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The Hero of Ages: Book Three of Mistborn (edition 2009)
Amazon.com Product Description (ISBN 0765356147, Mass Market Paperback)
Who is the Hero of Ages?
(retrieved from Amazon Fri, 04 Jan 2013 17:19:51 -0500)
Tricked into releasing the evil spirit Ruin while attempting to close the Well of Ascension, new emperor Elend Venture and his wife, the assassin Vin, are now hard-pressed to save the world from Ruin's deadly Inquisitors, the insidious lethal mists called the Deepness and the increasingly heavy falls of black ash that threaten to bury the land and starve its inhabitants.
(summary from another edition)