Tartarus was the deepest realm of the Underworld as well as the third of the Primordial deities. Some sources describe Tartarus as “the unbounded first-existing entity from which the Light and the cosmos are born” (thank you wikipedia).
It would take nine days to fall to Tartarus from the earth (which was a nine day fall from heaven). Tartarus is separated from the rest of the Underworld by a river of fire called the Phlegethon ( tangent: Styx and Phlegethon were in love but were eternally separated. In the Underworld one flows to the other so they can always be together).
Tartarus had many notable residents, most of whom I’ve discussed in other mythology Mondays. The cyclopes, the giants, and the Titans of course all reside within the depths of Tartarus. But a few humans made their way there as well. Sisyphus, Tantalus, Ixion, and Salmoneus. Most of the humans that ended up in Tartarus did something to upset Zeus.
Tartarus was much more active as a place than a deity, but Rick Riordan described Tartarus as a place as well as a deity best in House of Hades. The primordial deities are difficult to visualize, but his depiction felt spot on.