Obviously, Percy is the central character. But often, Annabeth can also play an important role, even if she's not physically in Tartarus with Percy in some stories. She might be trying to find a way to rescue him from the outside. Another common character is Nico, who has some knowledge of the Underworld and Tartarus and could potentially be a key figure in helping Percy either survive or escape.
One popular story might be 'Percy's Descent' where he faces various challenges in Tartarus. Another could be 'Percy in the Abyss of Tartarus' which focuses on his survival and growth during his time there. There's also 'Tartarus Trials of Percy' that delves into his encounters with the dark creatures of Tartarus.
His friends like Annabeth or Grover could be the ones to rescue him. They might discover a way into Tartarus and brave all the dangers to get to Percy.
One popular type of fanfic could be about Percy and Annabeth's journey through Tartarus. It might focus on their struggles, like how they face the various terrifying monsters there. Maybe they use their unique powers and the bond between them to survive. For example, Percy's control over water could be crucial in crossing some of the dangerous rivers in Tartarus, while Annabeth's wisdom helps them make smart decisions when choosing paths in that dark and chaotic place.
In some fanfictions, Percy might be more withdrawn.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?
One common theme is identity struggle. Percy has to figure out who he is now that his parentage has changed. He was known as a son of Poseidon, and suddenly being the son of Tartarus throws his whole self - concept into chaos.
One interesting 'Twilight Spell Tartarus' fanfiction could be about a character using twilight spells to escape from Tartarus. Maybe they have to face various challenges and monsters within Tartarus while harnessing the unique power of the twilight spells.
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.