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 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 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.
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.
Percy could gain power by defeating the most powerful beings in Tartarus. By taking down the toughest Titans or ancient monsters that lurk there, he could absorb their power or gain respect from the other inhabitants, which would help him rule.
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 possible source is the dark magic or ancient spells that exist in Tartarus. Percy might stumble upon a spellbook or be taught by a long - forgotten spirit in Tartarus. Another source could be the unique minerals or substances found in Tartarus. If he consumes or uses them in some way, they could enhance his abilities. For example, there could be a crystal that amplifies his water - based powers.
There's no way to tell who the author is just from the title 'percy is tartarus fanfiction'. It could be written by any fan of the Percy Jackson series who wanted to explore this particular scenario in their own writing. It might be someone new to fanfiction writing or an experienced writer in the Percy Jackson fan community.