Maybe Percy uses the Sword of Achilles to defeat a new and powerful enemy that the regular weapons couldn't handle. For example, a super - strong monster that was immune to normal Celestial Bronze weapons.
In the fanfiction of the Curse of Achilles in Percy Jackson, many authors like to play with the idea of Percy's vulnerability. Even though he has the curse which makes him almost invincible, there are always ways around it that the fanfic writers come up with. This could be related to his emotional state or some hidden loopholes in the magic of the curse.
Well, another place could be some Percy Jackson - related fan communities on social media platforms like Reddit. There are often threads where fans share their own fanfictions or recommend good ones, so it might be worth a shot to search there for stories about Percy having the Sword of Achilles.
Another theme is identity. With the curse, Percy is somewhat changed. He's no longer just the regular demigod. Fanfiction may delve into how he reconciles this new, more powerful self with his old identity. For instance, he might struggle with the idea that he's now a 'super' demigod while still wanting to be seen as the same Percy by his friends. This can lead to a lot of internal conflict within the character in the fanfiction.
One possible plot could be Percy discovering his connection to Achilles while on a new quest. Maybe he has to face a powerful enemy that only his newfound link to Achilles can help him defeat. For example, he might gain some of Achilles' combat skills or invulnerability in certain situations.
Percy might become more confident in his abilities. Knowing he is related to Achilles, a great hero, could give him a boost. He may also become more strategic in combat, learning from Achilles' military tactics.
If Percy was the son of Achilles, his relationship with Annabeth might become more strained. Percy could become more headstrong and impulsive because of the influence of Achilles' traits, which would clash with Annabeth's more strategic and cautious nature. For example, in battle situations, Percy might rush in without a proper plan, causing Annabeth to worry more about his safety and get frustrated with his new behavior.
One interesting plot twist could be that Percy discovers he has some of Achilles' more negative traits along with his powers. For example, Achilles was known for his pride, and Percy might start to struggle with excessive pride that gets in the way of his relationships with his friends at Camp Half - Blood. Another could be that the gods, fearing Percy's new connection to Achilles, send out a series of trials that are specifically designed to test his loyalty and his ability to control the power of Achilles. Maybe he has to face off against a powerful monster that can only be defeated if he uses his Achilles - like invulnerability in a very specific and dangerous way.
I'm fairly certain there is. Fanfiction writers often like to cross - over different fictional universes, and Percy Jackson and Excalibur are both rich sources of inspiration. Writers could explore how Percy, with his demigod abilities, might interact with the power and mystery of Excalibur. There could be all kinds of plotlines, from Percy finding Excalibur on a quest to it being a key element in a battle against a new, powerful enemy.
It could be an old enemy of the gods who sees Percy as a threat because of the Curse of Achilles. Maybe a Titan who escaped from Tartarus and wants to use Percy's power for their own gain. If they can control Percy, they could have a powerful weapon against the gods.