This is a very well written fiction. the idea is original and convincing. I will divide my review in good points, and points that could get better
The characters are fantastic. probably some of the best in the entire site. Their motivations are clear, their actions make sense and they grow and change with the history, they are incridible dinamic.
Right now, my favorite character is probably the MC's mother. I found it amazing the way you made her balance between the love for her child and the love for her husband and the rest of the family. I find her the most complex character.
The MC is also great. He is cold but not heartless, in an easily relatable way, his personality is cleary understandable, taking in account his past.
I like cold characters, but a lot of people take it too far, sometimes the character doesn't even have a reason for their personality.
The worldbuilding is not bruttaly extensive. There's no new races, but some new perspectives in classical fantasy races. Its taylor made for the casual reader of royal road that have no patience to climb steep learning curves.
Not much more to add, awesome story and plot
Points that could get better:
The one thing that I really think you should change are the prologues. They are boring to read and the first time I read your work, I just skipped them. I am afraid a lot of readers don't read your work because they get stuck in the prologues.
The start of the book needs to be great, more then any other part, it is your story's visit card. There are two ways you could fix this:
One of them is just to delete the prologues and fill the reader with the background information during the actual story.
The second is to create one or two prologues with actual scenes and POV to help the reader understand the world. for exemple. you could write an entire chapter about king Lionel III and the battle against the fenix, focusing on the characters and his motivations, making the reader relate to them, instead of focusing in the background information.
The grammar also could get better, but this is a minor problem. Doesn't bother at all.
In any way, these are just ideas. The hard part about writing a fiction you got down extremily well.
I am no writer yet, so you shouldn't take my criticism too seriously. keep the chapters coming, I really love your work.