It could be a powerful demigod who was once rejected by the Camp Half - Blood. This demigod, filled with resentment, forms a mafia - like group to seek revenge on those who he believes wronged him. He uses his powers, which could be something like controlling shadows, to create an air of mystery and fear around his mafia operations.
Maybe a half - Titan who has managed to survive and has decided to use the chaos of the mortal and demigod worlds to his advantage. He creates a mafia in the underworld, recruiting all kinds of dark creatures. He has his eyes set on overthrowing the Olympians by first destabilizing the demigod community through the mafia's illegal activities.
A group of rogue nymphs could be the main antagonists. They are usually associated with nature, but in this fanfiction, they've turned to a life of crime. They form a mafia that controls the trade of rare magical plants. Their leader, a particularly devious nymph, has the power to manipulate plants to attack their enemies. Percy has to deal with their tricky plant - based defenses and their ability to disappear into the forests when threatened.