(Before I begin this theory, I would like to say that I have indeed broadened the scope for the theories, animation was big, but a lot harder to be original. Now, with nostalgic, anything from 2015 on back is fair play for this. Look out for a lot more updates to this because it will get a lot more very soon.)
Well, this was part of a question I saw on Reddit multiple times. Anyone who doesn't know, I am a big fan of anime, Percy Jackson and the Olympians, and a lot of early 2010's, 2000's, and late 90's shows and movies.
The question was how does being in Tartarus allow Percy to control any liquid. In the end, I saw it a different way than we saw from one person's point of view. I have two instances where we got to see the effects on demigods in detail, and one where the effects are implied heavily. The demigods in question are three of the greatest in the story's canon. Annabeth Chase the Daughter of Athena, Percy Jackson the Son of Poseidon, and Nico Did Angelo the Son of Hades.
We'll start with Percy thanks to him being the one who's perspective we see throughout the main overpowered moments in Tartarus. He ends up being able to control any liquid in Tartarus, from the Underworld rivers to poison from various plants. When you think of his father's abilities that Percy is confirmed to have, this is not actually supposed to be possible within the realm of his father's control. It is stated multiple times that Poseidon controls the Sea not all water everywhere, but Percy is known for controlling Fresh Water, as well. Though, any water under the control of a minor god/goddess has never been able to be controlled without forcing it on that deity. When each Underworld river is known as the blood of Tartarus (god) it doesn't make sense for him to control any if them in Tartarus(place). Though, when he entered Tartarus, his entire mood changes more often. This is something common to various children of the gods thanks to them having what is known as separate moods. Children of Poseidon are known for being born with the mood of the god at the time of birth. We have seen Percy's (various) godly siblings, and they are all across the board, though the ones who are monsters seem to have a more angry and destructive nature. Is it odd that Percy begins to show signs of the same mood throughout his time in Tartarus (place)? His first combative action was followed by a noticibly dark phrase about his enemy "dying too easily" and that monster "deserved worse". Yet, during his fight with Akhlys, he was able to control poison to nearly consume the goddess of poison herself. The only thing that stopped him was Annabeth becoming intensely frightened. Even during the fight at the Doors of Death, he was controlling every Underworld river despite being at the heart of Tartarus (place) and having the god Tartarus himself before him. Amazing how he was there for what seemed to be around a week in the mortal world, but already had such a boost in power that he could take on an entire army of everyone he ever fought from Titans to basic monsters. All of which were incredibly hard fights in single combat in the mortal world and were supposed to be stronger than ever thanks to being in their birthplace. All this could point to Tartarus (place) strengthening the abilities of demigods after prolonged exposure and also slowly making them the equivalent of their godly parent.
Next is Nico Do Angelo, and boy does Death Boy end up getting the short end of the stick with Tartarus. So, he actually got through most of Tartarus alone. Something that shouldn't be possible in any sense of the word. Yes, this kid was captured, but that could be attributed to fatigue from going alone. His time in Tartarus (place) actually might have caused PTSD of the darkness. Oddly, this may be his father's reaction to being forced to the Underworld for so long. Plus, his power was dramatically increased to the point of being able to shadow travel halfway across the world in around 8 days time with only a day of forced rest in between each jump. Just trying to shadow travel once a year before would have put him in a comatose state. Another thing he obtained after returning from Tartarus (place) was an odd affinity with other children of the Underworld. Even to the point of enhancing and helping his sister Hazel control her powers while the same worked in reverse. The final upgrade to his powers was some measure of dream control, albeit not in regards to his sister's side of control as far as we know. From sending dream messages to causing mortals to dream "deeply", this kid from the 1940's grew stronger at an insane level after returning from Tartarus (place) and it could only be attributed to his time there. In other words, the same thing as Percy, though he kept his strength at that level longer.
The final character is Annabeth Chase the Daughter of Athena. (As a fellow Cabin Six member, I hope this part is true thanks to the overpowered nature of the upgrade.) Okay, before Annabeth fell to Tartarus (place), we were told the Children of Athena do not obtain any special powers, and are basically the equivalent of a Olympian athlete in mortal terms. Yes, they may not get any powers in the mortal world, but there are a surprisingly many times where Tartarus (place) affects her in extremely good ways for her combat style. Among these, she gained a sixth sense upon entry to Tartarus (place). When Percy killed his first monster there, she had sensed it coming for her just before Percy killed it. Later on, she was blinded and still capable of fighting multiple enemies even proving capable of defeating at least three alone before being "saved" by Percy. During the time Percy was controlling the poison, she was still suffering from some side effects and ended up sensing Percy's entire mood and thought process before seeing what was happening and calming him easily. Not only that, she confirmed she knew what he thought by saying, "Some things aren't meant to be controlled." This quote was an odd choice, but could be explained by simply saying that Athena kids in Tartarus (place) end up getting an upgrade to their mind that borders on reading others thoughts. Since their abilities aren't considered noticable, and they are children of the mind of Athena, they are likely to have that part of themselves get an upgrade rather than any physical or new abilities based on warfare. In the end, Annabeth Chase actually got mentally upgraded, yet didn't keep it at all after that. Almost like Athena wasn't a permanent change thanks to being the goddess of wisdom and battle strategy and dealing with ideas and thoughts rather than abilities like the rest of the gods and goddesses.
Overall, I would like to go through the next few theories to explain what other demigods would likely get as upgrades in Tartarus (place), and that would be extremely original in terms of what each one would acquire. So, next week, I will do Hermes, Dionysus, and Ares.