This is book six in the Septimus Heap series. Here you can read about book five.
Marcia Overstrand (the ExtraOrdinary wizard) accidentally banishes the ghost of Alther Mella while banishing the evil ghost of Tertius Fume. Septimus’ Darke week is coming up, which means that while doing his apprenticeship he has to do a Darke task. He decides that for this task, he will retrieve Alther Mella from the Darke Halls.
After Septimus begins this journey, Merrin Meridith (an evil wizard) sets up a Darke Domain (an evil cloud that takes people’s energy), and floods the castle with it.
I didn’t like this book as much as the others because it felt more difficult to read with the darker theme.