Another possibility is that there is a prophecy involved. The prophecy might state that Percy will gain the power of his enemies at a crucial moment. This could lead to a series of events where he is unknowingly put in a position to inherit or absorb their power. It could be something like he has to face a great danger where the only way to survive is to take on the power of those who oppose him.
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.
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.
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.
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.
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.
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.
In some fanfics, Naruto might gain new powerful jutsus through intense training with a hermit or by unlocking some hidden bloodline limit. Maybe he discovers a new form of chakra control that gives him an edge over his enemies.
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.
Well, he might become more confident. With new powers, he has more abilities at his disposal, which could make him bolder in facing challenges.
In some fanfictions, Percy might gain power in Tartarus through facing and overcoming numerous deadly challenges. For example, by defeating powerful monsters that inhabit Tartarus, he could absorb some of their essence or gain knowledge from them which then transforms into power. Another way could be through making pacts with certain entities in Tartarus in exchange for power, although this would likely come with a price.
In many fanfic stories, Percy and Annabeth often face various enemies. Some common ones might be new monsters created by the author, or perhaps other demigods who have turned to the dark side. These enemies add drama and action to the stories.