So, for magic systems, I have a lot of trouble. It's still incredibly fun, I just haven't had enough practice creating magic systems of my own. And to be honest, this one is only kind of a magic system, primarily due to the fact the "magic" is actually just a mind-controling fungus with the consistincy of oobleck.

Because the world this magic system is going in is specificly for a story I'm working on, it's a bit more work to get it how I want, because I have to primarily focus on the characters instead of the worldbuilding and make sure the worldbuilding ties back to the characters. But, the basic way this "magic system" works is that there is THE OOZE. THE OOZE is a parasitic fungus. The only creatures it has a symbiotic relationship with is the carbohydrates.
When THE OOZE gets into any creature that is not the carbohydrates, it doesn't do anything at first. But when the creature's fight or flight responce activates, that gives THE OOZE enough energy to release magical powers as some sort of attack. When THE OOZE releases magic, it worms its way deeper into the system of the creature it's infecting. Once a creature is infected, they can also cause THE OOZE to release magic volintarily. When THE OOZE fully gets into the brain, it starts to remove what makes the creature an individual, and it turns it into part of THE OOZE's hivemind. (The carbohydrates are only partly part of the hivemind btw).