Most Percy Jackson fans would likely be appalled. The original series is about adventure, friendship, and heroism among kids and young adults. A fanfic with such a inappropriate concept would go against what they love about the series.
He might be shocked and a bit disturbed. Percy is a heroic character, and seeing a 'dark' version of himself in fanfiction would be quite jarring. It's like looking into a distorted mirror of his identity.
One interesting Percy Jackson reaction fanfiction could be where the characters react to modern technology. For example, Percy seeing a smartphone for the first time and being completely baffled by it. It adds a humorous twist to the story.
Fanfiction based on 'Percy Jackson: The Last Olympian' can be quite diverse. Some might focus on the relationships between the characters, like Percy and Annabeth's relationship evolving in new ways. Others could explore the idea of new threats emerging in the demigod world. It's also a great way for fans to connect with each other, sharing their ideas and interpretations of the story.
He might be quite intrigued. Harry is used to his own magical world, and reading about Percy's adventures would expose him to a different kind of magic system. He'd probably compare the demigod world to the wizarding world, like how they both have special powers but in different ways.
There's a fanfiction where the demigods react to a group of mortals who know all about them. It's a very interesting plot as the demigods have to deal with the fact that their secrets are out and how these mortals might use that knowledge. It has a lot of mystery and suspense throughout.
He might find it really interesting. Since both Percy Jackson and Carter Kane are from different but equally rich myth - based series, he could gain new perspectives on how to blend modern settings with ancient myths, which is something his own series also does.