One possible reason could be that Percy feels betrayed by the Olympians first. Maybe he discovers some dark secrets about them that make him lose faith. For example, if he finds out that they have been using demigods unfairly in their battles or politics.
Well, usually it starts with Percy having some sort of conflict with the Olympians. Then he starts to make plans in secret. He might gather a group of supporters, like other disgruntled demigods or even some mythical creatures who also have a grudge against Olympus.
In this fanfiction, perhaps Percy is influenced by some dark force. This force convinces him that the Olympians are corrupt and need to be overthrown. He then betrays them by stealing important artifacts from Olympus that are crucial for its protection. He might also try to turn the demigods against the Olympians.
One possible reason could be to create a tragic story. Writers might want to show how the world of the story would be affected by the loss of such a central character like Percy.
The Olympians themselves could be the main antagonists in a sense. Since Percy is betraying them, they would be the ones trying to stop him. Zeus, with his power and pride, would likely be at the forefront, seeing Percy's betrayal as a direct threat to his authority.
It's not likely that the actual Olympus (in the context of the Percy Jackson series being a place of the gods) reads fanfiction. Fanfiction is a creation of the fans in our world, and there's no indication in the books that the gods or Olympus as an entity would engage with it.
In fanfiction, the politics and intrigues within Olympus might be depicted in a way that makes Percy disgusted. The backstabbing and power - hungry nature of some of the Olympians could be too much for him to bear, so he hates Olympus.
His friends from Camp Half - Blood would be his allies for sure. Annabeth, with her strategic mind, would be invaluable. Grover, the satyr, could help Percy communicate with nature spirits which could be useful. But also, some of the minor gods who feel left out by the major Olympians might side with Percy. For example, Hecate could use her magic to assist Percy, seeing him as a chance for a new order in the Olympian world. There's also Tyson, Percy's cyclops half - brother, who would be a strong and loyal ally.
Annabeth could definitely be one of his main allies. Her intelligence and strategic planning would be invaluable in a fight against Olympus. Also, Grover might be there, using his skills as a satyr to gather information and support from other nature - related beings. Some of the less - well - known demigods from Camp Half - Blood could also join in, seeing Percy as a chance for a new order.
One great fanfiction is 'The New Quest of Percy Jackson'. It continues Percy's adventures in a very believable way, with new challenges and old friends by his side. The author really captures the essence of the original series.
A lesser - known but really interesting fanfiction is 'Annabeth's Quest for Wisdom'. This story follows Annabeth as she goes on a solo quest to gain more knowledge and wisdom. It shows a different side of her character, her determination and her thirst for knowledge in a very engaging way.