The Deep Ones ("Sea Devils") are a species of aquatic/semiaquatic humanoids with both anurid and ichthyic characteristics (simply put, frog-fish men), dwelling in cities at the ocean's bottom. These creatures were introduced in H. P. Lovecraft's story "The Shadow over Innsmouth".
Described as both froglike and fishlike, slimy anthropoid creatures with spiny ridges down their back and bulging lidless eyes, the Deep Ones, despite their preferred marine environment, can survive on land for extended periods. Deep Ones are capable of hybridizing with humans, resulting in a person with the "Innsmouth look", which will eventually metamorphose into a complete Deep One and return to the sea. Deep Ones do not die natural deaths but are effectively immortal and will only die of accidents, war, or disease. Deep Ones apparently are found all over the world, but few know the correct method to contact them. Deep Ones can also hybridize with the Yuggs, sentient flatworms.
The Deep Ones revere both Mother Hydra and Father Dagon, powerful demigods who may both be, in fact, enormous Deep Ones.
The city of Y'ha-nthlei is the only Deep One city mentioned by Lovecraft, and this was torpedoed by the US Government in 1928.
"'...Obed learned that there are things on this Earth most folks never hear about - and wouldn't believe if they did hear. It seems the Kanaky natives were sacrificing their young men and maidens to some kind of god-things that lived under the sea, and getting all kinds of favour in return. They met the things on the little islet with the queer ruins, and it seems the awful carvings of frog-fish monsters were supposed to be pictures of these things - maybe they were the kind of creatures that got all the mermaid stories and such started....'" - H.P. Lovecraft, "The Shadow Over Innsmouth"
"...That damnable fishy odour again waxed dominant.... They would perhaps be the worst of all Innsmouth types.... The horde was very close now - air foul with their hoarse snarlings, and the ground almost shaking with their alien-rhythmed footfalls.... I saw them in a limitless stream - flopping, hopping, croaking, bleating - urging inhumanly through the spectral moonlight in a grotesque, malignant saraband of fantastic nightmare. And some of them had tall tiaras of that nameless whitish-gold metal ... and some were strangely robed ... and one, who led the way, was clad in a ghoulishly humped black coat and striped trousers, and had a man's felt hat perched on the shapeless thing that answered for a head.... I think their predominant colour was a greyish-green, though they had white bellies. They were mostly shiny and slippery, but the ridges of their backs were scaly. Their forms vaguely suggested the anthropoid, while their heads were the heads of fish, with prodigious bulging eyes that never closed. At the sides of their necks were palpitating gills, and their long paws were webbed. They hopped irregularly, sometimes on two legs and sometimes on four.... Their croaking, baying voices... held all the dark shades of expression which their staring faces lacked...."
- H.P. Lovecraft, "The Shadow Over Innsmouth"
- Ramsey Campbell - "The Room in the Castle"
- Basil Copper - "Beyond the Reef"
- August Derleth - "The Shuttered Room"
- August Derleth and H. P. Lovecraft - "The Survivor"
- Ruthanna Emrys - "The Innsmouth Legacy" series
- H. P. Lovecraft - "The Shadow Over Innsmouth"
- Brian Lumley - "The Return of the Deep Ones"
- Guy N. Smith - "Return to Innsmouth"
- Brian Stableford - "The Innsmouth Heritage"
- Charles Stross - The Jennifer Morgue
- James Wade - "A Darker Shadow Over Innsmouth"
- James Wade - "The Deep Ones"
- Jack Yeovil - "The Big Fish"