One common plot twist is that Percy discovers his mother isn't really dead. Maybe she was taken by a powerful being and is being held captive in some hidden realm. Percy then embarks on a secret quest to rescue her, which could involve making deals with some very dangerous entities.
A plot twist might also involve Percy finding out that his mother's death was part of a much larger prophecy. He then has to figure out what the prophecy means and how he can fulfill it while also dealing with his grief. This could lead to him having to make some very difficult choices, like choosing between saving the world or avenging his mother.
A possible plot in such a fanfic could be Percy trying to bring Sally back from the dead. He could go on a dangerous and forbidden quest, defying the gods and the natural order. This would put him at odds with many, but his love for his mother is so strong. He might encounter dark magic or make deals with shadowy figures in his desperation, all while his friends are worried about him losing himself completely in this quest.
In this fanfiction, it could be that Artemis is in a situation where she is about to be killed or lose her powers forever. Percy, being the selfless hero he is, takes on a deadly task to prevent that from happening. Maybe it involves facing an ancient and malevolent god who has a vendetta against Artemis, and Percy uses his unique abilities and the element of surprise to try and stop this god, but at the cost of his own life.
One possible plot twist could be that Percy, with Zeus as his father, discovers he has the power to control the skies in a way that even Zeus is afraid of. This newfound power could put him at odds with the other gods who see him as a threat to the established order.
Perhaps Sally starts a support group for the parents of demigods. This could lead to some interesting interactions and new friendships. Percy could be involved in helping her set it up and deal with the various challenges that come with it, like keeping it a secret from the mortal world and protecting it from monsters.
In some fanfictions, Percy's half - brother Tyson might be made to betray him. This could be due to being tricked by some sea monster that convinces Tyson that Percy is a danger to their kind. Also, Grover could be lured into a situation where he has to choose between saving the wild and betraying Percy. It often makes for a very heart - wrenching story as Grover and Percy have such a strong bond. Sometimes, the betrayal comes from an unexpected source like the gods themselves. They might use Percy as a pawn and then turn on him when he's no longer useful.
One plot twist could be that the male OC discovers that his reincarnation has caused a disruption in the balance of the demigod world. His very existence might be threatening the safety of Camp Half - Blood or other important places. Or perhaps he finds out that his past - life memories were deliberately altered by a mysterious force, and as he uncovers the truth, it leads to a whole new set of challenges and revelations. Also, what if his reincarnation is a way to revive an ancient and powerful being? That would be a huge plot twist with far - reaching consequences.
Well, a frequent plot twist is related to Percy's origin. In some fanfictions, it turns out that his assassin - god powers are not a result of some new blessing but are actually related to his long - lost family heritage. Maybe he has a secret relative who was an assassin - god in the past. Also, there can be a plot twist where the entire concept of the 'God of Assassins' is a ruse created by a more powerful god to manipulate Percy. He might think he's on a path to become this new entity, but in reality, it's all part of a bigger scheme. Another interesting plot twist could be that the mortal world starts to notice Percy's assassin - like activities, and he has to deal with both the godly and mortal consequences.
A theme of growth is also present. Through his mother's death, Percy has to grow up faster and become more independent. He has to make decisions on his own without his mother's guidance. Also, the theme of family is redefined. With his mother gone, Percy's concept of family expands to include his friends at Camp Half - Blood more deeply.
One common plot twist is the discovery of a hidden power. For example, the reincarnated character might suddenly find out they have a unique ability related to a minor god that no one expected. Another is a change in the prophecy. Instead of the original prophecy being fulfilled in the expected way, it takes a different turn because of the reincarnated character's presence. And sometimes, there's a betrayal from an unexpected source, like a character who was thought to be a friend turns out to be working for an enemy due to some dark secret from the past.
One common plot twist could be Percy discovering that his connection to Artemis brings some unforeseen powers. For example, he might gain the ability to communicate with certain animals that are sacred to Artemis. Another could be that there's a prophecy about him that was hidden until a crucial moment, changing the course of his journey. Or perhaps he finds out that his parentage is more complex than he thought, with some dark secret related to Artemis that others have been hiding.