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.
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.
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 might face extreme physical challenges like battling powerful monsters on his own. In Tartarus, there are many dangerous creatures that could be a huge threat.
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.
His water - based powers might intensify as he is in a place filled with so much negative energy. He could learn to control the strange, dark waters in Tartarus, which are probably more powerful and unpredictable than normal water. Maybe he can use them as a shield or a weapon in ways he never thought of before.
A really cool plot could be that Percy is alone in Tartarus and he starts to lose his memory due to the dark magic there. As he forgets who he is, he still has his instincts. He begins to form an unlikely alliance with some of the less evil creatures in Tartarus in order to survive. He has to then rediscover his identity while trying to find a way out of this hellish place.