).
I got inspired to make this post today because I’m working on brainstorming a magic system for one of my newest storyworlds. It got me thinking about magic (or any other supernatural-type system that doesn’t exist in our world, like element bending), and how to make it believable. Luckily, people can suspend a whole lot of their belief when reading a fantasy novel, especially when it comes to magic. You still can go wrong with magic, though, and you don’t want to leave people tilting their heads and going “ehhh” because your world’s magic doesn’t make sense.
One of the things that bothers me when I read a story that executes its magic badly is when things like enormous spells happen and require no effort (or garner no consequences). Your character probably shouldn’t be shooting enormous dragons out of her wand just because she can. Since magic is a type of power, at the very least, it should require effort to execute it correctly. If your character is casting a big spell, it should take a whole lot of concentration, or exert some sort of physical effect on your character. In Harry Potter, for instance, you need to be very focused if you’re casting an intense spell. Magic could also take a physical toll on your character, since it's the process of manipulating and using energy.
The effect of magic on your character probably depends on how the magic is integrated into the world in the first place. In Harry Potter, it seems like magic is just something that exists, and those with certain traits (probably something genetic that makes you a wizard) can have a certain degree of control over it. And when they practice, they can get better at tapping into that resource. The same goes with bending, in the Avatar universe. The ability to bend appears to be genetic, and even those who first discovered they could bend had to learn from creatures who were experts in it. Even if magic doesn’t have a defined source, it should still take concentration and skill to use it.
Another way to have magic in your story is to give it a defined source. Magic could come from somewhere within the planet, for instance. It could be a resource that humans (or whatever sentient species you’re writing about) taps into, and it could be finite or infinite. It could be something spiritual, an energy you can only access when you’re more in tune with the universe (bending is like this; the Avatar’s bending is a lot more powerful when the Avatar is in tune with their spiritual side).
Also, if your character has a better ability than most when it comes to magic, give them a reason for it (and don’t make them absurdly powerful either, unless you can explain it well. It also makes it more believable if the absurdly powerful person suffers some sort of toll when they use excessive amounts of magic). With someone like the Avatar, their bending/magic ability is there because they are the spirit incarnate of the planet itself. And within that storyworld, it makes sense. In the Lost Years of Merlin, Merlin has strong genetics for magic ability and will become extremely powerful, but it takes him an immense amount of time to hone those skills. Within that storyworld, magic takes a lot of energy out of you, which makes it more believable. Merlin’s not just throwing huge amounts of magic at things with no consequences.
Aside from all this, here are some things to think about when you’re brainstorming a magic system:
Magic is a lot of fun to write about, and when written well, it can really make a story memorable. Hopefully this will help you out a bit, when you’re writing about magic. Good luck!