Perhaps Poseidon is trying to protect Percy in a very strange way. He might think that by 'betraying' him, he can keep Percy away from a greater danger that he can't directly oppose. It could also be that Poseidon has some prophetic knowledge that Percy is going to be involved in something that will have a huge negative impact on the gods, and he believes that betraying him now is a lesser evil.
One possible reason could be power politics among the gods. Poseidon might see Percy as a liability in some complex power play. Maybe other gods are pressuring him to distance himself from Percy for some hidden agenda. Another reason could be a misunderstanding. Percy might have unknowingly done something that Poseidon misinterprets as a threat to his domain or power. For example, Percy's actions in the mortal world might have unintended consequences for the underwater realm.
One possible plotline could be that Poseidon, under some sort of dark influence, publicly disowns Percy. This would leave Percy feeling completely alone and betrayed, as his father's support had been a major part of his identity. He might then have to struggle not only with the regular threats he faces but also with the emotional turmoil of his father's betrayal. Another plotline could involve Poseidon stealing Percy's powers for some self - serving reason, perhaps to gain more power in the pantheon, forcing Percy to find new ways to survive and fight.
One possible reason could be jealousy among his so - called friends. Maybe someone was envious of Percy's powers or popularity and decided to frame him, leading to his betrayal and subsequent sending to Tartarus. Another reason might be a misunderstanding. For example, if there was some false evidence planted against Percy, his comrades could have been misled into believing he had done something unforgivable and sent him to that horrible place.
One possible reason could be a misunderstanding. Maybe Artemis misinterprets Percy's actions as being against her or her hunters. For example, if Percy accidentally endangers one of her hunters during a quest, Artemis might see it as a betrayal of her trust in him.
One reason could be for a fresh take on the story. Writers might want to break away from the typical demigod narrative and explore a darker, more supernatural side of the Percy Jackson universe. Another reason could be to create a more complex and morally ambiguous character. By making him a demon, they can introduce new conflicts and relationships.
Perhaps it's part of a grand plan that Poseidon has, one that he believes will ultimately benefit Percy but in the short - term causes him pain. For example, he might be trying to toughen Percy up or make him prove himself in a more extreme way, thinking it will help Percy in the long run.
Well, in the original Percy Jackson lore, Percy is the son of Poseidon and Sally Jackson. But in a fanfic where he's the son of Artemis and Poseidon, it would be a wild departure. Artemis is a virgin goddess, so this goes against the traditional mythology. However, in fanfic, anything is possible. Maybe the story could start with some kind of magical interference that results in this unexpected parentage. The plot could then explore how Percy deals with being the son of two such powerful and different deities. He might struggle with the expectations of both Artemis' domain of the hunt and the sea - related powers of Poseidon.
Well, in fanfiction, one reason could be the loss of his friends. Percy has been through so many battles, and if he loses someone close like Annabeth or Grover, it could send him into a deep depression. Another reason might be the burden of his prophecies. He's constantly expected to save the world, and that kind of pressure can wear a person down over time.
In some fanfictions, Percy might be banished because he goes against the traditional ways of the demigod society too much. For example, if he tries to form an alliance with a group that the other demigods consider enemies. His actions could be seen as a threat to the stability of their world, so they banish him to keep the peace.
Well, it could be that Hestia betrays Percy for power. In the fanfiction world, she might see Percy as a threat to her own influence among the demigods or the gods. So she makes a deal with another powerful entity to get rid of Percy. For example, she could give false information about Percy's next move to a group of monsters, causing them to ambush him.
Some authors might write it to add a new and unexpected twist to the Percy Jackson story. They may want to see how the other characters would react if such a thing happened. Also, it could be an attempt to explore the psychological impact on Percy in a very different and extreme situation, though this is a rather controversial approach.