Fire Wizard. Ghostspeaker Chronicles, #4

Johanna, Roald, Nellie and Loesie return undercover to the devastated city of Saardam. In the months since they fled, the Fire Wizard Alexandre has ruled the town with campaigns of death and fear, focused on the eradication of the Church of the Triune, the former state church of Saarland that doesn't allow magic and that is more popular with the common people than with nobles. Johanna meets up with underground members of the church, even though her little gift of magic makes her feel ambivalent about it.
She knows that ousting Alexandre will require a powerful magician, something she and her followers do not have. What is more, Alexandre's men discover the group, and the band of resistance fighters faces the showdown with the tyrant long before they're even close to ready for it.