It's okay for a 70s B movie, and probably close enough to the original story if you don't want to read it (edit: actually, reading the description again, it's further from the story than I remembered. It's been a while since I saw it).

The Dunwich Legacy acts as a sequel, so knowing the source is beneficial rather than spoilery.

There's a more recent movie also on Prime, and also starring Dean Stockwell, this time as Armitage in a modernized retelling. It's pretty bad but has some moments. It deviates from the story quite a bit.https://www.amazon.com/Dunwich-Horror-Sarah-Lieving/dp/B07Z8BF5T5