It gives fans a chance to explore the darker side of the mythology that the original books might not have delved into as deeply. For example, the idea of the Underworld having even more terrifying secrets or the gods having a more malevolent side. Also, the element of fear adds an extra layer of excitement to the story. It makes the readers wonder how the characters they love will survive these new and scary challenges, which keeps them engaged and interested in the fanfiction.
One example could be a fanfiction where the monsters from the Percy Jackson universe are far more menacing and numerous. They could overrun Camp Half - Blood in a way that the demigods struggle to fend them off. For instance, an army of empousai attacking at night when most of the campers are asleep.
Well, it could be the way it twists the character of Percy in a scary way. Maybe it gives him dark motives or puts him in really creepy situations that are far from his original portrayal, which can be quite off - putting.
There might be a 'Percy Jackson Scary Fanfiction' where the gods turn against Percy in a really spooky way. They use their powers to isolate him and send all kinds of dark omens and curses his way. It could show Percy trying to survive in a world where the very beings he once thought were allies are now his enemies. This would create a very tense and scary atmosphere as he has to figure out how to deal with these powerful deities on his own.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of Percy Jackson fanfictions, and you can use the search filters to look for ones tagged as'scary' or 'horror'.
I don't know for sure as I haven't read this particular fanfiction, but usually in fanfiction, Percy could be a character from a popular book, movie or TV series.
I'm not sure who Percy specifically is without reading the fanfiction. It could be an original character created by the author, or perhaps a reimagined version of a known character from a particular franchise that the fanfiction is based on.
In Percy tired fanfiction, Percy often shows signs of physical exhaustion. His usual energetic actions might be slower, like his sword - fighting skills not being as sharp. For example, in some fics, he might stumble during a fight instead of his usual confident moves.