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.
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.
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.
There's a chance that Percy meets the personification of Tartarus itself. It could be a shadowy, all - powerful being that tries to break Percy's spirit. Percy, being the brave hero he is, would have to resist and find a way to either outwit or overpower this entity in order to continue his journey in Tartarus.
There might also be a lost spirit or a demigod who was previously trapped in Tartarus and has managed to survive. This character could share their experiences and skills with Percy, and together they could try to find a way out or at least make the best of their situation in Tartarus.
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.
Maybe it's the Titans who are still lurking in Tartarus and don't want Percy and Zoe to succeed in whatever mission they have. The Titans could see them as a threat to their potential future plans of rising again.
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.
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.
It could be another powerful god or goddess who wants to use Percy's betrayal of Tartarus for their own gain. Maybe a god of chaos who sees this as an opportunity to create more disorder in the world. They might manipulate Percy into the betrayal and then turn on him later.