Giant Devil in Parallel Dimension

by Steve
(Connecticut)

What is the name of a film that used to be shown frequently in the 1960s-70s?

The ending has a giant devil creature that kills a teenager; earlier a cross pendant is crushed by a evil guy who is the devil; a giant cat also appears. The ending is on a beach with a cave.

Somehow a group of teenagers got into something that sent them to a parallel dimension where magic is real...

Does anyone know this one?