The Deep Ones were a race of underwater beings who worshipped Dagon. Some sources clarified that they were the Sea Devils. They could interbreed with humans.
The Seventh Doctor recollected from his practical theology course at the Time Lord Academy that the Sea Devils worshipped Dagon, one of the Great Old Ones. (PROSE: All-Consuming Fire [+]Loading...["All-Consuming Fire (novel)"])
From an early age, H. P. Lovecraft had visions of "terrifying" underwater beings calling out to him in his dreams. His mother told him that he had "come from them", and that he was a Guardian of the Gate, tasked with mediating between humanity and beings outside its comprehension. She encouraged his imagination, and as he became a science fiction writer, Lovecraft gradually realised that he was channeling true, subconscious insights through his fiction, in order to warn humanity. (AUDIO: Guardian At The Gate [+]Loading...["Guardian At The Gate (audio story)"])
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One account referred to a Sea Devil encountered in Atlantis by Martha Jones, Liz Shaw and the Thirteenth Doctor as a "Deep One". (GAME: Lost in Time [+]Loading...["Lost in Time (video game)"])