The Greek primordial deity who personified the deep abyss of the underworld was Tartarus. In Hesiod’s Theogony, Tartarus appeared among the earliest cosmic beings, alongside Chaos, Gaia, and Eros. The name referred both to a divine personification and to the vast, gloomy region located beneath the underworld.
Tartarus was portrayed as unimaginably deep. Hesiod says that an object falling from heaven would take nine days to reach Earth, and another nine days to reach Tartarus, emphasizing its distance below the mortal world. It later became the prison of defeated Titans such as Cronus and his allies, guarded in some traditions by powerful divine forces.
Tartarus is not simply another name for Hades. Hades was both the god ruling the realm of the dead and, by extension, the underworld itself. Tartarus was a deeper and more dreadful section, associated with confinement and punishment. Chaos was the primordial chasm or first cosmic condition, Nyx was Night, and Erebus was primordial Darkness; all are related to the early cosmos but do not match the question’s specific abyss.