The Final Empire by Brandon Sanderson

For a thousand years the ash fell. For a thousand years, the Skaa slaved in misery and lived in fear. For a thousand years, the Lord Ruler reigned with absolute power and ultimate terror, divinely invincible. Every attempted revolt has failed miserably.

Yet somehow hope survives. A new kind of uprising is being planned, one that depends on the cunning of a brilliant criminal mastermind and the courage of an unlikely heroine, a Skaa street urchin, who must learn to master Allomancy, the power of a mistborn.

What if the prophesied hero had failed to defeat the Dark Lord?