An installment in the continuing story. With spoilers ahead for the earlier ones. Though this one ends on a much sharper transition than the last one, or indeed any of the earlier ones except volume 3.
It opens not with Agatha but with the Baron. He had discovered the new revenants, which do not look like shambling monsters, but act like normal -- until the Other needs them. And he's very afraid that Agatha may be connected, or even be the Other.
Meanwhile, in the castle, Lucrezia/the Other is pushing Agatha's body hard in attempt to carry out her plan. Agatha herself is able to break through now and again, and try her plan. And a rescue party is going into the sewers after Agatha. And Anveka is carrying out her plans.
This all collides, in spades. One thinks rather better of the Baron at the end of it all, seeing what he has to deal with; his behavior makes rather more sense. And alas -- it is a painful work, because on several occasions you see reasonable people making deductions that you know are wrong, but which are indeed reasonable and demand action at once.
At the end of it, Agatha has a new determination, and strong reasons for it.