The Hero Of Ages (Mistborn Book 3), Brandon Sanderson

A new emperor; but can there be a new world? Tricked into releasing the evil spirit Ruin while attempting to close the Well of Ascension, new emperor Elend Venture and his wife, the assassin Vin, are now hard-pressed to save the world. This adventure brings the Mistborn epic fantasy trilogy to a dramatic and surprising climax as Sanderson's saga offers complex characters and a compelling plot, asking hard questions about loyalty, faith and responsibility.