Daughter of Captain Hook Book 3. The Dark Oracle.

Bea and Iris are reunited in Camelot. But when Bea discovers that Merlin is trying to rid dark magic inside his wooden prison, she tries to help. Merlin teaches Bea how to become a part-time oarcle, but when Emma comes as the dark one, and her father realizes what she's done, she's in for a lot of trouble.