Well, one possible plot could be Percy facing his inner demons in Tartarus. Since he's alone, he might have to rely solely on his own skills and memories. Maybe he'll encounter some long - forgotten monsters that test his strength and willpower. He could also stumble upon some hidden secrets about the place that no one has ever known before.
It could be Nyx. She is a very powerful primordial goddess in Tartarus. Since Percy is alone, she might see him as an easy target or a threat to her domain. Nyx could send all sorts of dark creatures and illusions to stop Percy from achieving whatever goal he has in Tartarus.
He might face numerous terrifying monsters. Tartarus is full of dangerous creatures in the fanfic world, and Percy would have to fight for his survival all by himself.
Bob the Titan could potentially be an ally. Since he has some knowledge of Tartarus and has shown kindness before, he might come to Percy's aid again. He could help Percy navigate the dangerous areas and avoid some of the more powerful threats in Tartarus.
One possible plot development could be Percy facing off against Tartarus' own unique and terrifying monsters all by himself. He might have to rely solely on his wits and the powers he has developed over his previous adventures. For example, he could use his water - controlling abilities in creative ways to navigate the strange and dangerous terrain of Tartarus.
He uses his combat skills. Percy is a skilled fighter, so he'll likely rely on his sword - fighting abilities to take on the monsters he encounters in Tartarus.
Well, one possibility could be Percy facing his inner demons. In Tartarus alone, he might have to deal with the memories of his past failures and the fear that comes with being in such a desolate place. He could encounter strange and terrifying creatures that test his combat skills and his will to survive.
One popular Percy Tartarus Alone fanfic could be 'Surviving Tartarus: Percy's Solitary Struggle'. It might focus on Percy's inner strength as he faces the terrors of Tartarus alone. Another could be 'Percy in the Depths of Tartarus', which may explore how he uses his skills and wits to overcome the various challenges.
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.
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.