The Queen's Secret (The Queen's Assassin Duology, Bk. 2)

Though Lilac's birthright made her the Queen of Renovia, and marriage made her the Queen of Stavin, she soon learns that being a ruler does not mean making the rules. For Lilac, taking the throne means giving up the love of her life, the kingdom's Chief Assassin, Caledon Holt. Worse, Cal is forced to leave the castle when a horrific set of magical attacks threatens Lilac's sovereignty.

Now Cal and Lilac will have to battle dark forces separately, even though being together is the only thing that's ever saved them.