Shadow over Innsmouth

A young antiquarian sees an odd piece of jewelry in a museum and cannot place where it came from. His search for its origin takes him to Innsmouth, a decaying seaport on the New England coast that most people avoid talking about.

In town, an old drunk named Zadok Allen tells him about Obed Marsh, a merchant who made a bargain with an amphibious race called the Deep Ones. Wealth came to the town in return for human sacrifices and interbreeding. Out of that pact grew the secretive Esoteric Order of Dagon.

The narrator is stuck in Innsmouth overnight, and the town stops being merely strange. Its inhabitants, hybrids of fish and human, hunt him through empty streets, and his escape is a narrow one. The book also reveals what he learns about his own family afterward, a discovery tied to the bloodline behind the town's horror. That part is left for the reader to find.

This is a horror novella from 1936, told by its narrator in the first person. It starts as an investigation, settles into slow dread as the town's history comes out, and ends in a frantic chase. It is counted among the author's most influential works, and it established the Deep Ones and the Esoteric Order of Dagon as lasting parts of Cthulhu Mythos fiction.