Decades ago I went to a double-feature and I only remember the name of the 1st movie, Phantasm. This had to be around 1979/1980 when Phantasm was released.
The other movie though, I can't recall the name and only saw the beginning. The scene I remember is, it's dark, maybe in Europe in some catacombs or a cathedral and a young man (tall, thin, blond, jeans and short sleeved striped shirt, I think) is following a woman. She may have been thin, short reddish/blonde hair, freckled. I believe he's following her because they made eye contact and she was playing coy, getting him to follow her.
She finally stops and he approaches her. They kiss and she may have disrobed (most likely, it is a 70's movie) suddenly we see her leave and he's lying there looking up, but as the camera gets close, you see his head is turned around backward.
That's about all I got. I think it was supernatural and witchcraft based, but my sister took the popcorn and said we have to leave; she didn't want to watch a "dirty" movie (I was 10, she was 15).
I've looked for years, even scrolled through every 1979/1980 horror movie listed on IMDB but nothing seemed familiar.