The Shadow over Innsmouth

The Shadow over Innsmouth is a horror novella by H. P. Lovecraft, written in November–December 1931. It forms part of the Cthulhu Mythos, using its motif of a malign undersea civilization. It references several shared elements of the Mythos, including place-names, mythical creatures and invocations.
The narrator is a student on an antiquarian tour of New England. He sees a piece of exotic jewelry in a museum, and learns that its source is the nearby decrepit seaport of Innsmouth. He travels to Innsmouth and observes disturbing events and people.
It is the only Lovecraft story which was published in book form during his lifetime.
According to L Sprague de Camp, Lovecraft distrusted his ability to narrate action, and the story is unusual in that Lovecraft includes in chapter IV a sustained and effective piece of action writing.