After a long flight you'll fall into the underground, right into the Styx. Follow the current.
During the bath with the other convicts' souls you'll lose all your powers. Your energy bars will be shortened, and you'll have to build up Kratos' powers for the 4th time.
Blades of Exile
After getting to the shore, use whatever's left of your strength, get out and talk with Athena (who will give you your old-new weapon).