Difficult. I can give you some single pieces of music, however.
Johann Sebastian Bach's Toccata d-minor BMV 565 is a classic, also the title track from Phantom of the Opera.
Carmina Burana from Carl Orff has aside from the title track the excellent "Fortune Plango Vulnera", a truly scary choir piece.
Sergei Prokoffieff's Romeo and Juliet has the "Dance of the Knights", an equally dark piece.
Otherwise, though it's not reall organ music, there's always soundtracks. Poltergeist, Exocist, The Fog, The Ninth Gate, Halloween are all great for Ravenloft.
I hope you find what you're looking for,
Berandor