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Since discovering Ulduar in the Storm Peaks, adventurers have journeyed to the titan city to learn more about its mysterious past. Further exploration has revealed that the magnificent complex is above all a prison, intended to permanently confine the Old God of death, Yogg-Saron. This ancient horror has corrupted its guards, and now it is breaking through the last of its restraints.Faced with the peril of Yogg-Saron's imminent freedom, a band of mortals has made preparations for a sweeping assault on the city. Overcoming the legions of iron minions and towering defenses at the city's entrance will take tremendous strength, but even that great victory may not be enough to overcome the madness that dwells below."
via Battle.net