Zeus is the King of the Greek Gods and the Father of Olympus.
Thousands of years ago, Zeus lead the Olympians in their war with the Titans. However, Hades, Zeus' brother, made a pact with the Titans, and, with the unwitting help of his lover Hippolyta, betrayed the Olympians to the Titans. The Titans were defeated, and Zeus banished Hades to Tartarus. Hippolyta was commanded to forever guard the gates of Tartarus for her role in the betrayal.
In the 21st century, Felix Faust overthrew Hades, threatening the balance between order and chaos. To prevent this, Zeus decreed that Wonder Woman was to go to Tartarus, defeat Faust, and restore Hades.