It could be a very dark and complex story. Percy might be constantly at war with himself, having grown up in a more 'good' family environment but having this dark connection to Voldemort. He could be hunted by both sides - those who fear he'll turn out like his 'grandfather' and the remnants of Voldemort's followers who might want to use him for their own ends.
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.
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.
Well, in such a fanfiction, the story could start with a big revelation. Harry might initially be in denial. Then, the plot could involve him exploring his new family connection, maybe even trying to find out if there are any other relatives. The wizarding world would be in shock too, and there would be a lot of mistrust towards Harry from some. Also, it could show how Harry uses this knowledge to try and defeat Voldemort in a different way, like by understanding his enemy on a deeper level.
It could be really exciting. Maybe Percy discovers some hidden magic or a secret weapon that gives him the edge over Voldemort. He might use his intelligence and knowledge of the wizarding world's rules to outwit Voldemort in a final showdown.
It would be extremely different from the original story. The relationship between Harry and Voldemort would be completely redefined. Instead of pure hatred, there could be a sense of family - related conflict. Harry might struggle with the fact that he is related to the most evil wizard.
Percy might be a bit curious too. He's used to strange things, and meeting someone related to such a powerful dark wizard could pique his interest. He could try to figure out if the grandson has the same evil intentions as Voldemort. He'd likely keep his distance at first, but as he observes the grandson, his reaction could change.
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.
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.
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.
There could potentially be a 'Percy is the grandson of Voldemort' fanfiction. Fanfiction writers often explore very strange and creative ideas. However, without actively searching through the vast expanse of fanfiction websites, it's hard to say for certain. It might be a very niche concept where the writer is trying to subvert the normal good - vs - evil family trees in the Harry Potter universe. For example, they could be exploring how Percy's character might be different if he had such a dark family connection, or how the wizarding world would react to such a revelation.