Zeus lay with Alcmene, the mother of Hercules, in the guise of her absent husband Amphitryon and begot Hercules, a circumstance revealed by the Seer Teiresias. The jealous Hera tricked Zeus into swearing that whoever was born first on a certain day would become the King of the House of Perseus. Hera then laid spells on Alcmene to delay the birth of Hercules, until the son of Sthenelus, whose name was Eurytheus, was already born. Zeus, who had intended the Kingship for Hercules, could not renege on his oath, but he persuaded Hera to agree that, if Hercules would perform 12 labors set upon him by Eurytheus, Hercules would then be allowed to become a god.
Later it was prophesied: "No man alive shall ever kill Heracles; a dead enemy shall be his downfall." (Robert Graves, Greek Myths, v.2, p. 202)]