Perhaps it was merely all the death from war and disease that open a portal some dread place, or maybe it was the purposeful act of malevolent intelligence of the Outer Dark? Whatever the cause, at the end of the Great War a strange mist settled over much of Europe. Supernatural beings of legend were again free to walk the Earth...
The idea is to take the style and ahistorical setting of the Universal and RKO horror films of the 30s--what in Shadows Over Filmland Robin Laws called the "Backlot Gothic"--and apply it to Ravenloft, whether the Masque of the Red Death version of Ravenloft or the original recipe would be up to you, but I think recasting various Ravenloft denizens as "off-brand" Universal Monsters stand-ins would be the way to go.