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.
If Percy were alone in Tartarus in fanfiction, the story might focus on his inner struggle and growth. He'd face extreme dangers and might discover hidden strengths within 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 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.
In this fanfiction, Percy going through Tartarus alone could involve him exploring the darkest parts of the underworld. He could discover secrets about the origin of the monsters there. Maybe he stumbles upon an ancient power that he has to either resist or use to his advantage. Along the way, he might have visions of his loved ones which both give him strength and also make him homesick.
Well, in these fanfictions, a recurring theme is Percy's perseverance. Tartarus is full of terrors, but he doesn't give up. It's like he's constantly fighting against the odds. And there's often the theme of hope. Even though he's alone, he holds onto the hope of getting out or making a difference. Sometimes, there's also the theme of facing one's past traumas. Tartarus might bring up things from his past that he has to deal with in order to move forward.