Half-remembered fantasy film - please help!

This is a desperate plea to all you fantasy cinephiles out there who post here!

I have a vague half-memory of a fantasy film, in which all I clearly recall is that people were possessed by evil spirits; they appeared normal when you looked at them, but, if forced to gaze into a mirror, their reflection would show their evil nature. If the mirror was shattered, the spell was broken, the character would swoon, and, upon re-awakening, would be their normal selves again.

I seem to remember that the setting was vaguely medieval; and the ambience seems to suggest something that MGM would have made in the 1950s.

The clues are sparse (and perhaps misleading!) - but do any of you think you know what the film might be?