No, it's not that common. Most fanfictions in the Percy Jackson fandom tend to focus on the positive relationships between the characters, like Percy and Annabeth's love story or the adventures of the demigods together. However, every now and then, some darker or more alternative takes like this one might pop up, but they are definitely in the minority.
One possible plot could be that Annabeth is under some kind of mind control. Maybe a powerful enemy has taken over her mind, forcing her to turn against Percy. Another could be a misunderstanding. For example, she might think Percy has betrayed her or their friends in some way, leading to a tragic confrontation.
One possible plot development could be that Annabeth is filled with guilt and remorse. She might isolate herself from the rest of the demigod community. For example, she could leave Camp Half - Blood and go on a self - imposed exile. She would constantly be haunted by memories of Percy and struggle to come to terms with what she has done.
She would definitely have a downward spiral at first. Her confidence would be shattered as she has always been seen as a strong and intelligent character. But then, she could gradually start to rebuild herself. Maybe she would find new purpose in trying to atone for her mistake, like protecting Percy's legacy or mentoring new demigods in his name.
Well, it could be a sign of his protectiveness towards Annabeth. Spiders might be something she fears or dislikes, and Percy, being the hero he is, takes it upon himself to get rid of it. It shows his love for her in a simple yet powerful way.
In a large number of Percy and Annabeth fanfictions, they are indeed presented as soulmates. Their shared adventures, the way they understand each other without many words, and the deep loyalty they have for one another all contribute to this perception. Fanfiction authors often build on their existing relationship from the original works and enhance it to the level of soulmates. They might show it through things like Percy being able to sense Annabeth's distress from afar, or Annabeth always knowing exactly what Percy needs in difficult situations. This soulmate - like relationship makes for very engaging and heart - warming stories in the fanfiction realm.
It could be that Annabeth has a misunderstanding about Percy. She might think he's a threat to her position or status at the camp, so she bullies him to try and keep him in check.
It could be Chiron, the centaur. He's always keeping an eye on the campers and would likely be concerned if he caught them up to something. He might give them a stern talking - to and some extra chores as punishment.
Percy might be a bit flustered and embarrassed. He's protective of his relationship with Annabeth, so having Magnus walk in might make him feel like their privacy has been invaded. He could quickly try to shield Annabeth or just look really shocked.