Perhaps Percy Jackson's grandson is sent on a quest by the gods. This quest leads him to the world of Harry Potter where he finds out that Voldemort is a threat not only to the wizarding world but also to the demigod world in some way. So he has to figure out Voldemort's plans and put an end to them. He might use his knowledge of Greek mythology and his unique powers to outwit Voldemort.
The plot could start with Voldemort seeking some ancient power that is related to the Greek gods. Percy Jackson's grandson gets wind of this and tries to stop him. There could be a race between them to find certain artifacts or secrets. The grandson would have to navigate the wizarding world, make new allies, and use his powers like his grandfather's control over water or his own special abilities to take on Voldemort's dark magic and his army of Death Eaters.
One possible plot could be that Percy Jackson's grandson discovers a portal that leads to the wizarding world. He soon learns about Voldemort's reign of terror and decides to use his demigod powers to stop him. Along the way, he teams up with some young wizards and witches who are also against Voldemort.
In the fanfiction, perhaps Voldemort's grandson is not as evil as his grandfather. He could be trying to redeem his family's name. Percy Jackson might initially be suspicious of him but then they team up. The grandson could teach Percy some basic magic, and Percy could teach him how to be brave in the face of real danger, not just the kind wizards face. They might go on an adventure together to retrieve a powerful artifact that could change the fate of both their worlds. This would be a great way to explore the idea of good and evil not being so black and white.
Percy would probably rely on his friends. In the Percy Jackson series, his friends were always there for him. In this fanfiction situation, he'd need them more than ever. They could help him stay grounded and remind him of his true self, not the dark legacy of Voldemort. Also, he might learn more about his demigod side to overpower the dark side in him.
One way could be to start with Percy discovering this strange connection. Maybe he has some strange dreams or visions that lead him to investigate. Then he could travel to the wizarding world to learn more about Voldemort. Along the way, he'd face opposition from both demigods who think he's getting too involved with dark magic and wizards who are wary of his demigod status.
There might be very few fanfictions on this exact topic as it combines two very different fictional universes. But if it exists, it could explore how the powers from the Percy Jackson world, like the ability to control water or superhuman strength, would interact with the magic of the Harry Potter world where Voldemort resides. The grandson could be on a quest to stop Voldemort from getting some powerful demigod - related artifact that could enhance his dark magic.
Well, the plot could be centered around Percy's attempts to bridge the gap between his father Poseidon and his grandfather Zeus. There have always been tensions between the two gods. Percy, using his unique position, tries to bring peace. But there are forces at work, like some rogue demigods who don't want peace between the two powerful gods. They try to sabotage Percy's efforts, leading to a series of adventures and challenges for Percy.
One possible plot could be Percy discovering his relation to Dumbledore. Maybe he starts having strange dreams or visions that lead him to the wizarding world. He then has to learn about his new family connection while also trying to keep his demigod identity a secret from most of the wizards, as they might not understand it.
Well, one possible plot could be that Percy discovers his new relation to Hephaestus later in his adventures. He might then start to have an affinity for creating and fixing magical items, which could be a new skill for him in battles against monsters. He could also meet other descendants of Hephaestus who teach him more about his heritage.
It could be a wild mash - up. Percy might inherit some of Voldemort's dark magic tendencies while still having his own demigod powers. Maybe he'd struggle with the dark side of his heritage while trying to be a hero like he is in the Percy Jackson series.
One possible plot could be that Voldemort's heir somehow discovers a way to cross into the Percy Jackson universe. They might target the demigods because they see them as a source of power, perhaps thinking that the demigods' connection to the gods could be harnessed for their own dark magic. Percy and his friends would then have to figure out who this new threat is and how to stop them.
Well, it might start with Percy discovering his strange connection to Voldemort. There could be a lot of confusion and disbelief from Percy's side as he tries to understand how he could be related to such an evil wizard. And in the Harry Potter world, the characters would be equally shocked to find out about this new relation. It would be a story full of mystery and trying to figure out the how and why of this relationship.