User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-24138807-20130922230638/@comment-3146930-20130923222056

As seen in TLT and MoA, Tartarus has many entrances. Any soul in the Underworld could enter Tartarus if they wanted. I mean if the only reliable exit from the Underworld is the Doors of Death, then evil souls are most likely jumping into Tartarus in order to escape the Underworld faster.