Ghost Night book 6. Serious spoilers ahead for the earlier books.
Which is inevitable when the book opens right after the cliff-hanger of the last one, with Caina and Kylon talking with two others in the room with the dead emperor.
His successor will be elected. And one likely candidate is a grave danger because he will make peace with the Umbranian Order -- which, given their evil and sorcery, will be a disaster.
They agree on the spot to ensure an election that will end with victory for the war, pick the candidate then and there, and set about ensuring it. Starting with going to tell him (and protect him) and hiding the emperor's death until then.
It includes an assassination attempt already planned, Ilona doing a dance to distract, a pretext of a hunting party, a larger scale assassination, the deduction that the emperor died, and more.