Well, there was a 'fandom horror story' in the gaming fandom. A player was so into an online game that he believed he was actually a character in the game. He started to act out the game's violent scenarios in real life, which was really terrifying for the people around him.
One horror story could be about overly possessive fans. There were cases where some fans would attack others who didn't share the same 'ship' (relationship pairing) in the Hetalia fandom. They would send mean messages and create a very hostile environment for those with different opinions.
A frequent theme in 'fandom horror stories' is the sense of entitlement. Some fans feel they are entitled to special treatment or access just because they are fans. This can lead to them doing unethical or scary things, like hacking into a star's personal accounts to get exclusive content. Also, there is often a theme of isolation. Fans get so deep into the fandom that they isolate themselves from real - world relationships and this can have some very strange and sometimes horror - like consequences.
There's this one story where Twilight Sparkle's library is taken over by a dark force. The books start to whisper strange things and Twilight finds herself trapped in a never - ending maze within the library. As she tries to find her way out, she encounters shadowy figures that look like her friends but are really manifestations of her fears. It's a great horror story as it plays on the idea of a place that was once safe and full of knowledge becoming a place of terror.
In a roleplay horror story, some teens role - played a haunted house scenario. They set up different 'haunted' rooms. However, during the roleplay, one of the teens accidentally knocked over a candle and started a small fire. The panic of the fire combined with the horror roleplay made it a very chaotic and scary experience. They quickly called the fire department and learned that even a simple roleplay can turn into a real - life emergency.
The overzealous theory - fans are also a common type. They can be so pushy with their ideas that it scares off others who just want to enjoy the fandom casually.
One horror story could be about a really intense shipping war within the Homestuck fandom. Fans of different ships were so aggressive towards each other that it created a really toxic environment. People were sending mean messages and threats just because of their preferred character pairings.
There are horror stories where the magic in Equestria goes horribly wrong. Like a spell that was meant to do good but instead creates a nightmarish dimension where ponies are constantly in pain and fear. This is terrifying as it shows how the very thing that usually brings harmony can be so destructive. Also, stories about ponies being haunted by unseen forces that slowly drive them mad are quite common and spooky.
A common one is shipping wars. Fans who supported different character pairings would get into huge arguments. They'd attack each other on social media, which made the fandom environment very toxic. It was really scary for those who just wanted to enjoy the show and the fan creations in peace.
In the Steven Universe fandom, there were also instances of fans gatekeeping. They made new fans feel unwelcome if they didn't know all the details or hadn't been in the fandom from the start. This made the fandom seem like a hostile place for some, which is a horror in its own way as it goes against the spirit of a friendly fan community.
There are stories where fans imagine Naruto in different historical settings. For example, a story could be set in a steampunk world where Naruto uses his chakra in new and creative ways to operate steampunk - like machinery. His friends also adapt to this new world, and they all have to fight against an evil force that wants to take over this unique version of the Naruto universe.