
(Not really spoiler, but it's more explained in the novel)
Trudy's fairy blood naturally helps him (and other relatives in the Fleurette lineage) to have a high resistance to poison and heal faster. That's why it was so hard to kill him, that they needed the academia doctor to make the poison. The only thing getting "activated" when he reaches age 10 is the potency of his fairy blessing. This is important bcs future emperors have to receive blessings from the Fairy King before they're crowned. Renato has to protect him until he's 10 because by then, his fairy blood will be blessed by the Fairy king and his ascension to the throne would be undeniable. And from the flashback, we recall that Maximo didn't get the Fairy blessing. Trudy getting the fairy blessing(turning 10) is pivotal to their success.
Just some inconsistencies that bother me
At first it says Trudi died because his fairy blood hadn't activated before he is 10. But then suddenly there's an apothecarist who could neutralize the fairy blood that's why trudi died?