From the back of the book:
A Century ends... and Horror begins.
Long ago, the Mists of Ravenloft parted for a brief moment in time, and a terrible force invaded Earth. Now, in the waning years of the 19th century, its carefully woven plans come together. Only a few brave souls risk their lives to prevent the conquest of darkness.
Within these pages lies an alternate Earth of the 1890s, filled with wondrous inventions, dynamic energy, a thirst for knowledge, rare magics - and untold terror! Featuring a comprehensive campaign setting, new prestige classes, revised magic rules, lairs of evil and numerous adventure hooks, Masque of the Red Death allows you to visit the world of Arthur Conan Doyle, Edgar Allan Pe and Jack the Ripper. Explore the horrors of Ravenloft in a new time and place!