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 God of War: Chains of Olympus, Kratos is killed by Charon the ferryman, and upon being killed 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.