The first generation of gods came out of Chaos: Eros (Love), Gaea (Mother Earth), Uranus (Father Sky), and Tartarus (Underworld)
Uranus imprisoned his and Gaea's first six children (the Hundred-Handed Giants and the Cyclopes) in Tartarus, so Cronus, one of the thirteen Titans, told Gaea he would punish Uranus and take over ruling
Cronus was just as cruel a ruler as his father, and he did not release his siblings. He also ate all of his and Rhea's children so his rule would last forever.
Rhea hid one of her sons, Zeus, in a cave and tricked Cronus into eating a rock instead.
Zeus came back to Mount Olympus as an adult to save his siblings from Cronus and become the new king. Zeus and siblings fought the Titans for ten years, but still no one could win.
After freeing the hundred-handed giants and cyclopes from Tartarus on the advice of Gaea, Zeus and his siblings won the war against the Titans and Zeus became the new ruler.