Anti-Climax Boss: Wilbur, the hybrid son of an Eldritch Abomination, is unceremoniusly killed by a guard dog of all things. A single guard dog, not even a pack, and judging for the description of the scene, the guy (who is even revealed then to be grotesquely inhuman) barely put a fight at all.
Complete Monster: Old Whateley, the patriarch of the sinister, inbred Whateley family, is an ancient cultist with a storied history of Black Magic. As vile in his twilight years as he ever was in his youth, Old Whateley is hinted to be behind the violent death of his wife, and later turns over his sickly daughter Lavinia over to be impregnated by Yog-Sothoth itself in a hideous ritual. Old Whateley conditions one of the resulting twins, Wilbur, from youth in order to carry out Old Whateley's demands past his death; the entire Earth handed over to Yog-Sothoth. Old Whateley attempts to orchestrate the destruction of all life on the planet for a nameless purpose he takes with him to the grave.
Fridge Logic: Where did Wilbur's Y chromosome come from? Males inherit that from their fathers (females have XX).
If Wilbur is anything like Helen Vaughan from The Great God Pan, then he might not actually be biologically male—he could have assumed a shape agreeable to most people's image of physical maleness, just as Helen Vaughan wears a pretense of physical femaleness but is, in fact, an eldritch monstrosity that changes sex according to the situation.
Going by real(-ish) biology, the alien who fathered him must have human-like chromosomes to have viable offspring with a human woman. Of course, a very inhuman but sufficiently powerful Eldritch Abomination could also simply fashion whatever genome (and anatomy, for that matter) that was needed for the task.
Conversely (and perhaps more likely, given his own decidedly inhuman physiology and metabolism), Wilbur Whateley may have had a more exotic genome himself. It's not as though his DNA was ever tested in the story.
There's also a theory that Yog-Sothoth possessed Whateley Senior to impregnate Lavinia.
Describing a fight with an invisible monster is hard enough to do seriously in third person narration. It becomes even worse when the reader's viewpoint doesn't actually make it to the climax of the story and is instead forced to settle for listening to a description of the fight from a guy watching it through a telescope, making the ending come across like listening to someone describe a bad mime routine to a blind person.
The story's horror is somewhat lessened when described in exaggerated backwoods New England Funetik Aksents.
Ugly Cute: The Horror himself. He's particularly unsightly even by the standards of Lovecraft's creatures, being a house-sized blob of animal body parts with a human face, but it's also abundantly clear that he's essentially a giant child mentally and may indeed have little to no idea what's actually going on, unlike his brother and grandfather. His last words are a pained cry for his father Yog-Sothoth to save him.
The Woobie: Lavinia. She's ugly, deformed and ill physically and mentally. Isolated in squalor, she depends on her batshit insane and abusive father for everything. The same father who is implied to have killed her mother and sacrificed Lavinia to Yog-Sothoth in the first place. When he dies, she finds herself at the mercy of her demon-spawn son who openly hates her for no reason (implied to be because she is human) and soon enough kills her. To make matters worse, it seems that while her father and son's souls escape the demonic whippoorwhills at their deaths, they catch hers. Even after death, she can't catch a break.
YMMV for the 70s film
Awesome Music: The soundtrack of the film by Les Baxter is just a treasure trot of catchy psychedelic rock music, not to mention a rather cartoonish, but memorable main theme.
Complete Monster: Wilbur Whateley, child of Lavinia Whateley and Yog-Sothoth, seeks to unleash the Old Ones on humanity. Practicing Human Sacrifice and intending on making young student Nancy the victim to open the gateways, Wilbur seduces her, mind controls and rapes her, killing his grandfather when the man tries to stop him. Unleashing his horrific twin on the town of Dunwich, Wilbur attempts to kill Nancy to return the Old Ones and annihilate humankind.
Cult Classic: If you close your eyes to the fact that the movie is loved by fans of the 60's and psychedelic culture of that time, it is still a perfect example of Narm Charm.
Try not to laugh, watching the incredibly theatrical and eccentric Large Ham play of Dean Stockwell, making him look like a drunken student who is trying hard to keep his sanity and not to lose consciousness.
Willbur's fight with the guard is incredibly false, including obvious stunt performance and theatrical punches.
His grandfather should be perceived as an anxious, insane old man, but he is more like a funny eccentric grandfather.
Incredible ending of the film, including color hallucinations, newsreels depicting "weather special effects" and a burning mannequin falling into the abyss.
Narm Charm: Most of the viewing will seem to you that the film rather wants to give you psychedelic experiences under LSD, than to scare.
