Throne of Shadows and Hearts

Han Chan had been saved once by Prince Ning, a mild, decent man, and she was sent to destroy him. The man who ordered it was the enemy who had wiped out her people, and he held her younger sister's life over her to make her do it. She went into Prince Ning's household as a spy, a piece in the fight over the throne. She was a cold assassin by trade and found she could not move against him easily, so she played at seduction and cruelty while feeling neither. The more she was drawn in, the worse the things she had to do for her sister's sake. Then the truth came out and the real monster showed himself, and she understood she had had right and wrong backwards.