The Mouth of Kopth is Book 4 of The Emperor’s Foul Legacy.
Eight years on from the events in The Tower of Sheagil, Falia is an avowed priestess dividing her time between the infirmary and the palace. But people remember that Kopth once spoke through her, giving her more prestige than she is comfortable with. She is the Mouth of Kopth. So the Emperor asks the Mouth of Kopth to travel south to look into the nature of the strange creatures there. Are they humans or just a forest animal?
The journey takes her to places further than the Emperor, or anyone, dreamed of. She finds something much more menacing than forest creatures. And she finds that Kopth still speaks through her in ways she never thought possible.
This is the kind of story that has appendices and a map.