User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-198.228.228.30-20130210195334/@comment-3146930-20130211025231

Thanatos brings souls to the Underworld and makes sure they don't escape. However, even the most evil souls only go to the Fields of Punishment, meaning Thanatos doesn't go to Tartarus to complete his job. As Nico said, he is only the border patrol.