The Hero of Ages by Brandon Sanderson

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.

Having escaped death only by becoming a Mistborn himself, Emperor Elend Venture hopes to find clues left behind by the Lord Ruler that will allow him to save the world. Meanwhile, Vin is consumed with guilt at accidentally releasing the mystic force known as Ruin from the Well.