The Antyre Chronicles, book 1 : MaledicteLane Robins2007

Miranda, a bastard from the slums of Murne, capital of the Kingdom of Antyre, loses her lover, Janus, when his father, the otherwise heirless Earl of Last, abducts him. Miranda swears by Black-Winged Ani, god of love and vengeance, to reclaim the son and kill the father. Aided by dissolute Baron Vornatti, the earl's rival, Miranda develops the persona of Maledicte, a mysterious and dangerous cavalier. Eventually she reencounters Janus, but as Last's heir, he is two steps from the throne and has begun to develop ambitions. Miranda/Maledicte, torn between Janus' demands and the vow to Black-Winged Ani, is almost destroyed before she recognizes truth and loyalty again.