"The Last Hero: A Discoworld Fable" by Terry Prachett

Cohen the Barbarian may be old but one thing he's not is satisfied with being the Emperor. He's got a bone to pick with the Gods, so he and his Silver Horde are on a mission and if they should die a glorious death so much the better. However there are those opposed to this plan since it could mean the end of magic in Disc World, not in time but in seconds which would mean goodbye to wizards, dragons, or actually, anything. Can the wizards make it to the top of the Hub in time to stop Cohen or will the saga of Cohen and his Silver Horde be the most famous one never sung.