The Shades of Elphame. Gallowglass, #1.5

Robert will do anything to keep Karina safe, even if it means mastering the shades of Elphame. Robert has had many titles: folklorist, scholar, and most recently, assassin. He is the gallowglass, the most feared warrior in Elphame, and his sword belonged to the Seelie Queen. Then an American woman-his dearest Karina-freed not only his body but his soul from the queen's thrall. As Robert and Karina begin their life together, terrifying shades from Elphame attack them at home.
The shades focus their anger on Karina, but before they can be dealt with, Robert is summoned back to Elphame. The shades follow, and Robert is faced with a choice: destroy the shades, or become their master.