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.
Nico di Angelo might be an unexpected choice. He has his dark side and his powers related to the Underworld. If he were somehow tricked or forced into becoming an assassin, his powers combined with a special shield could make for a very interesting story. He could be in a moral dilemma as he battles Percy and his friends.
It could be a group of mythical creatures that have been experimented on by Shield. They've become more powerful and angry, and they see Percy as a threat. They target him and try to destroy him, as well as causing chaos within Shield. These creatures could be something like mutated centaurs or enhanced harpies.
Well, it could be about Percy using his demigod powers in the world of Shield. Maybe he's on a mission to uncover a conspiracy within Shield that threatens the demigod world. He could use his water - controlling abilities to his advantage, like creating distractions during battles or escaping tricky situations.
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.
It could be a new demigod who has a grudge against the Olympians. This demigod might be using Percy as a tool to carry out their assassination plans.
A powerful sorcerer from another realm who uses assassins as tools. This sorcerer could see Percy as a threat to their plans of taking over the demigod world. They send their best assassins after Percy and also try to manipulate other demigods against him. They are extremely cunning and have a vast array of dark magic at their disposal.
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.
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.
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.
A powerful dark sorcerer could be the main antagonist. He might be seeking to use Percy's skills for his own evil purposes, and when Percy refuses, he becomes the target. The sorcerer would have a vast array of dark magic at his disposal, making him a formidable foe.
Obviously, Percy Jackson and Artemis. Percy is the main character of the original series, and since he's the 'assassin' in this fanfiction, he's central. Artemis is the one he's wrongly accused of trying to assassinate, so she's also a key character.