Well, sometimes in such fanfictions, their relationship might be put to an extreme test. The harsh environment of Tartarus could cause tension between them. Maybe Percy has to make a tough choice to save Annabeth or vice versa, and this creates a rift in their relationship at first. But as they work through it, their bond becomes even stronger, which is a really engaging plot development.
In some fanfictions, a possible plot development is that Percy and Annabeth discover a long - lost secret or power within Tartarus. This could be something that gives them an edge in their fight to escape or a knowledge that changes their perspective on the whole Greek mythology world. As they explore Tartarus, they may find ancient artifacts or meet forgotten gods who have been trapped there for ages. These encounters can lead to unexpected alliances and revelations that add depth to the story.
They could discover a lost civilization in Tartarus. This civilization might have a way to manipulate the very fabric of Tartarus itself. Percy and Annabeth would then have to learn the ways of this civilization in order to use it to their advantage. Maybe they have to earn the trust of the civilization's leaders by helping them with their own internal problems first.
In this type of fanfiction, it could be that the very essence of Tartarus affects the pregnancy in strange ways. Maybe the unborn child starts to show signs of being influenced by the dark magic or the primordial forces there. This could lead to a race against time as they try to find a way to counteract these effects. Also, the relationship between Annabeth and Percy would likely be put to the ultimate test. They would have to rely on each other more than ever, and their love for each other and the baby would be what drives them forward. Their friends on the outside might also sense something is wrong through some sort of magical connection and start their own quest to try and reach them in Tartarus.
One interesting plot development could be that during the duel, they accidentally discover a new power or ability. For example, Percy might find a new way to control water that is triggered by the intensity of the duel with Annabeth.
One interesting scenario could be Percy and Annabeth finding a hidden, powerful weapon in Tartarus that gives them an edge against the monsters there. Maybe it's a sword that can cut through the very fabric of Tartarus's darkness.
In this fanfic, Percy could develop a darker side. The horrors of Tartarus might make him more ruthless when it comes to protecting himself and Annabeth. Annabeth, on the other hand, may become more empathetic. She could see the pain and suffering of the other beings trapped in Tartarus and try to help them, which would shape her character in a new way. Also, their relationship would develop further. They would have to trust each other completely, and this could lead to a more mature and deep connection between them.
There could be some minor gods or spirits that have been trapped in Tartarus but are still benevolent. These could be beings that Percy and Annabeth come across and who decide to help them in their journey. For example, a spirit that has knowledge of the layout of Tartarus could guide them away from the most dangerous areas. Another possibility is some half - forgotten heroes who have been in Tartarus for a long time and have developed unique skills to survive, and they join forces with Percy and Annabeth.
In this fanfic, they might face terrifying monsters. Tartarus is full of dangerous creatures, so they could be constantly fighting for their lives, like battling against ancient Titans or monstrous beasts that are native to that hellish place.
One possible plot development could be Percy having to face various ancient and terrifying monsters on his own in Tartarus. He might have to use his wits and limited resources to survive. For example, he could encounter the monstrous Titans that were imprisoned there long ago.
Perhaps Percy forms an unexpected alliance in Tartarus. There could be other lost souls or even some ancient beings that have been trapped there. Together, they could plan an escape or a way to overthrow the dark forces that rule Tartarus. This alliance could test Percy's beliefs and values as he has to work with some rather shady characters.
The plot could involve them losing their way in Tartarus, both literally and metaphorically. They get separated from their usual support systems and have to rely solely on each other. As they journey deeper, the darkness starts to change them. For example, Annabeth might become more aggressive in her decision - making, which is a departure from her normal self. Percy struggles to keep up with this new side of her while also fighting off the demons of Tartarus.