Follow Your Own Rules
You can do anything—ANYTHING you want—in a fantasy novel. That’s the whole point of writing a fantasy novel. You’re making an entire world with its own magic system, someplace new and special and amazing. And if you write well enough, I will probably be willing to follow along with whatever strange, wonderful, amazing, magical thing you come up with.
There’s only ONE thing you can’t do: Break your own rules.
When I’m reading a fantasy novel, I’m building a world—your world—inside my head. And every word that you write, every description you add, every seemingly random sentence you throw in, becomes law. If you tell me something, that is the way it is. It’s canon. It’s part of the world.
So if you tell me that the super-magical MacGuffin (don’t click that if you don’t have like five hours to spare :P) is kept on top of a mountain guarded by seven dragons, and can only be accessed with the blessing of a goddess, then that is Truth. So, if 200 pages later, our hero(es) arrives at the mountain, defeats the dragons, and wanders off with the MacGuffin, but doesn’t ALSO have the magical blessing of the goddess, then either it can’t be the real MacGuffin or the book is broken.
This is a crazy example off the top of my head, but hopefully you get my point. When you start a fantasy novel, you have nothing but possibilities. But every word you write limits what you can do afterwards, because I will believe anything you tell me until you break your own rules. And once that happens, as a reader, my suspension of disbelief is broken, and I’m tossed out of your world.
So, in order to make sure this doesn’t happen, it can be very beneficial for you as a writer to create a style guide/bible for your fantasy world. Keep track of any prophecies, magical rules, landscape features, or any other details that need to remain consistent throughout. Remember that your fantasy audience is probably made up of geeks, and when geeks fall in love with something (the way we’re going to fall in love your book, because it’s awesome obvs), we can get a little obsessive and nitpicky. Alpha and betareaders can also be really helpful here.
The rules for your fantasy world can literally be anything. All you have to do is follow them!