@Chttrbx_ He IS the rightful owner if the nobles decide to support him. That's how it works. If Claude is accused of black magic, and is found to have traces of it on him, the nobles will denounce his claim on the throne. Athy, by extension, for covering it up, will also be denounced. That's the whole point. That's the whole reason that Athy has been fighting so hard to not let the others realize Claude is not only bedridden, and that's why the nobles in the background were already chattering nonsense against Claude. Because if there is any reason Claude is not squeaky clean of black magic in this scenario when he's already on his near-death bed, they might as well switch sides. It's either the "black magic user Claude who's almost dead and his kid who tried to cover up the whole thing from us" or "black magic user Anastacius who is frankly still alive and standing."
So the thing is: if Claude is diagnosed with having used black magic, the nobles will not support him, and as he is on his death bed anyway they need a new ruler. They cannot trust Athy because she clearly knew it was black magic and therefore is not appropriate. Anastacius, having experience, the bloodline, AND the power, is clearly who they would support.
If Athy revealed Anastacius to be the cause of her father being bedridden, then Anastacius will prob clap back with smth like "lol where's the proof" and it also does not erase the fact that he is at least alive and well, while Claude is in a whole-ass coma. Athy is doing well covering for her father, but Anastacius obv has more experience and can handle the country's matters better, the nobles themselves have experienced that. Ergo, they would appoint Anastacius over Athy.
It's not just as simple as "ooo anastacius obv so bad, claude is already ruler so we can't go backwards," the nobles need someone to competently rule the country and at the moment Claude has not for a couple of days already. Athy, while she has handled herself well, probably still does not have the speed and competence that Anastacius does considering how long he had the role for.