The Hero of Ages: A Mistborn Novel by Brandon Sanderson (Book 3)

Having escaped death at the climax of The Well of Ascension only by becoming a Mistborn himself, Emperor Elend Venture now hopes to find clues left behind by the Lord Ruler that will allow him to save the world. But with the return of lethal mists, increasingly heavy ashfalls, and ever more powerful earthquakes, Elend and Vin aren't sure that killing the Lord Ruler to end the Final Empire was the right thing to do after all.

To make matters worse, Vin is wracked with guilt for releasing a primal being called Ruin from the Well. Long ago, Ruin was promised the eventual right to detroy all things, and now that Vin has been tricked into releasing him, Ruin apparently intends to collect.