I've just re-read the entire thing, but I don't see where it was written that Kalian's mother is a magician? From the official trans uploaded here, he's a son from a "lowly concubine." She has orange-red hair color. And her name is Freya. There isn't any information about her being a powerful magician. The most powerful magician, though, is Syspania, the she-dragon, who was the first queen consort of the land. And were talking about the history of their kingdom. Like the first one, their ancestor. And only Kalian inherited her features (red eyes, black hair). You might've confused her as Kalian's mother? But I do agree that the King is pretty spineless.
The reason why Kalian's mother died is because of Silikie poisoning her, not because of being a monster, but because she just wants to eliminate as many people that would impede Plants road to the throne. It makes me wonder if she's making a move on the first Prince's side...
I hate how mc speaks nicely with that emperor :/ he is a loser(emperor) let kalians mom got klled bc she was seen as a monster(strong magician banned) (if i remember correctly) ,and he treated karian as nothing bc it was his way to"protect" him ?? Bullsh1t man he is a loser and a powerless bstard till the end