It could be a brave knight. Knights are often protagonists in such stories. He might hear about the witch and the emerald and decide to go on a quest to stop the witch from using the emerald's power for evil.
Perhaps a young wizard or witch who has not yet been corrupted. They sense the danger of the emerald in the hands of the evil witch and strive to take it away. They might have some special powers that they can use against the more powerful and evil witch. They are driven by a sense of justice and a desire to protect the world from the horror that the emerald could bring if left with the wrong person.
In this story, the witch is surely a central figure. She could be using the emerald for her evil deeds, like casting dark spells. The emerald itself might have a backstory. It could have been stolen from a good wizard long ago, and now the witch has it. And there might be a hero or heroine in the story who tries to take the emerald away from the witch to stop her from causing more harm.
Obviously, the witch is one of the main characters. And since the diamond is so important in the title, it could almost be considered a character in a sense, as it drives the plot. There might also be some victims or people trying to stop the witch from getting the diamond, like a brave hero or a group of villagers.
The story could have a child finding the bottle with the witch and getting into adventures as the witch grants wishes or causes trouble. Maybe the witch has a backstory about how she ended up in the bottle and the protagonist helps her.
It could be a character who is an outcast. Given the unnamable aspect, someone who doesn't fit into the normal social or narrative structures might be the protagonist.
Maybe finding a bug in your fries. That would be really horrible.
It could be that in the '19 horror story', there are 19 victims of a serial killer. The killer has a very strange and terrifying modus operandi. He leaves a mark on each victim that is related to the number 19, and as the story progresses, the police are trying desperately to catch him before he gets to his 19th victim.
A woman bought a cupcake from a new bakery. When she got home, she noticed a strange symbol on the frosting. As she started to eat it, she felt dizzy and then passed out. When she woke up, she was in a strange room filled with cupcakes that seemed to be pulsating. She realized she was in some sort of nightmare world created by the cupcakes.
One possible horror story could be when someone accidentally sends a video or photo related to their private act to the wrong person. Maybe they were trying to store it privately on their device but due to a technical glitch, it gets sent to a family member or a friend. That would be extremely horrifying.
A possible Trump horror story in the eyes of some could be his tweets. His use of Twitter to make announcements, attack opponents, etc., created a lot of chaos. It was unpresidential according to many, and his tweets sometimes led to market fluctuations and international diplomatic tensions.
One possible laxative horror story could be that a person took a laxative that they were allergic to. They didn't know about the allergy. Immediately after taking it, they had a severe allergic reaction along with the normal laxative effects. Their skin broke out in hives, they had trouble breathing, and also had to deal with the diarrhea. It was a terrifying situation that combined multiple health problems all at once.