Equinox (1970) 
Equinox is a curious little film, which shares more than a few similarities with its 11 year junior The Evil Dead.
Two couples take a road trip into the woods one day to have a picnic with the intention of meeting up with a professor friend. They arrive to find the professor's cabin has been crushed and that there is no sign of him. Exploring the area for clues leads the quartet to stumble across an insane old man living in a cave who hands them an old book - which just happens to be The Necronomicon (or a variation of)...
As you can expect, the book is opened and passages recited which conjures up all manner of beasts - all of which are depicted via wonderful stop-motion model effects. It is here where the film really shines as our quartet of teens are assaulted from all angles by otherworldly creatures having also to contend with strange invisible barriers and a park ranger who isn't all he seems to be.
The film's main failing lies in its narrative structure which unfortunately takes the form of the sole survivor recanting the tale of what happened on the ill-fated picnic via flashback sequences to a psychiatrist. This device in many ways takes a large chunk of the mystery away from the tale and instead of building suspense and the viewer working towards who will live and who will die, this has all been neatly resolved for you in the first ten minutes. However, the ending does at least provide a little counteractive nudge more towards the mysterious, albeit if not entirely unexpected.
Overall, despite its low budget trappings, Equinox is highly enjoyable, both as a dark magic fueled tale of the supernatural and as a precursor to the raw and atmospheric The Evil Dead.