The Well of Ascension (Mistborn Book 2)

Vin, the street urchin who has grown into the most powerful Mistborn in the land, and Elend Venture, the idealistic young nobleman who loves her, must build a healthy new society in the ashes of an empire. Three separate armies attack. As the siege tightens, an ancient legend seems to offer a glimmer of hope. But even if it really exists, no one knows where to find the Well of Ascension or what manner of power it bestows.