This is a really interesting concept. In the original Percy Jackson series, Percy is the son of Poseidon. But in this fanfiction idea, if he were the son of Gaea and Tartarus, it would completely change his character and the story. His powers would likely be much darker and more primal. He might have an in - depth connection to the earth and the abyss, which could lead to some really unique storylines.
There could also be a group of demigods who are jealous of Percy's power due to his unique parentage. They might form an alliance against him, trying to steal his powers or at least make his life difficult. They could be led by a demigod who has a grudge against Percy or is power - hungry.
One idea could be Percy having to prove himself in the Underworld. Since he's the son of Tartarus, he might face unique challenges from the other denizens who fear or distrust him. Maybe he has to compete in some sort of underworldly tournament to gain respect.
The fanfiction could also explore Percy's identity crisis. He's known himself as the son of Poseidon for so long, but now being the son of Gaea and Tartarus would shatter that identity. He would have to re - evaluate who he is, what his purpose is, and how he fits into the world of demigods. This internal struggle could be a major part of the story, with Percy constantly fighting against the darker impulses that come from his new parentage while also trying to find a place where he can be accepted and still be himself in some way.
His relationship with Annabeth would be severely strained. Annabeth, who has always known Percy as Poseidon's son, would be confused and a bit distrustful. She might think Percy has been lying to her all this time. His friends at Camp Half - Blood would also be divided. Some would be scared of his new powers and his connection to Tartarus, thinking he could be a danger. However, some might be more understanding, especially those who have faced their own identity crises. His relationship with the gods would also change. The gods who were once somewhat friendly or at least neutral towards him might now see him as a threat. Zeus, for example, would be extremely wary of Percy's Tartarus - related abilities.
Annabeth could be a likely candidate. She's always been by Percy's side, and she'd stop at nothing to save him. Her intelligence and combat skills would be useful in the dangerous environment of Tartarus.
In some fanfictions, Percy might be more withdrawn. He has seen the horrors of Tartarus and it could make him less trusting of the world around him. He may also have new powers or a deeper understanding of his abilities that he gained from the experiences there.
Well, one aspect to consider in this fanfiction could be the character development. Percy Jackson is already a well - established character, but as the heir of Tartarus, he might face new challenges that force him to grow in different ways. Maybe he has to deal with darker powers within himself. Another point could be the world - building. Tartarus is a very dark and dangerous place in the Percy Jackson universe, so how the author expands on that and shows its influence on Percy and the story is important. Also, the relationships with other characters would be interesting. Will his friends still trust him when they find out he has this new 'heir' status? How will his enemies react?
Well, a plot twist could be that Percy's connection to Tartarus gives him the ability to control the very essence of the underworld. This means he can manipulate the shadows and the dark magic there. But because of his Lupa heritage, he also has a strong sense of loyalty and pack mentality. So, he forms an unlikely alliance with some of the more 'civilized' monsters in the underworld who also follow a code of honor. His friends from Camp Half - Blood are initially very skeptical and even a bit scared of him, but as they learn more about his new powers and how he's trying to use them for good, they start to come around.
One possibility could be Percy's struggle to survive the harsh conditions. He might have to find new ways to use his powers in Tartarus. For example, the strange and dark magic there could either enhance or limit his abilities. He could also form alliances with unexpected creatures. Maybe he teams up with some of the ancient and terrifying monsters that live there, but they have a common enemy, so they work together for a time.