Ghost Night book 5. Spoilers ahead for the earlier books, and the earlier Caina books.
It opens with the high Ghost circlemaster telling Caina that if she can not solve a mystery, the Empire is doomed.
While this does interfere with her getting the Sword back to safety, it is urgent: an ambassador, a king's brother, has vanished, and they need to get him back before his brother's kingdom switches sides in the war. This involves old friends -- Ark is astounded that she and Kylon married -- and enemies. Questioning the ambassador's servants brings up the ominous House of the Silent Door, in a manner which understates its problems. The Bronze Witch finds her efforts more difficult. A legacy of Moroaica causes many problems.