Hades' Soul is the soul of the God Hades. If a mortal were to obtain it and have it absorbed into his body, he can safely swim through the River Styx and use Hyperion gates because it takes a soul of a god to use the Hyperion gates.
God of War III
During the battle with Hades, Kratos managed to steal the God's weapons, the Claws of Hades, and, after a painful yet short bout, the ruler of the Underworld plummeted into the River Styx, absorbing its power and becoming even stronger, attacking Kratos yet again.
However, Kratos managed to knock Hades unconscious and latched the Claws onto the bleeding God's skull. Then, he tugged at the hilts of the Claws, which caused Hades' soul to emerge from its broken body. After tugging at the hilts with all of his might, Hades' soul was painfully pulled out of his body. Kratos then absorbed the soul into himself, while the souls of the Underworld escaped and laid waste to Hades' body.