This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Colour of Magic
While somewhat meager in comparison to later characters, the evil Sender of EIIIGNNnn... Bel-Shamharoth definately counts. His very presence makes Father Time stay away from an area around his temple (which is bathed in such an erie UV that everyone seems purple in its light), and also keeps the nearby dryads alive as worshippers. It's a mass of tentacles, mandibles, and one great eye in the center. It's the reason wizards cannot say the number 8, and it's one of the many creatures "that died out at the beginning of time". Though it's a little bit of a Nightmare Retardant when it's Twoflower's camera that sends the Dark God fleeing so fast he leaves some tentacles stuck in the portal.