User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-6068707-20130708185831/@comment-3146930-20130709005857

Exactly. The Underworld is mostly a place for mortal souls to go when they die. Tartarus on the other hand is below the Underworld. It is the deepest most evil part of the Underworld that is actually designed to be a prison. Another way you can look at it is that the Underworld is made up of Erebus, the black walls of the Underworld. Tartarus is Tartarus, a separate entity that is part of the Underworld, but separate from Erebus.