House of Ice & Shadows by Sherrilyn Kenyon

TAKE BACK YOUR KINGDOM

Gisela Bloodthorn is trained to kill. She has spent her life enslaved as an assassin in the court of the Centaurs, where only the strongest survive.
Now, the Centaur queen has a final assignment for her: protect the Oath Keeper who holds the contract that will keep their enemy at bay and kill the queen’s enemies and Gisela will go free.
But when Gisela discovers her first Outlaw known as Xaydin, Gisela learns that the Outlaws aren’t what they seem. Nor are they easily slain.
Xaydin isn’t just a bastard half-troll, he’s an assassin who has dedicated his life to ridding the Thirteen Kingdoms of the Oath Keepers, especially the one who holds the contract Gisela has been sworn to protect.

But trust doesn’t come easy to either of them. If they can’t find common ground and fight their real enemy, both of them will lose everything and everyone they care for.