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.
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.
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.
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.
Perhaps Percy has to go to Tartarus alone because he believes it's the only way to save his friends. In Tartarus, he might encounter old foes who are now even more powerful. He has to rely on his wits and his skills with his sword, Riptide. He could also make unexpected alliances with some of the creatures there in order to survive and achieve his goal.
He might face numerous terrifying monsters. Tartarus is full of dangerous creatures, and without his friends, Percy would have to rely solely on his own skills and the power of his demigod abilities to fight them off.
He uses his wits. Percy has always been clever, and in Tartarus, he'd use that to his advantage. He'd find ways to outsmart the monsters, like using the environment against them.