The Final Empire by Brandon Sanderson

Once, a hero arose to save the world. He failed.

Ever since, the world has been a wasteland of ash and mist controlled by the immortal emperor known as the Lord Ruler.

But hope survives. A new uprising is forming, one built around the ultimate caper, the cunning of a brilliant criminal mastermind, and the determination of an unlikely heroine: a street urchin who must learn to master the power of a Mistborn