The Rose of Shanhasson. The Shanhasson Trilogy, #1

A princess stolen by a barbarian warlord... As Our Blessed Lady's last daughter, Shannari must rule as High Queen or the Green Lands will fall into eternal darkness. Her destiny is to shine against the Shadow, protect the land and people with her magic, and keep the Blackest Heart of Darkness imprisoned. Her blood is the key, powered by the love in her heart. However, Shannari's heart is broken, her magic is crippled, and the nobles must have forgotten the dire prophesies, because everyone wants her dead. Only love can restore her magic, but her scars testify how love can be corrupted.
So when a barbarian warlord conquers her army and professes a love like no other, Shannari's first instinct is to kill the mighty Khul. Even worse, one of the Khul's guards used to be an assassin-a very skilled assassin, if the darkness in the Blood's eyes is any indication. The same darkness festers deep in her heart and draws her to the wickedly dangerous man as inexorably as his Khul's unshakable honor.
Her weakening heart is not only torn between love and duty, but also between two magnificent warriors. Yet neither warrior will be able to help her when Shadow calls her name.