Usurper - A King's Head, #1

Callin Vorst is the youngest member of the Kingdom's second family. Consumed with his conviction that his father should occupy the throne instead of the current incumbent, Rhomic Vandamm, he strikes a bargain with a creature of myth: the utterly misnamed 'Hag'. Before he can think again, the deal is struck and for him - not for his father. The Hag launches him on a career that will see him knighted and a national hero, triumphing against overwhelming odds - but at a terrible cost.
At once he is hero and villain - mighty warrior, gallant knight and secret murderer. It will also place him directly in the path of King Rhomic's beautiful daughter, Princess Avalind. Although they begin in friendship, their relationship is doomed to end in bitter opposition as the Hag exacts her price.