In the kingdom of Hallandren, an immortal God King presides over the Court of Gods — the Returned, people who died and came back with divine power and no memory of the life before. Peace with the neighbouring mountain kingdom of Idris rests on a treaty promising an Idrian princess as the God King's bride. When the day comes, the Idrian king sends not Vivenna, the eldest daughter raised her whole life for the role, but her youngest sister Siri, who was raised for nothing at all.
Siri arrives in the colour-drenched capital utterly unprepared, and finds that the God King is nothing the stories told her he was. Vivenna follows in secret to rescue her, falls in with the Idrian underground, and discovers that the politics of Hallandren — and her own convictions — are far murkier than she was taught. Watching from the Court is Lightsong, a Returned god who does not believe in his own divinity, and Vasher, a wandering man with a talking sword.
Warbreaker is a standalone novel built on Breath and BioChroma — a magic in which life force can be given away, hoarded, and used to Awaken inanimate things. Brandon Sanderson posted it chapter by chapter on his website as he wrote it, and it remains freely available; it was published in book form in June 2009.