This story of the war between the elder gods, also known as the Titans, and their younger siblings, the Olympians, is totally epic. It's also one of our best early examples of how a story can change (and boy do we mean change) when told by many different authors. The story of the Titan War was handled in bits and pieces by dozens of authors, and trying to read it is like listening to a bunch of old Greek dudes play Telephone.
Cronus took the throne from his father, but not before Uranus made a prophecy that his son would also be overthrown by his own sons. Afraid that he would lose the reign, Cronus turned into the same tyrant god that his father was; he put his brothers back into Tartarus, and also ate his own children, in an effort to prevent the prophecy from becoming true. However, his wife Rhea tricked him and saved her youngest child, Zeus, from his father's paranoia. She hid Zeus in a cave in Crete, where he was raised by a goat, Amalthea.
When Zeus grew up, he became his father's cupbearer, without revealing his true identity. Helped by Metis, the Titan goddess who later became Zeus' first wife, he gave Cronus a mixture of wine and mustard, causing him to vomit one by one the children he had swallowed. When all of his brothers and sisters were freed, Zeus gathered them and convinced them to start a rebellion against their father.
Thus started the Titanomachy. Zeus released the Hecatonchires and the Cyclopes from Tartarus and asked their help against their brother. They all agreed; the Hecatonchires started hurling rocks against the Titans, while the Cyclopes created the famous thunderbolts for their leader, Zeus. Themis and Prometheus were the only Titans that fought on the side of Zeus.
When the War ended with the Olympians on the winning side, all Titans except Themis and Prometheus were jailed in Tartarus and were guarded by the Hecatonchires. Zeus, along with his brothers Poseidon and Hades, divided the universe by drawing straws; Zeus won and became the king of the sky, as well as the ruler of mortals and gods; Poseidon became the ruler of the seas; while Hades, who drew the shortest straw, became the ruler of the Underworld. This was the dawn of a new era in Greek mythology.