Darklords (1991)

From the publisher:

There are hideous creatures roaming the mists of the Ravenloft dimension. These creatures have their own unique powers and histories. The Dark Lords accessory describes these creatures and their abilities.

From the back cover:
Within this volume are 16 dark lords and ladies, each a prisoner of Ravenloft. Some are new to the setting, yet all may seem hauntingly familiar, for they have been lurking at the edge of your nightmares, waiting for the chance to emerge in your campaigns.

The Headless Horseman, the Phantom Lover, the Vampire Panther - these and other malignant forces await. You'll read ghastly tales of their past, and grim accounts of their present. You'll visit their dark domains and tour the horrid sites portrayed in 10 full-page maps.

The shadows are lengthening; the air is growing chill. The Darklords are calling. Their voices are carried by the Mists of Ravenloft, and you have little choice but to answer.