The Dunwich Horror (1970), directed by Daniel Haller.
Lovecraft fans are not very happy with this AIP adaptation. We never get any metaphysical terror of the unearthly Old Ones; they are represented by glimpses of ecstatic dirty hippy folk, probably meant to represent their human cultists. The creature escaped from the attic is supposed to be -- what? The ending is an anticlimax.
The good points:
Sandra Dee is really very good as a young woman who submits to the overpowering will of another but never completely loses herself. We might expect "Gidget the Yog-Sothoth Slayer" but she is much better than that.
Dean Stockwell is intense and considered creepy by the local women, but he is charming to Dee and we believe in her attraction to him. He's the villain of the story but we can't come to dislike him; does that mean we also are getting pulled into his plans?
Supporting cast: Ed Begley (his last film), Lloyd Bochner, Sam Jaffe, Talia Shire.
Good photography, sometimes interrupted by smeared lens effects and bizarro solarization.
Produced by Roger Corman from a Curtis Hanson screenplay. Rather good Les Baxter score.
Available on Blu-ray from Shout Factory with a rather fine unprocessed image and one of those encyclopedic commentary tracks that would be better as a printed reference work. Murders in the Rue Morgue (1971) is on the same disc.