Doors of Death

The Doors of Death are Thanato's gateways out of and into the Underworld. During the events of The Heroes of Olympus, Gaea appears to be opening them, allowing anyone she chooses, to rise from the dead.

The Last Olympian
After Rachel Elizabeth Dare takes on the spirit of the Oracle of Delphi, she begins to say the next Great Prophecy. The last line of the prophecy says that foes will bear arms at the Doors of Death. No one knows what this means, but Apollo says that the prophecy may not come true during the demigods lifetime.

The Lost Hero
While Piper, Leo, and Jason are traveling on their quest to save Hera, they follow a sewer line into a shopping mall. A sales woman there begins to charmspeak Jason and Leo into fighting each other, but Piper figures out that the woman is Medea; a person that died during the time of the original Jason. Though Medea doesn't mention Gaea by name, she says that the dead are rising because the Doors of Death are opening to those with special abilities that could help Gaea in her plans. This is proven true as the three eventually meet King Midas and his son, Lit; two others that had been long since dead.

The Son of Neptune
While Percy is running from the two Gorgons, he claims he has killed them multiple time but they keep coming back to life after only a few hours. He later manages to kill them by drowning them in water. Later, during Camp Jupiter's War Game, Gwendolyn is impaled by a spear right through the chest and dies shortly after. However, a few moments later she springs back to life and explains that she was standing at the entrance to the Underworld, but there was an open door that she exited through, allowing herself to return to the land of the living. Mars then appears on the field and explains that the god Thanatos has been captured and eventually, even humans would start to return to life. It is around this time that Nico leaves to go back to the Underworld to see if he can find the doors from the other side, but was captured by Gaea.

As Percy, Hazel, and Frank continue on their quest to find Thanatos, they learn that Gaea fully controls these doors, anyone working for her, or any spirits/souls that finds these doors, can come back to life at an accelerated rate (sometimes instantly). This makes it hard for demigods as their enemies won't stay dead because Thanatos is unable to stop the souls from escaping as long as he is chained. After Frank manages to sent Thanatos free, he stops the spirits that Percy is fighting from returning.

Near the end, when Thanatos explains that he does not have the power to close the doors as he is no match for Gaea, but he can make it so that it takes longer for monsters to reform and possibly prevent souls in the Underworld from returning. He also explains that in order to close these doors, the demigods will have to go on a quest to the original Rome, and one of them must be able to read signs that lead them there.