Who were the beings that the Titans overthrew to gain power on Olympus? In the book of Encyclopedia of Mythology by Eric Flaum it discusses how Mother Earth conspired with Cronus to overthrow Uranus who was in power at the time. Who were these other beings before the Titans?
The Titans overthrew the primordials, the first beings that came into existence. More specifically, according to Hesiod the Titans, lead by Cronos, castrated Uranus (personification of the sky) at the behest of Gaia (personification of the earth):
(ll. 147-163) And again, three other sons were born of Earth and Heaven, great and doughty beyond telling, Cottus and Briareos and Gyes, presumptuous children. From their shoulders sprang an hundred arms, not to be approached, and each had fifty heads upon his shoulders on their strong limbs, and irresistible was the stubborn strength that was in their great forms. For of all the children that were born of Earth and Heaven, these were the most terrible, and they were hated by their own father from the first.
And he used to hide them all away in a secret place of Earth so soon as each was born, and would not suffer them to come up into the light: and Heaven rejoiced in his evil doing. But vast Earth groaned within, being straitened, and she made the element of grey flint and shaped a great sickle, and told her plan to her dear sons. And she spoke, cheering them, while she was vexed in her dear heart:
(ll. 164-166) 'My children, gotten of a sinful father, if you will obey me, we should punish the vile outrage of your father; for he first thought of doing shameful things.'
(ll. 167-169) So she said; but fear seized them all, and none of them uttered a word. But great Cronos the wily took courage and answered his dear mother:
(ll. 170-172) `Mother, I will undertake to do this deed, for I reverence not our father of evil name, for he first thought of doing shameful things.'
(ll. 173-175) So he said: and vast Earth rejoiced greatly in spirit, and set and hid him in an ambush, and put in his hands a jagged sickle, and revealed to him the whole plot.
(ll. 176-206) And Heaven came, bringing on night and longing for love, and he lay about Earth spreading himself full upon her (7).
Then the son from his ambush stretched forth his left hand and in his right took the great long sickle with jagged teeth, and swiftly lopped off his own father's members and cast them away to fall behind him. And not vainly did they fall from his hand; for all the bloody drops that gushed forth Earth received, and as the seasons moved round she bare the strong Erinyes and the great Giants with gleaming armour, holding long spears in their hands and the Nymphs whom they call Meliae (8) all over the boundless earth. And so soon as he had cut off the members with flint and cast them from the land into the surging sea, they were swept away over the main a long time: and a white foam spread around them from the immortal flesh, and in it there grew a maiden. First she drew near holy Cythera, and from there, afterwards, she came to sea-girt Cyprus, and came forth an awful and lovely goddess, and grass grew up about her beneath her shapely feet. Her gods and men call Aphrodite, and the foam-born goddess and rich-crowned Cytherea, because she grew amid the foam, and Cytherea because she reached Cythera, and Cyprogenes because she was born in billowy Cyprus, and Philommedes (9) because sprang from the members. And with her went Eros, and comely Desire followed her at her birth at the first and as she went into the assembly of the gods. This honour she has from the beginning, and this is the portion allotted to her amongst men and undying gods, -- the whisperings of maidens and smiles and deceits with sweet delight and love and graciousness.
(ll. 207-210) But these sons whom be begot himself great Heaven used to call Titans (Strainers) in reproach, for he said that they strained and did presumptuously a fearful deed, and that vengeance for it would come afterwards.