in chapter 20 or 21 (I can’t remember), diana asks ertha if she can use her but the reason isn’t explicitly stated until later. from the novel spoilers, ertha also makes an agreement with diana, who hates the royal family so much because they’re all crazy and that’s why she wants to destroy them. her plan is to help ertha to run away by accusing zahid of being a worshipper to a fallen angel (i’m putting this in very simple words but there’s a lot concerning the fallen angel story) and this is not allowed so then if zahid who is a strong person in the military for the empire is gone, she thinks naakal’s earth wizard will wipe out bahalun empire.