In Greek mythology, the primordial deities are the first generation of gods and goddesses. These deities represented the fundamental forces and physical foundations of the world and were generally not actively worshipped, as they, for the most part, were not given human characteristics; they were instead personifications of places or abstract concepts.
Hesiod, in his Theogony, considers the first beings (after Chaos) to be Erebus, Gaia, Tartarus, Eros and Nyx. Gaia and Uranus in turn gave birth to the Titans, and the Cyclopes. The Titans Cronus and Rhea then gave birth to the generation of the Olympians, Zeus, Poseidon, Hades, Hestia, Hera and Demeter, who overthrow the Titans, with the reign of Zeus marking the end of the period of warfare and usurpation among the gods.
Hesiod's primordial genealogy
Hesiod's Theogony, () which could be considered the "standard" creation myth of Greek mythology, tells the story of the genesis of the gods. After invoking the Muses (II.1–116), Hesiod says the world began with the spontaneous generation of four beings: first arose Chaos (Chasm); then came Gaia (the Earth), "the ever-sure foundation of all"; "dim" Tartarus (the Underworld), in the depths of the Earth; and Eros (Love) "fairest among the deathless gods". (Although in other myths, Eros was the name of Aphrodite's and Ares's son.)
From Chaos came Erebus (Darkness) and Nyx (Night). And Nyx "from union in love" with Erebus produced Aether (Light) and Hemera (Day). From Gaia came Uranus (Sky), the Ourea (Mountains), and Pontus (Sea).
Chaos
In Hesiod's creation myth, Chaos is the first being to ever exist. Chaos is both seen as a deity and a thing, with some sources seeing chaos as the gap between Heaven and Earth. In some accounts Chaos existed first alongside Eros and Nyx, while in others Chaos is the first and only thing in the universe. In some stories, Chaos is seen as existing beneath Tartarus. Chaos is the parent to Night and Darkness.
Gaia
Gaia was the second being to be formed, right after Chaos, in Hesiod's theogony, and parthenogenetically gave birth to Heaven, who would later become her husband and her equal, the Sea, and to the high Mountains.
Gaia is a mother earth figure and is seen as the mother of all the gods, while also being the seat on which they exist. Gaia is the Greek Equivalent to the Roman goddess, Tellus / Terra. The story of Uranus' castration at the hands of Cronus due to Gaia's involvement is seen as the explanation for why Heaven and Earth are separated. In Hesiod's story, Earth seeks revenge against Heaven for hiding her children the Cyclopes deep within her, Gaia then goes to her other children and asks for their help to get revenge against their cruel father; of her children, only Cronus, the youngest and "most dreadful" of them all agrees to do this. Gaia plans an ambush against Uranus where she hides Cronus and gives him the sickle to castrate him. From the blood Gaia again become pregnant with the Furies, the Giants, and the Melian nymphs. Cronus goes on to have six children with his sister, Rhea; who become the Olympians. Cronus is later overthrown by his son, Zeus, much in the same way he overthrew his father. Gaia is the mother to the twelve Titans; Oceanus, Coeus, Crius, Hyperion, Iapetus, Theia, Rhea, Themis, Mnemosyne, Phoebe, Tethys, and Cronus.
Later in the myth, after his succession, Cronus learns from his mother and father that his own son (Zeus) will overthrow him, as he did Uranus. To prevent this, Cronus swallowed all of his children with his sister Rhea as soon as they were born. Rhea sought out Gaia for help in hiding her youngest son, Zeus, and gave Cronus a rock instead to swallow. Zeus later went on to defeat his father and become the leader of the Olympians.
After Zeus's succession to the throne, Gaia bore another son with Tartarus, Typhon, a monster who would be the last to challenge Zeus's authority.
Sky and Earth have three sets of children: the Titans, the Cyclopes, and the Hecatoncheires.
Tartarus
Tartarus is described by Hesiod as both a primordial deity and also a great abyss where the Titans are imprisoned. Tartarus is seen as a prison, but is also where Day, Night, Sleep, and Death dwell, and also imagined as a great gorge that's a distinct part of the underworld. Hesiod tells that it took nine days for the Titans to fall to the bottom of Tartarus, describing how deep the abyss is. In some versions Tartarus is described as a "misty darkness" where Death, Styx, and Erebus reside.