City of Sorcery: The Hundred Halls Series Book Five

As the Hundred Halls heads towards civil war, the fate of the city-and possibly the world-depends on Aurie and Pi finding the truth about Invictus' disappearance. Following a trail of dangerous clues, the sisters delve deeper into the head patron's past, finding their family's history entwined with the university. As they get closer to the final answer, they must decide if saving the world is worth shattering the ideas of who they think they really are.