Dark Lord of Derkholm — Diana Wynne Jones

FOR FORTY YEARS, Wizard Derk's world has been devastated by Mr. Chesney's Pilgrim Parties -- packaged excursions for tourists from the next universe in search of adventure. When mild-mannered Derk is chosen to play the role of this year's Dark Lord, he is forced by the sinister Mr. Chesney to turn his bucolic country estate into a labyrinthine castle lit by baleful fires, manifest himself as a nine-foot-tall shadow with flaming red eyes, and lead his minions in a climactic battle against the Forces of Good.

Can Derk find a way to put an end to the evil Mr. Chesney and his Pilgrim Parties -- once and for all?