The underworld abyss personified as a primordial deity is Tartarus.
In Greek mythology, Tartarus is both a cosmic place and a divine personification. Hesiod’s Theogony, composed in the late eighth or early seventh century BCE, names Tartarus as one of the earliest beings, following Chaos and Gaia and preceding Eros. In this genealogical role, Tartarus fathers the monstrous Typhon with Gaia.
As a location, Tartarus is an abyss far beneath the earth. It serves as a prison for defeated Titans and, in later traditions, as a place of punishment for particularly wicked souls. Hesiod gives it immense depth, imagining that a bronze anvil would fall for nine days from heaven to earth and another nine days from earth to Tartarus.
Tartarus is often confused with Hades. Hades can mean both the god who rules the dead and the broader realm of the dead, whereas Tartarus is a deeper, darker region associated with confinement and torment. Nyx and Erebus are primordial deities linked to night and darkness, but neither is the personified abyss itself.