Deep Ones

"“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. I was somehow glad that they had no more than four limbs. Their croaking, baying voices, clearly used for articulate speech, held all the dark shades of expression which their staring faces lacked... They were the blasphemous fish-frogs of the nameless design – living and horrible.”"- H. P. Lovecraft, “The Shadow over Innsmouth”

The Deep Ones are a race of ocean dwelling beings, having numerous cities in the seas and oceans around the world.

They resemble fish-frog-human like creatures and are immortal (but can be killed through violent means). Father Dagon, Mother Hydra and Great Cthulhu are central deities in their religion.

History
Strangely, the origins of the Deep Ones is unclear. One theory suggests that they followed Great Cthulhu to Earth from the Outer Space. Thus they served as allies of Cthulhu and his kin – Star Spawn of Cthulhu, probably fighting numerous wars with rivaling species. Following the defeat of Cthulhu and the sinking of the city/continent R’lyeh, they were left alone and traveled around the world, establishing city-like colonies. When humans evolved they started to occasionally make pacts with them, giving them large supplies of fish and sometimes even gold. In return they were offered human sacrifices. Sometimes the relationship between human and Deep One society would become so connected that they would start mating with each other. This practice resulted in human-Deep One hybrids, who looked like normal humans in youth, but would eventually evolve into immortal Deep Ones.

This kind of pact was made with one of the tribe in the Polynesian islands. This tribe, known as Kanakies, were eventually discovered by a ship captain from the town of Innsmouth, his name was Obed Marsh. Obed soon learned about their golden treasure, which Kanakies acquired from the Deep Ones, consequently he would trade with them for many years onwards. Tribe’s chief, Walakea, eventually spoke to Obed about the Deep Ones, even teaching him how to summon them. Walakea then proceeded to even give him disks necessary for the summoning ritual.

When Obed returned to the islands, he couldn’t find any trace of the Kanakies left. Now with his trade gone, he returned to Innsmouth disappointed and worried. Wanting to bring good to the town, Obed went to Devil’s Reef in hope of summoning the Deep Ones. Shocked by their presence, he entered a pact similar to the one that Kanakies had with the alien race.

Obed and some of his followers bought the old Masonic Hall and converted it to the Esoteric Order of Dagon, which eventually attracted more and more citizens. Those who followed Obed started prospering and he eventually opened the gold processing Marsh Refinery. Meanwhile, around the year 1840, Deep Ones built their city of Y’ha-nthlei on the ruins of a prison for the Flying Polyps.

In 1846, Marsh and some of his followers were arrested. In the following weeks, Deep Ones came to aid their ally, killing more than half the town’s population that day. Those who survived were forced to take the Oaths of Dagon, the First Oath of loyalty, the Second Oath of secrecy and the Third Oath (not mandatory) of mating with a Deep One.

Society, religion and culture
"“... their history stretches back beyond the remotest human origins, into unthinkably remote antiquity. A few individuals are so incredibly old that they have seen continents rise and fall, for they do not die of old age as humans do. Father Dagon and Mother Hydra are such individuals, and are greatly revered for their age and size.”"- The Book of Dagon The Deep One society has a very hierarchical structure. At the top are Father Dagon and Mother Hydra, who serve as both monarchs and deities, below them are priests who make sacrifices and lead the rituals in their honour, and at the bottom are “common” Deep Ones.

Central to their religion is Great Cthulhu, whom they worship as their primordial deity. The greatest religious artifacts of the Deep Ones are the Tablets of Dagon. These tablets speak of the coming of the Great Cthulhu, the destruction of the old world and the rebirth of a new one.

Deep Ones also seem to be great craftsmen and architects, creating beautiful jewelry from their whitish gold and building cities capable of resisting the pressure of the deep ocean.

The Deep Ones are described as being pretty much everywhere around the seas of the world, waiting for the rise of Cthulhu to take place by his side. It is unknown, though likely, if the ones in Innsmouth survived the US torpedo attack of 1922 when Y'ha-nthlei was destroyed.

Trivia

 * It is possible that they cannot reproduce by themselves, so they use humans to further their race.
 * Another reason may be that they use humans to keep their progeny safe from themselves, as they have a tendency of killing it