"The Cruel Empress" also called (The Gruesome Empress)is the Fifth novels by author Lene Kaaberbøl
When Hay Lin receives an ancient lantern from one of her grandmother's friends, strange things begin to happen. She wanders into a store and finds a magical mirror, and then hears a mysterious call for help. The other Guardians are worried. Together, they try to find out who is trying to contact Hay Lin. To solve the mystery, the girls travel through a portal to a city where they face a cruel empress...and an unfulfilled promise!