I wonder: if medieval era sailors or researchers had somehow had the ability to go into and explore the deep sea, if they would think everything down there is just a physical incarnation of a demonic spirit:
Soulless black eyes? Check.
Charred/brimstone looking skin texture? Check.
Sharp, vicious teeth? Check.
Translucent/ghostly flesh? Check.
Live in the blackest, most remote area accessible? Check.
Said area feels closer to the center of the Earth than most other places? Check.