|“||And foes bear arms to the Doors of Death.||”|
The Doors of Death are the death god Thanatos' gateways in-and-out of the Underworld. During the events of The Heroes of Olympus, Gaea has opened them, allowing anyone she chooses to rise from the dead. And, others like Gwendolyn, come through too.
After Rachel Elizabeth Dare takes on the spirit of the Oracle of Delphi, she begins to recite 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' lifetimes.
While Piper McLean, Leo Valdez, and Jason Grace 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 who 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 able to help Gaea in her plans. This is proven true as the three eventually meet King Midas and his son, Lityerses; two people who had been dead for centuries. When the trio reaches Detroit on the way to Chicago, they are attacked by three cyclopes, Ma Gasket, Sump, and Torque. Leo manages to kill them, but notices they are quickly reforming due to Gaea's control over the Doors of Death.
While Percy is running from the two Gorgons, he claims he had killed them multiple times but they keep coming back to life after only a few hours, two at most. He later manages to kill them by drowning them in water that puts them off from coming back for further time, by scattering their essence. Later, during the Roman War Games, Gwendolyn is impaled by a spear right through the chest and dies shortly after. However, a few moments later she comes 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.
Later, Nico explains that while the Underworld has many exits and passages that allow those in the Underworld to escape (like Gwen), the Doors of Death are Thanatos' personal way in and out of the Underworld. However, Nico admits that even he doesn't understand the doors completely. It is around this time that Nico di Angelo 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 Jackson, Hazel Levesque, and Frank Zhang 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 set 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.
While visiting an island for supplies in Salt Lake City, Hazel Levesque and Leo Valdez find two souls that had escaped the Underworld using the Doors of Death. The two souls were Echo and Narcissus, but unlike other escapees they were not working for Gaea and Echo even helped Leo and Hazel escape the island.
Later, after rescuing Nico di Angelo, the group learns about where the two entrances to the Doors of Death are. One is in the city of Epirus in Greece in an underground temple called the House of Hades, while the other entrance is in the deepest parts of the Underworld, Tartarus. While Nico was searching for the entrance, he thought he could go to Tartarus because he was a son of Hades. However, the power of the pit pulled him in and he was captured by Gaea and her forces. Then when the time was right, Nico was transported through the doors to the world of the living, where Otis and Ephialtes used him as bait to lure the crew of the Argo II to them. Nico also informs them that Gaea's most powerful forces are on both sides of the doorway, so while Thanatos is able to return monsters to Tartarus, he is unable to stop them or close the doors.
After Annabeth Chase managed to defeat Arachne, the spider thrashed around, almost collapsing the room which would send them both to Tartarus. While most of the crew manage to escape after Arachne fell through the floor, a strand of spider thread began to pull Annabeth down toward the pit and Percy Jackson quickly ran over to catch her, resulting in being pulled into the pit himself. While they held onto a ledge, with Arachne trying to pull them with her, saying "I go to Tartarus, and you will come too." Percy called to Nico and told him to meet them at the Doors of Death, meaning that Percy and Annabeth would try and find their way from Tartarus to the Doors of Death. Nico understood, leading to Percy and Annabeth falling into Tartarus together.
Back on the Argo II, Nico explains that in order to close the doors, they must be in control on both sides, in Tartarus and in Epirus. While Leo plans on saving Percy and Annabeth, he knows that someone will have to be left behind to close the doors on the Tartarus side and wonders how Percy and Annabeth will survive long enough to make it to the doors. He says that if he has to make a grabber arm long enough to grab Percy and Annabeth out of Tartarus, he will.
- Nico di Angelo is the only mortal who has seen the Doors of Death from inside the Underworld.
- Echo, Narcissus and Hazel are the only known souls to escape the Underworld using the Doors of Death, but do not work for Gaea.
- While some souls that escape the Underworld using the Doors of Death appear to be more ghostly (like Medea and Otrera), while other seem to be completely indistinguishable from regular mortals (like Echo and Midas). It is unknown what causes this.