What do you do in the underworld on mythology island?

Take the pomegranates from the tree in the garden of the Sphinx to offer to Hades. Jump down the pit to reach the River Styx. Avoid the falling rocks, flaming skulls, and river monsters, until you reach Cerberus. Play the reed pipe, using the tune that you learned from Euterpe in the Temple of Muses (blue, green, yellow, blue, red, green) and get the Cerberus whisker.

You cannot open the throne room until Zeus has taken the sacred items and you have Hercules to help you.