Astarica is on the west coast of S America. There is a place to park your boat directly to the south. Then you can walk to the temple.
When you enter the temple, walk up to the alter in the middle of the room. You'll hear a voice telling you to drink from the goblet in front of you. There's no use fighting not drinking, you'll do it anyway as if your mind was controlled by someone else. After Ark drinks the liquid in the goblet, he passes out.
Lord Meila explains that they hold a holy ceremony once every year on this exact day. They place 6 goblets on the alter, which are filled with night dew that they have collected over the year. One of the goblets is said to make the drinker closer to the gods and must spend a year without food or drink, while the other 5 mean certain death. Today two of the priests have gone back to the goblet chamber, but they haven't returned yet. Lord Meila requests that you go investigate.
To enter the chamber, you must push the statue that is behind the alter with the two fire pots. The statue is directly north from where you woke up at and has a pink jewel in its forehead.
Just follow the pathway in this area. There's no way to get lost, but there a lot of monsters. You can also pick up a treasure and a MagiRock. Once you reach the end of the path, go into the next room.
Suddenly Roy and Fyla are surrounded by light. Roy demands to know what Elle is doing, and Fyla asks her what is wrong with a life that never ends. Elle then destroys the two of them, but not before Roy and Fyla tell each other they love one another.
Talk to Elle. She asks if you've come to gain an immortal body. Ark mentions to her that for an instant, her face looked like a demons. He recalls that it has been said the people have an external and an internal soul and perhaps the liquid in the goblet awakens the internal one. Elle then dares Ark to try it for himself.
It doesn't matter which goblet drink, even though the one that was directly behind Elle is not access able. Once you've taken your choice and have digested the contents, you pass out and wake up back at Astarica. There is now a treasure chest by the alter that holds the StarStone.