Mistborn trilogy Book 3. The Hero of Ages

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.