There's a chance that it could be a character from Percy's family. For example, a distant relative who has been corrupted by some dark magic. They might betray Percy for their own gain or because they are being controlled, and this betrayal could lead to Percy being killed.
The gods, especially Zeus. He has always been a bit suspicious of Percy. In fanfiction, he could easily be written as spreading lies about Percy to the other gods and demigods, leading to everyone turning against him. Also, some new characters introduced in fanfiction might be the ones who start the chain of events that lead to Percy's all - around betrayal.
The gods are always a possibility. Zeus, for example, has been distrustful of Percy in the past. If Zeus feels that Percy is getting too powerful or is a threat to his rule, he could engineer a situation where Percy is betrayed. He might use other demigods or even mortals to turn against Percy.
One character that could betray Percy is Thalia. She has her own issues with the gods and her place in the world. Maybe she gets jealous of Percy's growing popularity and power among the demigods. So, she decides to side with an enemy to take Percy down a notch. Additionally, some minor characters that Percy has helped in the past could be the ones to betray him. They might feel indebted to him but also resentful at the same time. An enemy could manipulate their feelings and turn them into traitors against Percy.
Ares could be a likely character. He's always been somewhat of a wild card in the Percy Jackson series. He might betray Percy for his own amusement or to gain more influence in Olympus. Another could be Zeus. He's often paranoid about his power and might see Percy as a threat and betray him to protect his throne.
One possible plot for this fanfiction could be that Percy is betrayed by a new character introduced in the fan - made story. This new character pretends to be his friend, gains his trust, and then reveals their true evil intentions. They lead Percy into a trap set by some ancient and deadly enemies. Percy, caught off - guard, is then killed. This would add an element of mystery and new drama to the familiar Percy Jackson universe.
Luke could be a likely candidate in some fanfictions. He already had his own issues and motives in the original story, and it wouldn't be a stretch for a fanfiction writer to take that further and have him betray Percy in a more elaborate way.
Perhaps she's a sister who was thought to be dead but actually survived. She could have been corrupted by the experiences she had while being 'dead'. When she returns, she blames Percy for not saving her properly. This false blame leads her to betray Percy. She could start spreading rumors about Percy being a coward or a bad leader among the demigod community.
In some fanfictions, Percy might betray Chaos if he discovers that Chaos's plans are more harmful than he initially thought. For example, if Chaos intends to destroy all of creation and remake it in a way that Percy believes is wrong.
Well, in some fanfiction scenarios, Percy Jackson might betray the Avengers due to a misunderstanding. Maybe he's been misinformed about the Avengers' intentions towards the demigods. For example, if he hears false rumors that the Avengers plan to control the demigods, he could turn against them in a fit of protectiveness for his kind.
He may also try to figure out what went wrong. Percy is a smart demigod, and he would likely not just accept the betrayal without trying to understand the cause. So, he might start investigating, talking to other characters who might have some insight, like the other demigods or even some minor gods who are friendly to him.