Nightmare Fuel / Andrew Doran

• The Child of Dreams, Annie, is someone who changes her age constantly as she moves in and out of time. She's quite insane and capable of erasing people from existence at will. The locals struggle not to offend her, lest they be destroyed or transformed.
• Olivia's Face–Heel Turn was bad from the Statement of Andrew Doran to Andrew Doran and The Mountains of Madness but she is turned into an insane twisted mess by her treatment at the hands of Timothy.
• The revelation of Erich Strobel being a shoggoth and his transformation into a proto-shoggoth are particularly nightmarish, especially when you note that he turned himself into the latter by using large amounts of body parts harvested from his men.
• The horrific death of Patricia in Andrew Doran and the Crawling Caves as she's torn apart by the Al Azif and then used to construct a new human skin body for her.
• I'm a Humanitarian has horrific effects in the Cthulhu Mythos as you're guaranteed to become a wendigo and stalk the living. This is what happened to Carol Berg and she was only brought out of her feral state by her husband (who she outlived and will by centuries).