Ah! I didn't know you weren't a native English speaker - I'm sorry if my feedback is a bit harsh. Honestly, for a non-native speaker your writing is quite advanced, especially your penchant for imagery and complex sentence structure. Please don't feel too discouraged for what I said, and just keep writing. You can only get better! :)
Since you asked, I'll give you an example of the sorts of errors in your story, and I'll quote phrases from the start of your story to do it.
Most of the errors in this story came in the form of either:
* the wrong word. For example: "The Kingdom of Resvenia has always despised magic despite never outright banning them." - 'them' should be 'it', as magic is a thing (it) and not a person (them). Or,
* odd or confusing phrasing in a sentence. For example: "His room you were guarding just before are now stifling, the air thin yet humming with rising pressure." Would read better as, "His room, before which you'd been standing guard, is stifling; the thin air around you humming with rising pressure." ... or something like that. Or,
* slipping between past and present tense. For example, "He pushed you aside, rushing into the room" (which is past-tense. pushed = past, pushes = present) is followed soon after by, "You follow him inside, rushing close behind." (which is present-tense. you follow = present, you followed = past). When writing a scene, it's important to stick to one tense only, which is usually the present-tense because the reader is usually reading of something as it's happening. The only exception to this is if, for example, one character is telling another about something that happened in the past, such as, "He walked right past me without even stopping to say 'hello'!". Past-tense can also be used when a character is recalling a memory with some distance, whereas if the memory feels as real as if it were happening now, present-tense is used. Or,
* punctuation. For example, "How can a curse, let alone magic even enter the palace?" - there should be a comma after magic, like: "How can a curse, let alone magic, even enter the palace?" Because in this phrase the "let alone magic" part is an additional thought added on to the main one which is, "how can a curse even enter the palace?", therefore it needs to be separated by a comma.
But in general your punctuation is pretty good, so that last one is not as prevalent as the other three.
I hope that helped in some way. Don't worry about it too much though, because as long as you get yourself a really good proofreader they'll be able to clear all of that up for you. Hopefully they'll explain the changes they make along the way too, so you can continue to learn and improve on your skills.
Good luck! :)