@Lioness94 Good point. The only way I can see it is if the poison created a genetic mutation/defect in him but that's some next level poison if that's the case. The other option would be that the poison impacted the inherited magical/holy power received by Ludan and that in turn resulted in a physical impact on his lungs. However, these kinds of explanations seem much more convoluted and improbable. It would have definitely made more sense if the mother had been poisoned.
Now the other thing I'm wondering about is the relation between the Emperor and Hilde. It seems like her father is implied to be a prince, and the Emperor was I think the son of another prince (the 3rd prince maybe?). In that case they'd be cousins or, if their father's were half brothers, half-cousins. That's if I'm understanding it right though, I don't think her father's position was exactly clear. If Hilde's father was not a prince but just another powerful noble, then the relation would likely be further or nonexistent.