The core of Hellenic cosmology comes from the poets Hesiod and Homer, whose epics were likely constructed from various bits of oral lore. Hesiod's Theogony is concerned with the origins of world and the gods.
First there is Khaos (Chaos) the first of the Protogenoi (primeval gods) at the origin of the universe. She is the void between heaven and earth and considered a goddess of fate. Then come Gaia (Earth), Tartaros (the Underworld) and Eros (Desire). From Khaos came Erebos (Darkness) and Nyx (Night), and from their union was born the goddess Hemera (Day) and god Aither (Air).
Alone Gaia birthed Ouranos (Heavens), Pontos (Sea) and the Ourea (Mountains spirits). With Ouranos, Gaia was mother to the Titans, (Okeanos, Koios, Krios, Hyperion, Iapetos ,Theia, Rhea, Themis, Mnemosyne,Tethys and Kronos), the Kyklopes (single-eyed giants), and the Hekatonkheires (hundred-headed giants).
Ouranos despised his monstrous children, the Kyklopes and the Hekatonkheires, and has them sealed inside Gaia's body. In pain Gaia called upon her other children to overthrow their father and free their siblings. Only Kronos, the youngest and most cunning of her children agreed to help, and when Ouranos covered Gaia in his sky blue cloak to couple with her, Kronos took the adamantine sickle Gaia had given him and castrated Ouranos.
Ouranos's severed genitals were thrown into the sea and from them emerged the beautiful goddess Aphrodite, carried on the foam to Kypros (Cyprus). The drops of blood from Ouranos's castration splattered onto Gaia and from them emerged the winged Erinyes (Furies), goddesses who pursue unjust mortals, and the Meliai (nymphs of Ash trees).
With Ouranos mutilated Kronos assumed the role of leader of the Titans and married his sister Rhea. Kronos ruled over a golden age, in which mankind needed no laws and there was no immorality, he rescinded on his promise to free his monstrous siblings and re-imprisoned the Kyklopes and Hekatonkheires. Perhaps in spiteful retribution Gaia informed her son that he himself would be replaced by one of his children.
Desperate to retain his kingship, Kronos devoured five of his children as they were born. However when Rhea fell pregnant with her sixth child she fled to Ouranos and Gaia who hid her in Crete where she gave birth to her son, Zeus. Gaia kept Zeus hidden in a cave on Mount Ida where he was suckled by the goat Amalthea. His cries were drowned out by the Kouretes, a band of warrior-dancers who clashed their swords and shields. In place of her son Rhea wrapped a stone in swaddling and presented it to Kronos, who promptly swallowed it.
When Zeus reached manhood he led a war against his father, the Titanomachy, fought in Thessaly over a period of ten years. During the war Zeus freed his siblings from Kronos's gullet, and the Kyklopes and Hekatonkheires from their prison in Tartaros. In gratitude they joined with him in the fight for Olympos, the Kyklopes gifting Zeus with thunder and lightning.
The younger gods defeated the Titans and imprisoned them as the Kyklopes and Hekatonkheires had been. The Zeus assumed the leadership of the Olympian gods, taking the sky and Olympos as his realm, with his brothers Haides taking the Underworld as his realm and Poseidon claiming the sea for his domain.
Prometheus and the Creation of Mankind
During Kronos's rule the first men were crafted from clay by the Titan Prometheus. These were men of a Golden Age, who lived an extended lifespan but did not became frail and old, never worked and mingled freely with the gods. When the men of the Golden Age died they became wise and benevolent daimons, who could be called upon to guard against evil and ills.
Subsequent generations of mankind were created by Zeus and paid tribute to the gods. However Prometheus, ever on the side of mortal men, tricked Zeus into accepting lesser sacrifices, saving the best portions for mankind. In retribution Zeus withdrew the gift of fire, spurring Prometheus to steal fire from Hephaestoss forge and return it to mortal man. This time Zeus punished Prometheus as well as man. The Titan was chained to a rock and his internal organs devoured by an eagle. Each night Prometheus's organs grew back, only to be eaten again the following day. To mortal men Zeus and Hephaestos crafted a beautiful woman, Pandora, and with her came all the ills that plague mankind.
Despite this the gods are still helpful to mankind and have their favourites. The hero Odysseus was aided by Hermes, Athene helped her half-brother Herakles in his labours and Artemis favoured the hunter Orion.