"The Dunwich Horror" is a short story by H. P. Lovecraft. Written in 1928, it was first published in the April 1929 issue of Weird Tales (pp. 481–508). It takes place in Dunwich, a fictional town in Massachusetts. It is considered one of the core stories of the Cthulhu Mythos.
Wilbur Whateley is the son of a deformed albino mother and an unknown father (alluded to in passing by the mad Old Whateley as "Yog-Sothoth"), and the strange events surrounding his birth and precocious development. Wilbur matures at an abnormal rate, reaching manhood within a decade. All the while, his sorcerer grandfather indoctrinates him into certain dark rituals and the study of witchcraft.
The plot revolves around the desire of Wilbur to acquire an unabridged Latin version of the Necronomicon — his imperfect English copy is ill-suited for his dark purpose — so that he may open the way for the return of the mysterious "Old Ones", whose forerunner is the Outer God Yog-Sothoth. Thus, Wilbur and his grandfather have sequestered an unseen presence at their farmhouse; this being is connected somehow to Yog-Sothoth. Year by year, this unseen entity grows to monstrous proportions, requiring Wilbur and his patriarch to make frequent modifications to their residence. People begin to notice a trend of cattle mysteriously disappearing. Eventually, Wilbur's mother also disappears. By the time Wilbur's grandfather dies, the colossal entity occupies the whole interior of the farmhouse.
Wilbur ventures to Miskatonic University in Arkham to procure a copy of the dreaded Necronomicon – Miskatonic's library is one of only a handful in the world to stock an original print of the frightful tome. The Necronomicon has certain spells that Wilbur can use to summon the Old Ones for dark purposes unfathomable to men. When the librarian, Dr. Henry Armitage, refuses to release the university's copy to him, Wilbur breaks into the library at night to steal the loathsome book. A guard dog attacks Wilbur with unusual ferocity, killing him. When Dr. Armitage and two other professors arrive on the scene and see Wilbur Whateley's partly non-human corpse, before it melts completely to leave no evidence, they realize that the youth was not wholly of this earth.
The story culminates with the actual Dunwich horror: With Wilbur Whateley now dead, no one can attend to the mysterious presence growing in the Whateley farmhouse. Early one morning, the Whateley farmhouse explodes as the thing, an invisible monster, rampages across Dunwich, cutting a path through fields, trees, and ravines, leaving huge "prints" the size of tree trunks. The monster eventually makes forays into inhabited areas. Part of the cattle of at least two farms, and two entire families (the Fryes and the Bishops), are attacked and devoured. The frightened town is terrorized by the invisible creature for several days, until Dr. Armitage, Professor Warren Rice, and Dr. Francis Morgan, all of Miskatonic University, arrive with the knowledge and weapons needed to kill it. In the end, its nature is revealed: it is the twin brother of Wilbur Whateley, though it "looked more like the father than Wilbur did."
"The Dunwich Horror" is one of the few tales Lovecraft wrote wherein the heroes successfully defeat the antagonistic entity or monster of the story, although the Horror itself is only the remainder of a far more fiendish plan thwarted by Wilbur's premature death.