First came the twelve Titans, next came the three one-eyed Cyclopes:

Then [Gaia] bore the Cyclopes, who have very violent hearts, Brontes (Thunder) and Steropes (Lightning) and strong-spirited Arges (Bright), those who gave thunder to Zeus and fashioned the thunderbolt. These were like the gods in other regards, but only one eye was set in the middle of their foreheads; and they were called Cyclopes (Circle-eyed) by name, since a single circle-shaped eye was set in their foreheads. Strength and force and contrivances were in their works. Following the Cyclopes, Gaia next gave birth to three more monstrous brothers, the Hecatoncheires, or Hundred-Handed Giants. Uranus hated his monstrous children, and as soon as each was born, he imprisoned them underground, somewhere deep inside Gaia. Eventually Uranus son, the Titan Cronus, castrated Uranus, becoming the new ruler of the cosmos, but he did not release his brothers, the Cyclopes and the Hecatoncheires, from their imprisonment in Tartarus.For this failing, Gaia foretold that Cronus would eventually be overthrown by one of his children, as he had overthrown his own father. To prevent this, as each of his children were born, Cronus swallowed them whole; as gods they were not killed, but imprisoned within his belly. His wife, Rhea, sought her mothers advice to avoid losing all of her children in this way, and Gaia advised her to give Cronus a stone wrapped in swaddling clothes. In this way, Zeus was spared the fate of his elder siblings, and was hidden away by his mother.
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