The sum of all men - David Farland

Prince Gaborn Val Orden is visiting Sylvarresta, whose Crown Princess, Iome, he is expected to marry. Speaking incognito to the people, he finds the matter is not as cut and dried as expected. However, that pales into insignificance when it becomes clear that Raj Ahten, overlord of the Southern Lands, is planning to invade the Northern Realms and add them to his power base. As the war goes on, so the teenage lovers must come to maturity too quickly. And the magical system by which people give their overlords 'endowments' comes to be used for horrific ends...