User blog comment:Menara/The Son of Sobek: The official PJ and Kane Chronicles Crossover/@comment-75.172.142.247-20130517193944/@comment-3146930-20130517210509

Thanatos is the god of death and prevents souls from escaping the Underworld. Nico explains that Thanatos is like border control, making sure no dead souls return to the land of the living. He also escorts dead souls to the Underworld if they have died. Yes souls do sometimes escape, but not very often. Thanatos' personal way in and out of the Underworld are the Doors of Death, which act like a super fast passage between worlds. Gaea has basically stolen the Doors of Death from Thanatos and is using them to allow souls and monster to quickly return to the Mortal World.