Terry Pratchett. The Last Hero - A Discworld Fable. (Full Audiobook)

A message, carried by pointless albatross, arrives for Lord Vetinari from the Agatean Empire. The message explains that the Silver Horde (a group of aged barbarians introduced in Interesting Times, wherein they conquered the Empire, and led by Cohen the Barbarian, now the Emperor) have set out on a quest. The first hero of the Discworld, "Fingers" Mazda, stole fire from the gods and gave it to mankind (analogous to Prometheus), and was chained to a rock to be torn open daily by a giant eagle as punishment. As the last heroes remaining on the Disc, the Silver Horde seek to return fire to the gods with interest, in the form of a large sled packed with explosive Agatean Thunder Clay. They plan to blow up the gods at their mountain home, Cori Celesti. With them is a whiny, terrified bard, whom they have kidnapped so that he can write the saga of their quest. Along the way, they are joined by Evil Harry Dread (the last Dark Lord) and Vena the Raven-haired (an elderly heroine who has now gone grey).