Interesting Times. A Discworld Novel: 17

The oldest and most inscrutable empire on the Discworld is in turmoil, brought about by the revolutionary treatise What I did on My Holidays. Workers are uniting, with nothing to lose but their water buffaloes. Warlords are struggling for power. War (and Clancy) are spreading throughout the ancient cities.
And all that stands in the way of terrible doom for everyone is:
Rincewind the Wizard, who can't even spell the word 'wizard'...
Cohen the barbarian hero, five foot tall in his surgical sandals, who has had a lifetime's experience of not dying...
...and a very special butterfly.