Creation of The World
According to Hesiod's Theogony, there was only Chaos in the beginning as a primeval state of existence. It was dark, silent, stationary and without any trace of life to infinity. Then out of silent void Gaea (the earth) and Tartarus (the sunless and terrible region beneath Gaea) appeared. They were not the only ones who appeared out of the void. Hesiod also talks about the deathless gods who hold the peaks of Olympus and Eros (the desire to re-produce) being the fairest among them. About the same time or soon after, out of Chaos, Erebus (the place of unknown where the death dwells) and Nyx (the night) were born. Erebus and Nyx in order to restore the balance gave birth to their opposites, Aether (the upper atmosphere just below heaven) and Hemera (the day). Meanwhile Gaea gave birth to Ourea (the mountains), Pontus (the sea) and Uranus (the heavens), with whom she had been secretly meeting in the edge place of Aether. They became a couple and gave birth to three Cyclopes, three Hecatoncheries and twelve Titans.
Uranus was displeased with his children and saw them as a threat to his throne. He decided to imprison Hecatoncheires and Cyclopes in Tartarus. Gaea became so mad that she started plotting against him and eventually convinced her children to join her plot. They came up with a plan to attack him with a flint sickle which Gaea made. However, all were too afraid except Cronus who decided to wield the sickle because he strongly desired the throne for himself. At night when Uranus laid to Gaea and embraced her with his coat, Iapetus, Crius, Coeus and Hyperion grabbed him and held him still while Cronus castrated him with the flint-bladed sickle and threw his genitals into the ocean. From his blood split on earth, the Giants, Ash-tree Nymphs and the Erinyes were born. According to Hesiod, Aphrodite was also born from the sea, where his genitals fell.
Reign of Titans
Cronus became the next ruler but he was no better than his father. He re-imprisoned the Cyclopes and the Hecatoncheires in Tartarus. In order to restore the throne he married his sister Rhea and together they ruled for ages. They had six children and because both Gaea and Uranus had prophesied that Cronus will be overthrown by a son, he decided to swallow each of his children at the same moment they were born. History repeated itself and Rhea was angry and started plotting against him. Upon arrival of her sixth child she hid herself, gave birth in secrecy and left the child to be raised by Gaea or variously by the Curetes. To hide her act, she wrapped a stone in a blanket and gave it to Cronus who swallowed it, thinking it was his child.
The child's name was Zeus. Once he had grown up, he consulted Gaea or, variously, Metis how to defeat his father. The goddess told him that should he stand a chance against him and his followers, he needs to make allies and, thereafter, prepared a drink designed for vomiting and gave it to him. Meanwhile, Rhea convinced Cronus to accept Zeus and made him return to Mount Othrys as a cupbearer. Once Zeus settled to his new profession, he used the special potion and forced Cronus into disgorging his brothers and sisters. Hestia, Hera, Demeter, Poseidon and Hades came back unharmed and, being thankful to their brother, named him their leader. Together they rebelled against Cronus.
Soon it all led to a ten-year war called Titanomachy. It was fought between the Titans and the Olympian gods. Cronus and other Titans except Epimetheus and Prometheus who sided with the Olympians, and Oceanus who declined to fight, fought to retain their power. Through Atlas as their leader it looked like they were going to win the war but Cronus made too many enemies during his ruleage. And Zeus was aware of this and therefore released the Hecatonchires from Tartarus. He also released the Cyclopes who later on, with the help of Hephaestus, forged three mighty items, Zeus's thunderbolt, Poseidon's trident, and Hades's helm of darkness. With extra numbers and strength they were able to defeat the Titans. In the final battle, the Hecatoncheires kept the Titans under constant pressure by throwing large rocks which, together with the other gods and their new weapons, especially with Zeus's thunderbolt, enabled them to finally overcome the Titans. Zeus then imprisoned them in Tartarus and made the Hecatoncheires as their guards and guards of Tartarus. It is also said that Poseidon built a gate of bronze and fixed it upon the entrance to provide extra barrier and prevent anyone ever escaping this horrible pit.
Reign of Olympians and Mankind
Finally the long war ended and the Olympians became rulers of the cosmos. The three brothers divided heavens, seas and underworld between them and Zeus decided to fill the earth with a new generation of mortal beings and give them the earth at their disposal. Although the brothers agreed that the gods will all be able to visit earth at any time and allowed to use their powers, Zeus ordered them not to interfere with men directly and let them write their own destiny. He ordered Prometheus and Epimetheus, as the only Titans left with old powers that he trusted, to create this new generation of men and animals. Epimetheus worked on animals as Prometheus was tasked to create men. Epimetheus was so dedicated that he completed his work while Prometheus was still in the process of going through ideas which treats to give to his clay figures. After his work Epimetheus informed his brother that he foolishly used all the treats and gifts on animals. Angry Prometheus, agitated by his brother for using all the gifts, decided to steal back the godly fire, previously banned by Zeus for being tricked by Prometheus, and gave it back mankind. When Zeus discovered his deeds, he ordered him to be chained to a mountain for eternity. However, still unsatisfied Zeus decided to punish mankind also, creating a woman of great beauty called Pandora. Each of the gods gave her some of their powers. Zeus himself gave her a box which he ordered never to be open. Zeus married Pandora to Epimetheus with whom she lived a happy life for a while, until her curiosity got the better of her and she decided to open the box.
Out of the box escaped all the horrors such as greed, envy, pain, illness which are present since then. There would be more but Epimetheus and Pandora managed to shut the box before these terrors could escape. Later that day they heard a voice coming from the box saying it was hope. They decided to take a risk and released it and gave mankind hope.