Herakles was born to a mortal woman but had the god Zeus for his father. To protect his son Zeus gave him the gift of great strength. Hera, Zeus' wife was jealous and plotted revenge for his birth. Herakles grew into a tall strong man. He married and had three sons. He led a very settled peaceful life with his family but Hera's plot involved him having a dreadful dream. In that dream he killed his family.
Herakles was devastated. Finally he went to the Oracle to ask the gods what he had to do. He went as a slave to serve King Eurystheus who set him twelve labours which would free him from the burden of his family's death. If he complete all of the tasks he would once again be a free man. Herakles was a strong, powerful slave and the King did not want to lose him.
King Eurystheus set Herakles the task of killing the lion of Nemea. To everyone's amazement Herakles came out of it alive.