"Here where the souls of the wicked are tortured in the pits of Tartarus, the wailing and agony of the damned, resonated deep throughout the bowels of the Underworld." - Gaia
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 its where Zeus imprisoned the Titans. In some variations of the myths, tartarus is also a titan being like Gaia. He also made love to Gaia and became the father of Typhon.
In God of War: Chains of Olympus
In God of War: Chains of Olympus, Kratos is defeated by Charon the ferryman, and upon being defeated is thrown into the prisons of Tartarus. As he attempts to make his way out of the depths, Kratos fights his way through hordes of enemies, finds the prison of the Titans, and obtains the Gauntlet of Zeus which also helps him escape.
In God of War III
In order to craft a special weapon for the Spartan warrior, Hephaestus tells Kratos to retrieve the Omphalos Stone for him. The stone, however, is said to dwell deep within the Pits of Tartarus, deep below Hephaestus' forge. Kratos enters Tartarus, and battles his way through its dark depths, until ultimately coming face-to-face with Cronos the mighty Titan. After a vicious battle, Kratos slays Cronos and retrieves the Omphalos Stone that Hephaestus had requested.