Obviously, Percy would be one of the main characters. Since it's about him training in Tartarus. And Annabeth might also be involved. Maybe she's helping him in some way, or she's waiting for him outside Tartarus while he trains.
Well, in most fanfictions where Percy trains in Tartarus, it often involves him facing extreme challenges. He might be learning to control new powers or just surviving the harsh environment. For example, he could be training to fight stronger monsters that lurk in Tartarus.
Annabeth might also be a main character. She could be either with Percy in Tartarus or be a driving force for him from outside, like trying to find a way to rescue him or communicate with him in some way.
Obviously, Percy is one of the main characters. Given the 'dark' aspect, he may have a more brooding and conflicted personality compared to his normal self.
Obviously, Percy is one of the main characters. Since he's the one sent to Tartarus. There might also be some of his old friends who play important roles, like Annabeth. Maybe she's on a quest to save him, so she becomes a central figure in the fanfiction.
In some more creative fanfictions, new characters might be introduced. For example, a powerful demigod who was born in Tartarus and has unique abilities. These new characters can add a fresh twist to the story, interacting with Percy and the others in unexpected ways, and often bringing new plotlines related to the mysteries of Tartarus.
Obviously, Percy Jackson himself is a main character. Annabeth might also be very involved, either helping him escape or being there to welcome him back. Grover could be another main character, perhaps using his nature - related powers to assist in some way.
There could also be some characters unique to the fanfiction. Maybe a new demigod who has some special power related to Tartarus and ends up joining Percy on his quest. Or perhaps an ancient being who was imprisoned in Tartarus long ago and has knowledge that Percy needs to complete his mission.
Obviously, Percy would be the main character. He's the one stuck in Tartarus, so the story would revolve around him. Maybe Annabeth could also be an important character, even if she's not physically in Tartarus with him. Her thoughts and attempts to rescue Percy from afar could be part of the story.
He would surely interact with Annabeth. Their relationship is central in the Percy Jackson series, so in a Tartarus fanfic, she'd likely be there to support him or be in a similar situation as him.
There might be a monstrous creature that Percy befriends instead of fighting. This creature could have unique abilities that help Percy survive, like being able to sense danger from afar.