Ok, so my story is high fantasy, set in medieval times. My MC is the crown prince and only child of the king (he is 15). Horse back riding is a big deal in the kingdom, the army is all mounted (no foot soldiers) military states is also a big deal. My MC falls of of his horse during the Games (a big festival held every year, to celebrate the military.) He sustains a bad injure to his back, as well as a head injury. The head injury causes him to have seizures. Basically I ruin my MC's life. I need to know if what I have come up with is even possible. And if it even sounds like a good idea.
Sounds possible to me. Horseback riding accidents were the car accidents of pre-modern times, i.e. a common way people were injured or killed. And head injuries can indeed cause ongoing problems with seizures. Is it a good idea? Dunno, that totally depends on whether it works in your story.
I think it sounds like an interesting idea. There have been plenty of conquering heroes, or acrobatic thieves, or mystical magicians in the world (not that there's anything wrong with that). I think it would be a cool idea to explore a high fantasy world from the point of view of someone who is disabled. I'm not a medical expert, but I'm fairly sure an injury to the head or back could lead to seizures, and it's totally possible for even a seasoned rider to fall and get hurt.
Pretty straight forward: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4516165/ https://msktc.org/tbi/factsheets/seizures-after-traumatic-brain-injury http://www.braininjury.com/seizures-and-head-injury.shtml
It depends on your magic system. If mages can heal instantly then heal him up. But if magic is more or less in the background I would spend some time on this. Maybe have the "healers" tell the king his son will never walk again and spend some time showing how, through will alone, the prince learns to walk again and ride again. This will show your character's determination to succeed, a trait that rulers must have if they are to be effective. This will be a slow process, and he'll want to feel sorry for himself. Come up with a way to get him out of that funk and inspire him to do whatever it takes to be able to recover.