throughout the game after zeus takes your items athena will talk to you and tell you to speak with hercules and he wil finally help you and he will move the rock from hades throne.
herculese opens it after Zeus takes all your items
You give them to Hercule's and get him to move the rock in front of Hades throne room. Hope it help :)
get the five sacred items, give them to Zeus, go to Hercules, and use the mirror to get there.
herculees does that
After you collect all five sacred items, return to the Tree of Immortality. Zeus will take them. Then you can go to Hercules and talk to him. Use the Touchscreen Mirror to teleport to Hades throne room and Hercules will move the boulder.
Get Hurc to do it
You cannot open it until you have the aid of Hercules, and he will not do so right away. Once you have all 5 sacred objects, you have to bring them to Athena at the Tree of Immortality, where their message can be revealed. Then they will be stolen by Zeus. Now Hercules will agree to help you, and he can move the boulders blocking you from Hades and from Poseidon.
The entrance to the Underworld (realm of Hades) is in the Temple of Hades. Put pomegranates (from the garden of the Sphinx) on the altar to open the door. You cannot move the rock from the throne room. Only Hercules can do that, and not until you have collected the 5 sacred items and returned them to Athena at the Tree of Immortality.
To move the big rock in front of Hades' throne room on Mythology Island, you need to use the Mirror of Truth from Athena's statue. Stand on the circle with the cracked floor and reflect the light towards the rock. This will cause the rock to shift and grant you access to the throne room.
Once Zeus has taken the 5 sacred items, Hercules will agree to help you, and will move the rocks blocking the way to Hades and to Poseidon. (You will need the Touchscreen Mirror from Aphrodite in Poseidon's realm.)
You don't need to get the 9 drachmas; he lowers the price and you have to do the other challenges first you must go to Olympus village where you must speak to Athena who will get Hercules to help you move the rock in front of Hades' throne room. :)
When you get to Posidon's (the sea God) little castle front (by Hades' one, it's to the right) there will be an empty pedastal, put the starfish there and you will open a door, allowing you to move on,