Prometheus was being punished for giving fire to mankind. His punishment was to be chained to a mountain where every day an eagle would come and eat his liver. Every night his liver would grow back. After thousands of years of torment, the gods asked Zeus to end his punishment and suffering.

Zeus sent Hermes, the wing-footed messenger of the gods, to Prometheus with the message that he would release him if he revealed to Zeus the secret that only Prometheus knew.

Prometheus, whose talent was foresight, could see the future and knew the only person who could destroy Zeus. That person/god was the future son of Thetis, a sea nymph.

To ensure this future would never come to pass, Zeus and his brother, who both wanted Thetis for themselves, agreed to have her marry a mortal. This way, her children would not inherit the strength of the gods and would never be able to overthrow them the way Zeus had overthrown his titan father.

For more information on Zeus, Prometheus and Hermes, see the links below.