A plot idea might involve the Evil God creating doppelgangers of the main characters. These doppelgangers would cause chaos among the demigod community. Percy would have to figure out how to tell the real from the fake. Also, the Evil God could be slowly poisoning the Camp Half - Blood environment, making it a more hostile place for the demigods to live and train.
Well, the Evil God could start a war between different cabins at Camp Half - Blood. By spreading lies and envy among the demigods, he creates a rift. Percy has to use his wits and leadership skills to stop this civil war. The Evil God might also have an alliance with some ancient and powerful monsters, like the Titans that were thought to be defeated, and they together plan to destroy the world of demigods.
One idea could be that the Evil God manipulates Percy's friends against him. For example, making Annabeth doubt Percy's loyalty through false visions. Another could be that the Evil God corrupts the demigod world's magic, turning it against the heroes. Maybe the water Percy usually controls starts to harm him instead of help.