Warbreaker

This is a story of two sisters - who happen to be princesses, the God King one of them has to marry, a lesser god, and an immortal trying to undo the mistakes he made hundreds of years ago.
Theirs is a world in which those who die in glory return as gods to live confined to a pantheon in Hallandren's capital city. A world transformed by BioChromatic magic, a power based on an essence known as breath. Using magic is arduous: breath can only be collected one unit at a time from individual people.