Tartarus (primordial)

"You cannot kill me! I am the pit itself. You might as well try to kill the Earth. Gaea and I - we are eternal. We own you, flesh and spirit!"

- Tartarus to Iapetus Tartarus is the primordial Greek god of the abyss, the third one to manifest in the yawning void of Chaos. When a monster is slain, it's non-spiritual essence is sent to Tartarus until they reform, for a time ranging from days to sometimes centuries. Only immortal enemies (such as Kronos) are sent here; the mortals (such as Tantalus) are sent to the Fields of Punishment. Tartarus is also the deepest, darkest pit in the Underworld.

History
Tartarus is the father of Typhon and the Giants, of whom Gaea is the mother. He is seen as the "prison guard" of Tartarus, the darkest pit in the Underworld where monsters go to reform once they die.

The House of Hades
In the climax, Tartarus takes on a mortal form, remarking on how wise his consort Gaea was to consider waking up fully. Annabeth Chase notes that the only reason she, Percy Jackson, and Bob were still alive was because Tartarus was too busy marveling on his mortal form. She realizes that Protogenoi like Tartarus are unlike their descendants, that not even a Titan could stand alone against Tartarus. To her dismay, she notes that this was what it would be like if Gaea were to awake fully. Feeling hopeless, Annabeth and Percy resign to fighting to the death against Tartarus. They are saved by the arrival of Tartarus' own son, the Gigante Damasen. He and Iapetus sacrifice themselves to allow Percy and Annabeth to escape. They were likely destroyed by Tartarus.

Trivia

 * Along with his father Chaos, Tartarus is the only known figure to be both a place and a protogenoi, with the possible exception of Gaea.