Tartarus is both a being and a place. Being the first celestial, the light inside the cosmos. Tartarus was torn apart by its own expansion, becoming the later celestials as component parts began to differ in their experience of existence.
Tartarus was opposed by the power known as Erebus, and it is this conflict that caused the cracks that would later fragment into independent awarenesses.
While the entities awareness was supplanted by the Celestials it became, Celestial Realm is still completely comprised of its 'flesh'.