According to Greek mythology, the first two entities to come into existence were Uranus, the lord of the sky and the heavens, and Gaia, the personification of the Earth. These two mated and had many children together, including the Titans, the Hecatoncheires and the Cyclopes.
However, Uranus hated his offspring and, against Gail's wishes, imprisoned them all in the Pit of Tartarus as punishment for displeasing him.
Deeply hurt by this turn of events, Gaia constructed a scythe for her children, and begged them to castrate her partner in order to free her imprisoned young. One of the Titans, Cronus, the only one with the will to carry out his mothers wishes, ambushed Uranus and sliced off his genitals. From the blood which was spilt sprung forth the Giants, the Furies and the Nymphs, and the severed parts which were thrown into the ocean grew into the goddess Aphrodite.
Afterwards, the defeated Uranus prophesied that Cronus would in turn be overthrown by his own children, and this caused Cronus to become every bit the tyrant that his father had been.