From the back of the book:
There are realms and realms incomprehensible to the puny mortals who dwell within this cage that they term the Land of the Mists!
— The Madrigorian, Book I, Chapter I
Fiends have com forth from the Abyss and into Ravenloft! Masters of deception and manipulation, these ancient evil beings stalk the Lands, roaming freely in pursuit of their foul ends, and warping the very Land around them to suit their needs.
Dr. Rudolph Van Richten has set himself on their trail and presents here his preliminary findings and theories on these beings from places of unimaginable horror.
Dr. Van Richten exposes the nature, origin, strengths and weaknesses of the Fiend, and how it reaches the Land of the Mists. He explores the character of these beings, and their connections to the people and the Land itself. In the ultimate irony, these awesome creatures are as trapped by the Land as any mortal, seduced by prospects of power and glory!
Beware the Fiend! For those caught in the web of a Fiendish plot, nothing is as it seems. To defeat a Fiend, a quick wit and a strong heart are more important than a strong arm or flashing blade!