In Greek mythology, the Giants were the children of Gaia or Gaea, who was fertilized by the blood of Uranus, after Uranus was castrated by his son Cronus.
Cronus secured his power by re-imprisoning or refusing to free his siblings, the Hecatonchires and Cyclopes, and his (newly-created) siblings, the Giants, in Tartarus. Afterwards, Cronus and his Titans lost the battle to his son Zeus.
Gaea, incensed by the imprisonment of the Titans in Tartarus by the Olympians, incited the Giants to rise up in arms against them, end their reign, and restore the Titans' rule. Led on by Alcyoneus and Porphyrion, they tested the strength of the Olympians in what is known as the Gigantomachia or Gigantomachy. The Giants Otus and Ephialtes hoped to reach the top of Mount Olympus by stacking the mountain ranges of Thessaly, Pelion, and Ossa, on top of each other.
The Olympians called upon the aid of Heracles after a prophecy warned them that he was required to defeat the Giants. Athena, instructed by Zeus, sought out Heracles and requested his aid in the battle. Heracles responded to Athena's request by shooting an arrow dipped in the poisonous blood of the dreaded Hydra at Alcyoneus, which made the Giant fall to the earth. However, the Giant was immortal so long as he remained in Pallene. Athena advised Heracles to drag Alcyoneus outside Pallene to make the Giant susceptible to death. Once outside Pallene, he was beaten to death by Heracles. Heracles slew not only Alcyoneus, but dealt the death blow to the Giants who had been wounded by the Olympians. The Giants who died by the hero's hands were Alcyoneus, Damysos, Ephialtes, Leon, Peloreus, Porphyrion and Theodamas, giving Heracles the most kills of the Gigantomachy.
The Olympians fought the Giants with the Moirae aiding them before the aforementioned prophecy was made, meaning the Giants would have overcome the combined efforts of both Olympus and the Sisters of Fate had Heracles not fought.
"Power is latent violence, which must have been manifested at least in some mythological once-upon-a-time. Superiority is guaranteed only by defeated inferiors," Walter Burkert remarked of the Gigantomachy.
This battle parallels the Titanomachy, a fierce struggle between the upstart Olympians and their older predecessors, the Titans (who lost the battle). In the Gigantomachy, however, the Olympians were already in power when the Giants rose to challenge them. With the aid of their powerful weapons, the Moirae and Heracles, the Olympians defeated the Giants and quelled the rebellion, confirming their reign over the earth, sea, and heaven, and confining the Giants into Tartarus. The only Giant not slain in the conflict was Aristaios, who was turned into a dung beetle by Gaea so the Giant might be safe from the wrath of the Olympains. 
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