The Deep Ones are the ocean dwelling beings. They are immortal and fish-frog-human like creatures. They follow and worship their monarchs, Father Dagon and Mother Hydra, who are the main antagonists of the game.
Deep Ones probably followed Dagon along with Hydra and Cthulhu to conquer the Earth from outer space. Following the defeat of Cthulhu and the sinking of R'lyeh, they were left alone. Much of their history was unknown until Obed Marsh, a ship captain from Innsmouth, had brought them to his home town from the Polynesian islands in the Pacific Ocean. He entered into a pact with them and offered human sacrifices in exchange for fish and gold. The Deep Ones soon built their city Y'ha-nthlei on the ruins of the prison for Flying Polyps. When Marsh was arrested in 1846, the Deep Ones came to aid their ally, killing most of the town's citizens that day. Those who survived were forced to interbreed with the Deep Ones, creating the human-Deep One hybrids that would one day evolve and live forever in Y'ha-nthlei. The Deep Ones are waiting for the rise of Cthulhu to take place by his side. It is unknown if they survived the U. S. torpedo attack of 1922 when Y'ha-nthlei was destroyed.