That could actually make a lot of sense. Tartarus is older than even the titans, I’m pretty sure he’s even slightly older or about as old as Gaia. It would also connect to the pit in Makrinaos, as Tartarus is often described as a pit. It would also mean that The Seekers of the Sight’s master wouldn’t be Hades, which is a definite plus.
Though the main counterargument would be motivation, as I can’t see exactly how the state of the world benefits Tartarus in any way. Though Vetex could ascribe any motivation to this entity as currently we don’t have any clues to their identity (or if we do I’m not aware of them.)
Edit because I just realized this: the pit leading to Tartarus makes a lot more sense than just the Underworld proper, since the Underworld ain’t that scary. Sure it’s scary to think about death, but the place itself aside from Tartarus is rather peaceful of a bit dreary. The Underworld classically (Riordan added some bits, so I’m going off of Wikipedia to be more in line to the original mythology) consisted of 3 to 4 parts. The Asphodel Meadows, where most mortals went. Elysium home of the honored heroes and Tartarus where the particularly wicked were held and punished. In some interpretations there’s also the Isle of the Blessed, for those who were pure but not heroes, at least best I can guess, Wikipedia just says those who are particularly pure. I realize this is a tangent, but any chance I get to talk about how Hades doesn’t make sense as a villain is one I will take.