In the Cthulhu Mythos, The Deep Ones are a race of amphibious beings, somewhat similar in appearance to the Creature From The Black Lagoon, though with more frog-like features. The Deep Ones live in great underwater cities and worship a trinity of three deities, Father Dagon, Mother Hydra, & Great Cthulhu.
Father Dagon and Mother Hydra are Great Old Ones who dwell under the sea. Their appearance is akin to gigantic forms of the Deep Ones, and rightfully so, as the Deep Ones may be the offspring of Dagon and Hydra.
The Deep Ones are known to interbreed with Humans. The resulting hybrids, a.k.a. fishmen, look and act like normal humans when young. As the hybrids mature they begin to take on "fishy" features. Eventually the hybrid can no longer live on land and will retreat to the sea to join the Deep Ones.
Lore: The Deep Ones come to a new location and bribe the inhabitants with bountiful fish and gold from the sea, which is described as having an unusual color. (Their underwater cities are said to be made of this gold.) The Deep Ones then set about establishing the cult of Dagon, Hydra, & Cthulhu, as well as producing hybrids with the natives. Standard human sacrifices and murders abound.
Powers: The Deep Ones are incredible strong and known to kill people when not mating with them. They have power to increase the harvesting of fish and may have other powers over the ocean.
Defense Against: Practically none, save the Elder Sign which repels them. The Deep Ones are said to fear the "old ones", which may be a reference to the Elder Things, which likewise are said to live under water in their own cities.