User blog comment:Menara/The Son of Sobek: The official PJ and Kane Chronicles Crossover/@comment-5885060-20130511082742/@comment-3146930-20130512171859

Also, Nico never mentions the Doors of Death in The Demigod Files. He mentions the Keys of Hades, which act as more of a way to send a person's soul to or from the Underworld. The Doors of Death are more of a passageway between the Mortal world and Underworld and are pretty much sealed up unless Thanatos is using them (or some primordial goddess traps Thanatos and forces the doors open, but what are the odds of that?)