The Veiled Diplomat (The Reluctant Assassin Book 3)

Even a master assassin can be foiled.
Divination—the magic of seeing the future—provides peace in the realms, because no one can launch a war or assassination without being discovered. But when a strange building appears in the city of sorcery, everyone believes it's benign—everyone except Zayn and his teammates. To save the Hundred Halls, they must use all their talents to deceive and uncover its purpose before it's too late.