After solving the bones puzzle and climbing up and down some chains, you reach another door. Enter it. You come across the red-eyed snake puzzle:

"The red-eyed snake now holds your fate. Catch her thrice to raise the gate"

Different snakes will pop out of different holes. You are interested in the snake with red eyes. Click on his its head as soon as you see it come out of the hole. You need good reflexes for this. Do this three times, and  the gate opens. Exit through the door and talk to the minotaur again.

You can tell him:
I completed the Labyrinth
So... Can I have the ring now?
This ring is one of the sacred items I need to give to Zeus

Tell him that you completed his labyrinth. He cannot believe you are still alive. Ask him if you can have the ring and he gives it to you:

Sacred Item
Ring from the Minotaur
Whoever Wields

When you tell him about Zeus wanting that ring, he wants you to be careful. Zeus cannot be trusted.

Go right and exit the labyrinth.

Go to the garden of the Sphinx and speak to the Spinx. She tells you that to gain one of her flowers you must pass her test. You ask her what you have to do. She tells you that what was once wet is now dried up. When once again the water flows then she will grant you one red rose.

Jump up and climb on the rope that says "Aqueducts"  Dont activate the levers yet. Jump through the platforms first and lower the bridges down. Then, activate the levers one by one starting from the one at the top. The water will start flowing and will reach the flower at the base of the Sphinx. Jump down and get it:

Sacred Item
Rare Flower
All of

The Sphinx ells you that your fate awaits you. Before leaving the garden, climb up some platforms to a tree with fruit and a sign that says "You pick'em, you keep'em". Grab a Pomegranate from that tree.

Go far to the right. A satyr you meet tells you that to appease a god, you must leave an appropriate offering upon their altar. Keep going right and descend below the ground. A guy there asks you if you can help him clean the Graffiti. He will pay you one Drachma. Clean the graffiti which says "Down with Hades"  and another one on top of the door which says "Hades is a deadbeat", and some red graffiti to the right, that says "Hades has Deathly Breath".

The dude thanks you for your help in cleaning the graffiti and wishes that Hades looks favorably upon you.

You recieve:

This is a Greek coin.

Enter through the door. There is another door inside, but it does not open. Place the Pomegranate on the altar near the door and the door will open. Go inside. There is a sign that says "The descent into Hades is easy. The way lies open to all."  There is some flying creature there which you can click but you say that it seems harmless enough.  It follow you around. You reach another door, but it does not have a handle. It must open from the other side.

Jump down and you will reach the Underworld.