In fanfiction, if Percy Jackson uses Mjolnir, it could lead to some really interesting power combinations. Percy already has his water - based powers from being a son of Poseidon. Mjolnir gives the power of thunder and is a symbol of Thor's might. Percy might be able to use the hammer to control storms in a different way, perhaps combining water and lightning to create more powerful attacks. He could also use it as a physical weapon, which would be a change from his usual sword, Riptide.
If Percy Jackson uses Mjolnir in fanfiction, it would likely disrupt the balance of power in the fictional universe. Percy is already a very powerful demigod, and Mjolnir is one of the most powerful weapons in Norse mythology. He could potentially use it to defeat enemies that were previously unbeatable. However, it might also attract unwanted attention from other powerful beings, such as the Norse gods or more powerful Titans.