In the beginning there was Chaos...
Ritual, Prophecy, Incest, Cannibalism, Patricide, Fratricide and Infanticide were all foundations of the emergence of the gods and titans. The first deity was Chaos, out of Chaos was born Gaia (Mother Earth) gave birth to Uranus (heaven), they then became husband and wife and Gaia gives birth to Cronos and Ria who are brother and sister and also become husband and wife.
Its important to remember the foundations of the twelve Olympian deities as this gives us an idea of what drove them to reign Olympus.