In Greek mythology, Tartarus is the darkest depths of the Underworld. It is where the souls of the evil are sent to for eternal torture, and it is where Zeus imprisoned the Titans. In mythology, Tartarus is a part of Gaia, near the regions of her stomach, while in other variations of the myths, Tartarus is also a Titan being like Gaia. He made love to Gaia as well, having Gaia birth to Typhon as a result.
In God of War Series
God of War: Chains of Olympus
In God of War: Chains of Olympus, Kratos was defeated by Charon the ferryman, and subsequently thrown into the prisons of Tartarus. As he attempted to make his way out of the depths, Kratos fought his way through hordes of enemies, came across the prison of the Titans, and obtained the Gauntlet of Zeus, which aided his escape.
God of War III
In order to craft a special weapon for the Spartan warrior, Hephaestus told Kratos to retrieve the Omphalos Stone. The stone, however, was said to dwell deep within the Pits of Tartarus, deep below Hephaestus' forge. In the caverns, on his way his to the doors, Kratos witnessed the freed souls of the Underworld in search of their redemption that is nowhere to be found. Then Kratos reaches the Gates of Tisiphone, which he first needs to solve the puzzle of the doors that will lead him to the Pits of Tartarus. He then battled his way through its darkest depths, until ultimately coming face-to-face with Cronos the mighty Titan. After a vicious battle, Kratos slew Cronos and retrieved the Omphalos Stone.