This film is not so much a horror story (based on something by H.P. Lovecraft) but a tale of one man's (Dean Stockwell) misguided efforts to practice his "devil" religion- or to steal an old book and trick Sandra Dee into sleeping with him - it's fairly off putting because of a few weird dream sequences featuring painted naked hippies, people in hooded robes and bad special effects- also there's a monster with heat vision, Thalia Shire (as a secretary/victim), a crazy lady in an asylum, angry ineffective townspeople run amok, an spell casting show down, and "spontaneous" combustion- an ending that gets wrapped up with an explanation that really doesn't explain ANYTHING- just an all around strange movie smh
Here are a few links
https://en.m.wikipedia.org/wiki/The_Dunwich_Horror
(The history)
https://www.rottentomatoes.com/m/dunwich_horror/
https://en.m.wikipedia.org/wiki/The_Dunwich_Horror_(film)