The Shadow over Innsmouth is great a horror novella by H. P. Lovecraft,
written in 1931, which forms part of the Cthulhu Mythos, using its motif
of a malign undersea civilization. It also makes references to several
shared elements of the Mythos, including place names, mythical creatures
and invocations.
The narrator, who is a student on an antiquarian tour of New England,
sees a piece of exotic jewelry in a museum, and learns that its source
is the nearby decrepit seaport of Innsmouth. Then he travels to
Innsmouth and observes disturbing events and people.
This is the only Lovecraft story which was published in book form during
his lifetime.