Title: Dark Lord of Derkholm
Author: Diana Wynne Jones
Publisher: Greenwillow Books, New York
ISBN: 0-688-16004-2

A world of magic under the tyrannical rule of a man with enormous powers: Mr. Chesney, a humorless business man. Doesn't sound quite like your typical fantasy novel, does it? And it's not. Mr. Chensey, who comes from a world with no magic, has somehow gained control over a small but incredibly powerful demon. He uses this demon to force the people of the (unnamed) world of magic to stage the typical "band of heros overthrows tyrannical Evil Overlord" stories, and then sends people from his world on "tours" through these staged stories, where the tourists get to play the parts of the main characters; he charges money for these "Pilgrim Parties", and lives in his non-magical world, getting rich. There's so many Pilgrim Parties that there are shortages of food (the Armies of Good and Armies of Evil trample the crops), kingdoms are going into debt (because the cost of rebuilding after the battles isn't as much as they get paid by Mr. Chensey), and the economy is in a sorry state (because of all the people are getting drafted into the Armies).

This has been going on for decades, and the people of the world want the Pilgrim Parties stopped. Querida, High Chancellor of the Wizards' University, takes a small group of important people to consult the Oracles. They are told that, to put a stop to Mr. Chesney, they must appoint the first person they meet to be this year's Dark Lord, and the second person they meet to be one of the Wizard Guides. The first people they meet are the wizard Derk and his son Blade.

Derk is a kind, peaceful bioengineer Wizard who's spent his life breeding flying horses, flying pigs, smart geese, and griffins with human intelligence; in fact, five of his seven children are griffins, with only two of them being human. Since he's the Dark Lord, he has to change his house into a dark fortress and be prepared to have both it and the nearby village repeatedly destroyed by wave after wave of Pilgrim Parties. (Poor Derk!) His family helps by creating and placing clues for the tourists, planning tour migration paths, and so on, but they have to take over the whole rigmarole after Derk is put out of commission by an angry dragon. And then things start getting even worse...