Well, a possible plot is that Percy has a flashback to a time in Tartarus when he witnessed a prophecy being fulfilled in a way he didn't expect. This could lead him to re - evaluate his understanding of prophecies in the present. Another idea is that his flashback reveals a secret passage or power source in Tartarus that he believes might exist in the mortal world too, and he sets out to find it. Also, the flashback could be about a time when Percy was separated from Annabeth in Tartarus for a brief moment, and that fear of losing her again triggers his current actions to protect her more fiercely.
A great plot could involve Percy finding a way to use the unique environment of Tartarus to his advantage. For example, there could be some sort of energy in Tartarus that he discovers he can harness to become stronger, but it has a dangerous side - like it might slowly corrupt him. He then has to struggle with the decision of whether to keep using it to get out or resist it.
Another plot could be that something from Tartarus has followed Percy out. A small, malevolent creature that starts causing trouble in Camp Half - Blood. Percy feels responsible for it and has to find a way to send it back to Tartarus without putting everyone in danger.
Tartarus is an abyss of nightmares. Picture a place where the very light seems to be devoured. The walls are made of jagged obsidian, so sharp that they seem to cut the air itself. There are strange, twisted formations of rock that look like grotesque monsters frozen in time. The heat is oppressive, not just a physical heat but a heat that seeps into your soul, making you feel like you're slowly being cooked from the inside. And there are echoes, the screams and moans of the damned that reverberate through the endless expanse.
In a Percy Jackson Tartarus fanfic, a great plot could be centered around Tartarus corrupting the very essence of Percy's powers. He starts to lose control of his water - based abilities and instead they become tainted with the darkness of Tartarus. Percy then has to go on a journey within Tartarus to find a way to cleanse his powers, all while being hunted by the most dangerous creatures that lurk in the abyss.
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.
Well, in many fanfictions, Percy might be on a mission to assassinate some powerful being in Tartarus. Maybe he has to face all kinds of hellish creatures and overcome insane challenges. It could involve him using his water powers in unique ways, like creating weapons out of water to fight the demons of Tartarus.
One great Percy Jackson Tartarus fanfiction is 'Into the Depths of Tartarus'. It really delves into the horrors Percy and his friends might face there. The author does a great job of describing the dark and dangerous environment, making it feel very real.
One plot idea could be Percy struggling with his new identity. Since Tartarus and Nyx are such powerful and dark entities, Percy might have to battle against the dark urges within him. He could be shunned by his friends at first, but then prove that he's not defined by his parentage. For example, he might save Camp Half - Blood from a new, powerful threat using his unique powers that come from his heritage.
Well, one possibility is that Percy has to deal with the various monstrous creatures in Tartarus. He might form alliances with some of the less evil ones to maintain order. Maybe he uses his powers over water in unique ways in the dark and chaotic environment of Tartarus.
One possible plot could be Percy having to face his inner demons in Tartarus. Maybe he has to battle new and terrifying monsters that are unique to that realm. He could also encounter old foes who have been made even more powerful in Tartarus.