The Harbringer was the sword of the Lich King. In his attempt to amplify its power, he corrupted the sword. You must retrieve the Cursed Harbringer from the king's chamber, and then uncurse the sword to receive the Blessed Harbringer. These quests are available in The Book of the Lich King. Uncursing the sword requires the Cursed Harbringer and 3 Ginseng.