The Dark Lord of Derkholm by Diana Wynne Jones
Everyone - wizards, soldiers, farmers, elves, dragons, kings and queens alike - is fed up with Mr Chesney's Pilgrim Parties: groups of tourists from the next-door world who descend en masse every year to take the Grand Tour. What they expect are all the trappings of a grand fantasy adventure, including the Evil Enchantress, Wizard Guides, the Dark Lord, Winged Minions, and all. And every year different people are chosen to play these parts. But now they've had enough: Mr Chesney may be backed by a very powerful demon, but the Oracles have spoken. Nw it's up to the Wizard Derk and his son Blade, this year's Dark Lord and Wizard Guide, not to mention Blade's griffin brothers and sisters, to save the world from Mr Chesney's depredations.