The Deep Ones are an immortal, fish-and-frog-like race living in cities on the ocean floor, serving Dagon and Cthulhu. They trade and interbreed with the humans of coastal towns, leaving half-human, half-fish descendants.
In "The Shadow over Innsmouth," those descendants lose their human shape with age and return to the sea. The horror is of an inhuman lineage latent within — an otherness that has already entered the self and slowly surfaces.