The Shadow Over Innsmouth by Howard Phillips Lovecraft

The story describes of a strange hybrid race, half-human and half an unknown creature that resembles a cross between a fish and frog, that dwells in the seaside village of Innsmouth (formerly a large town, but lately fallen into disrepair). The townspeople worship Cthulhu and Dagon, a Philistine deity incorporated into the Cthulhu Mythos.