The Gigantomachy was probably the most important battle that happened in Greek mythology. It was a fight between the Giants or Gigantes, sons of Gaia and Ouranos, and the Olympian gods who were trying to overthrow the old religion and establish themselves as the new rulers of the cosmos.
According to the most detailed source for this battle, what started the war was the Giant Alcyoneus stealing the cattle of god Helios. A prophecy had it that the Giants would only be defeated if a mortal was to help the gods. To protect her children, Gaea tried to find a plant that would shield the Giants from any harm; however, Zeus stopped Eos (Dawn), Selene (Moon) and Helios (Sun) from shining, and took every single plant for himself.
The Giants with their thousand hands darted rocks and burning oaks at the sky, and it is said that the most fierce among them were Porphyrion and Alcyoneus. The fight was so terrible that Hephaestus is said to have fainted, being rescued by Helios, who took him up in his chariot. The gods needed help in this fight, and they knew of an oracle that had declared that none of the Giants could perish at their hands, but that with the help of a mortal they would defeat them. Gaia, who also knew of the oracle, sought for a medicinal herb to protect their offspring from being destroyed by a mortal, but Zeus forbade Eos, Selene, and Helios to shine, and collected the herb himself. Then he sent Athena to summon Heracles to his help.
It is also told that during the war, Triton blew his shell-trumpet (which he himself had invented) against the Giants, putting them to flight. And the asses that the Sirens and the Satyrs rode when they came to assist Zeus in the war were so terrified that they let out a braying such as the Giants had never heard, thus frightening them. This is how the Giants were defeated, and the constellation Altar (Ara) is a memorial of the victory of the gods in this war. It is also told that during the fight the Giants threw a dragon against Athena, who having snatched it, threw it to the stars, and fixed it in heaven as the constellation Serpens.
Having come into battle, Heracles shot Alcyoneus, but as the giant, having fallen on the ground, revived, Athena told Heracles to drag him outside Pallene. For he was immortal only for as long as he remained in the land of his birth. Later, when Porphyrion attacked Heracles, Zeus distracted the giant by inspiring him with lust for Hera, and when the goddess, with torn robes, cried for help, Zeus smote him with a thunderbolt, and then Heracles shot him dead with an arrow. But others have said that Apollo killed Porphyrion, and also Ephialtes, whom the god shot in his left eye and Heracles in his right. And Dionysus killed Eurytus with a thyrsus, and Hecate slew Clytius with torches, and Hephaestus killed Mimas with missiles of red-hot metal. Athena drove her chariot against Enceladus, and when he fled she threw on him the island of Sicily. And the giant Pallas she flayed, using his skin to protect her own body during the battle.
In the meantime, Poseidon pursued Polybotes across the sea, and when they came to Cos (one of the Sporades Islands, now Dodecanese) the god broke a piece of the island (the one called Nisyrus) and threw it on him. Yet others say that the giant lies beneath Cos. Hermes, wearing Hades' helmet, slew Hippolytus; and Artemis slew Gration; and the Moerae killed Agrius and Thoas with brazen clubs; and the rest, they say, Zeus destroyed with his thunderbolt, all of them being shot, as they were dying, by Heracles. The last of the Giants, some say, were buried by Heracles beneath Myconos, the small island to the east of Delos.