User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-67.42.242.115-20130701044352/@comment-3146930-20130707145253

Nico states in The Mark of Athena that he thought he could go anywhere in the Underworld because he was a child of Hades (much like you said). However, even if Tartarus is in the Underworld, that doesn't mean Hades/Pluto has power over it. Tartarus is a living being too and much more ancient then Hades. It is a black hole that drags in anything that even comes close to it and never lets it go. Nico even says that not even the gods go down there. Thanatos doesn't go down to Tartarus, and he is the god of death.