just wake up the elf king(or was it elf prince, its the same person though) and he will
give it to you

he will be in elfhem

how i wake up elf king?

you go to the great marsh and in a room at b3f there will be a roonm with a chest
surrounded by stones, you walk up to it and you get attacked, after kiling the piscodemons
open the chest and find a crown, then you go to the abandoned keep which is farther north
from the marsh, there is a king looking for his crown, you give him the crown and he
reveals that he is the king of the dark elves astos, you defeat him and get a crystal eye,
you then go back to your ship and sail to cornelia, you go northwest from the bridge and
then u start to go north after you see the water, then you go into a cave and give the
person back her eye, she gives you the antidote for the prince, you go back to elfheim and
give it to the nurse, she gives you a mystic key.

sorry my last comment wasn't entirely true, its a tad bit difficult make sure you have
plenty of antidotes.

i dont know the game very well well actualy i dont know it at all but i can tell ya won
thing i will give u so many answers if i get the game kk? ok well bye bye 4 now!