I gotta say, Carmina Burana and Alexander Nevsky are probably going to be the tops…though Penderecki’s Canticum Canticorum Salomonis would fit the bill. Or his Dies Irae (which I’ve newly discovered!) which was written in honor of the Warsaw Uprising.
Heavy.
It isn’t exactly classical, but check out Nico’s Desertshore and Marble Index albums. They are awesome and have a spooky vibe that could work in an old Roger Corman or Euro horror soundtrack.
Songs like The Falconer (which has a beautiful piano piece in the middle) or Janitor of Lunacy I think might work for you.