User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-4902593-20121022064940/@comment-3146930-20121022103104

In order for the Doors of Death to be closed, they must be closed at both ends. Meaning someone has to close the doors in Tartarus and someone has to close them in the Mortal world. Only closing the doors on one side of the passage is impossible. As far as dying in Tartarus, it is possible that if Percy died in Tartarus, he would return to life like Gwen did. But it is also possible that his body would be killed in Tartarus, but his soul would go to the River Styx to be carried across by Charon. Lastly, Nico is the first demigod to survive Tartarus