Nightmare Fuel / The Colour of Magic

• While somewhat meager in comparison to later Discworld characters, the evil Sender of Eiiignnnn... er, six plus two, Bel-Shamharoth definitely counts. His very presence makes Time stay away from an area around his temple (which is bathed in such an eerie UV that everyone seems purple in its light), and also keeps the nearby dryads alive as worshipers. It's a mass of tentacles, mandibles, and one great eye in the center. It's the reason wizards cannot say the number that equals four times two, 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.
• The fire that rages through Ankh-Morpork becomes this, upon re-reading, for long-term fans of the franchise. Pratchett himself admitted that he couldn't possibly have pulled off such a catastrophe at later points in the series, when readers have actually gotten to know Ankh-Morpork's people so well and would legitimately fear for the lives of their beloved favorites.