He might face internal struggles. Since it's the power of his enemies, it could have a corrupting influence. He has to constantly fight against the darker aspects of that power, like the urge to use it for revenge or in a more aggressive and unethical way.
In other cases, it might be a result of a curse or a spell gone wrong. His enemies could be trying to harm him with a dark spell, but instead, it backfires and he ends up absorbing their power. For example, if they were trying to drain his own power but the spell reversed on them.
One great Percy gains power fanfiction is 'Percy's Ascent'. In it, Percy discovers an ancient power within himself through a series of trials. It's really exciting as it shows his growth from a normal demigod to a super - powered one. Another one is 'Power Surge Percy' which focuses on Percy getting a new kind of water - based power that no one has seen before.
Another way could be via some sort of magical artifact. For example, he discovers an ancient amulet that has the power to enhance his natural abilities. When he touches it, it starts to merge with his essence, gradually making him stronger and endowing him with new powers like the ability to control different elements related to water, not just manipulate it.
He could also face challenges related to the knowledge required to use his power effectively. In fanfiction, there might be ancient texts or prophecies that he needs to decipher in order to fully understand and utilize his new powers. But these texts could be written in a long - lost language or be guarded by powerful entities, making it difficult for him to access the necessary information.
One popular theme is self - discovery. As Percy gains power, he often has to figure out who he is now and how this new power fits into his identity. Another theme is the struggle for control. The new power can be overwhelming, like in 'Percy's Ascent' where he has to learn to control his newfound abilities to avoid hurting those he loves.
The new enemies could be the descendants of an ancient and forgotten race of sea - dwelling creatures who are jealous of Percy's growing power. They might see him as a threat to their long - lost dominion over the oceans. Another possibility is a group of rogue demigods who have been corrupted by dark magic and want to steal Percy's new powers for themselves. There could also be a powerful sorcerer from a far - off land who has heard of Percy's powers and wants to use them to break a curse that has been placed on him.
The power to manipulate elements like water or earth could also be really exciting. Since Percy already has some connection to water, adding earth - manipulating powers would make him more versatile. He could create barriers of earth, or use water and earth together in new and creative ways. It would change the way he approaches battles and problem - solving in the fanfic.
If Naruto gains the power of Beerus in fanfiction, he would become extremely powerful. His already impressive combat skills combined with Beerus' god - like power would make him nearly unbeatable. He could easily defeat the toughest of foes in the Naruto universe with a single blow.