The Nine Hells
Tartarus exists as the warped remnants of an ancient celestial prison, ruled by its former long time prisoner, Prometheus. While many other prisoners of old had long since vanished, forgotten to time, Prometheus existed out of pure spite and a longing to seek retribution on those he felt had wronged him.
If a soul is sufficiently evil, regardless of its proclivity for worship or its devotion to a God, it may still end up in Tartarus by some sort of cosmic law placed into effect long ago, perhaps even before the war between the Archons and the Absolutes.