They made their debut in Lovecraft's 1931 novella "The Shadow Over Innsmouth". They are described as being an ocean-dwelling race, as evidenced by their name, with an affinity for mating with humans.
Apparently, the Deep Ones have some command of fish, as they entered a contract with the people of Innsmouth to keep the fish plentiful in their area. They also seem to have a supply of gold artifacts of unearthly design. The Deep Ones revere Dagon and his consort Hydra, worshipping them under the names Father Dagon and Mother Hydra.
The Deep Ones are humanoid beings with fish, human and amphibian-like traits.
As part of their pact with the Deep Ones the residents of Innsmouth are forced to interbreed with them. Although the Deep One hybrid offspring are born with the appearance of a normal human being, they gradually assume the appearance of their Deep One ancestors. These traits include:
- Shrunken/nonexistant ears
- Glassy, bulging, unblinking eyes
- Narrow, hairless head
- Rubbery, blue-gray skin
- Folds on the neck which eventually become functional gills
- Webbing between fingers
When the hybrid becomes too obviously non-human, they go into seclusion in ostensibly abandoned buildings until fully capable of living solely underwater. The rate of progression varies with each individual, however it normally does not reach completion until the human middle age. Once this transformation is complete they are also functionally immortal. Apart from the physical change they also feel a scorn for humanity, an affinity for the Non-euclidean artwork of the Deep One race, and an increasingly powerful desire to abandon the human world and go to the Deep One city of Y'ha-nthlei.