I'm assuming you mean the elf prince in Final Fantasy. You need to get a Jolt Tonic (or an Herb in the original English translation) from Matoya, but before you can do that, you need to get her crystal eye back from Astos, but before you can do that, you need to retrieve his crown from the Marsh Cave. So:
- Go to the Marsh Cave and find the Crown. It's in a chest.
- Go to the Western Keep and give the Crown to the king. After defeating Astos, you'll receive the Crystal Eye.
- Go back to Matoya's Cave and give her the Crystal Eye, and she'll give you the Jolt Tonic.