The Well of Ascension: Mistborn Book Two by Brandon Sanderson
The impossible has happened. The Lord Ruler is dead has been vanquished. But so too is Kelsier the man who masterminded the triumph. The awesome task of rebuilding the world has been left to his protege Vin; a one-time street urchin, now the most powerful Mistborn in the land.
Worryingly for her Vin has become the focus of a new religion, a development that leaves her intensely uneasy. More worryingly still the mists have become unpredictable since the Lord Ruler died and a strage vaprous entity is stalking Vin.
As the siege of Luthadel intensifies the ancient legend of the Well of Ascension offers the only glimmer of hope. But no-one knows where it is or what it can do...