The Whateley family who live outside of the forgotten, decadent village of Dunwich in Massachusetts's Miskatonic valley are shunned as freaks and wizards–especially Wilbur Whateley, who was born of an insane albino mother and an unknown father. A profoundly ugly child, Wilbur grows and develops at an alarming rate. Before he's even a teenager, he's the size and maturity of an adult. And along with his barely human appearance, his grandfather is versing him in all manner of unspeakable occult lore that has something to do with shunned Sentinel Hill and the reason why more and more of the Whateley house boarded up and hollowed out as if to hide and make room for... something.
One of the defining stories of Lovecraft's Cthulhu Mythos, The Dunwich Horror is also one of the most accessible, having later been made into a 1945 radio play and a 1970 feature film. It's a frightening story of barely contained evil that is bent on breaking through and overwhelming our world, but its real power is the mounting revelation of the "other" as the oppressive decadence of Dunwich gives way to the evil of the Whateleys, who have sold out the human race, then the fate of Wilbur that uncovers his secret and then the even greater menace that this releases on the world. Though the climax is a bit underwhelming, owing to being related by witnesses who only see it from a distance, the outcome is saved by one of the most chilling final reveals in weird fiction made even more frightening by its matter of fact statement.