Sure. Given the popularity of the 'Percy Jackson' books, there are all kinds of fan - created stories out there. Some fans like to twist the original character traits and storylines. Making Percy a hired assassin is a way to put a new spin on the character. It could involve him using his demigod powers in a more morally ambiguous way compared to the original stories.
In some fanfiction, Percy Jackson could potentially be depicted as an hired assassin. Fanfiction often takes the original characters and puts them in new and different scenarios. However, this would be a very different take from the original Percy Jackson series which focuses on his role as a demigod with a different set of adventures and responsibilities.
One common theme is the struggle between Percy's normal life and his new assassin identity. He has to balance his old friendships and responsibilities with this new, dangerous role.
Well, in many fanfictions, Percy might be on a mission to assassinate some powerful being in Tartarus. Maybe he has to face all kinds of hellish creatures and overcome insane challenges. It could involve him using his water powers in unique ways, like creating weapons out of water to fight the demons of Tartarus.
Luke could be a great choice. He already had his moments of turning against the good side in the original series. He could be resurrected or something and become an assassin with a powerful shield, using his combat skills and knowledge of the demigod world to his advantage.
Perhaps the story involves Percy being framed as an assassin of Olympus. He then has to clear his name while dealing with the real assassin who is causing chaos among the gods and demigods. The plot might also include Percy's journey to discover who set him up and why.
A likely main antagonist could be a powerful demigod who has turned to the dark side and wants to use chaos for his own evil purposes. He might see Percy as a threat to his plans.
An ancient and forgotten god who has been awakened. This god could be angry at the modern demigods for ignoring the old ways and decides to send assassins after Percy as a warning to the rest. They might want to bring back the old order where gods were feared and worshiped more and see Percy as a symbol of the new, more independent demigods.
Well, it could involve Percy being tasked with protecting Artemis in a way that goes against his normal alliances. Maybe some dark force is after Artemis and Percy has to use his skills in a new and unexpected way.
There could also be some characters unique to the fanfiction. Maybe a new demigod who has some special power related to Tartarus and ends up joining Percy on his quest. Or perhaps an ancient being who was imprisoned in Tartarus long ago and has knowledge that Percy needs to complete his mission.