The Hero of Ages: Book Three of Mistborn

To end the Final Empire and restore freedom, Vin killed the Lord Ruler. But as a result, the Deepness--the lethal form of the ubiquitous mists--is back, along with increasingly heavy ashfalls and ever more powerful earthquakes. Humanity appears to be doomed unless Emperor Elend Venture can find clues left behind by the Lord Ruler that will allow him to save the world.