The Flame of the North (Last Life Book #4‪)‬

Konung Bjørn Sharptooth, ruler of Vintervald, has announced a Great Trial. Whoever emerges victorious shall be wed to the konung's daughter, Princess Astrid. In accordance with ancient custom, for the duration of the Great Trial, a ceasefire is declared in Northland. Sworn enemies in the midst of bloody war must put down their weapons, and all those worthy of the princess' hand may come to the capital to try their luck. Carl III, wanting to impede Northlander raids into his northern borderlands, decides to conclude an alliance with the konung and sends his youngest son Prince Louis to Vintervald to take part in the Great Trial. Impressed by Max Renard's mastery as a swordsman, Prince Louis invites him to join the northern embassy, to which Max Renard agrees. After all, the trip to the north with the embassy will serve as a great cover story for him to complete a secret mission from the Duke de Bauffremont. And while the powers that be think they've found an obedient agent, Max Renard will put his own plan into action.