User blog comment:SayuriDarling/The Mark of Athena Official Discussion/@comment-186.82.70.150-20121010002729/@comment-3146930-20121014214254

There are exceptions to the "permission" rule. For example, Hermes is the messenger god and is one of the few gods able to go in and out of the Underworld at will. As far as the original comment, I'm not sure what mythology you were reading, but I've never heard of the Doors of Death outside the series. Maybe the owl can travel in and out of the Underworld, but not even the gods go to Tartarus.