Quest to the Doors of Death

The Quest to the Doors of Death was undertaken by the crew of the Argo II in Greece, while Percy Jackson, Annabeth Chase, and Iapetus journeyed across Tartarus so that the Doors could be simultaneously closed from both sides.

Prophecy
While the quest had no accompanying prophecy, it fulfilled a line of the Prophecy of Seven: And foes bear arms to the Doors of Death. It took the combined effort of both Greek and Roman demigods (who are usually sworn enemies) to close the mortal side of the Doors. A team consisting of demigods (Percy Jackson and Annabeth Chase), Titan (Iapetus), and giant (Damasen) worked together to close the Doors from Tartarus, a notable feat considering that the three sides normally oppose each other.

The Son of Neptune
While on their quest to free Thanatos, Frank Zhang, Percy Jackson, and Hazel Levesque learn that Gaea has taken control of the Doors of the Death, allowing any deceased soul or monster in her cause to walk the earth once more. Upon releasing Thanatos in Alaska, the god reveals that he is unable to close the Doors while they are under Gaea's power. He explains that 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.

The Mark of Athena
Following the Mark of Athena beneath Rome, Annabeth finds the stolen Athena Parthenos statue in Arachne's lair, located above a pit to Tartarus. She manages to trick the weaver into trapping herself in a woven prison, and when the Argo II arrives to procure the statue, Arachne falls into Tartarus. She drags Annabeth with her, but Percy follows them as he does not wish to lose Annabeth again. Before falling into Tartarus, Percy makes Nico promise to lead the Argo II to the House of Hades in Epirus, where the mortal side of the Doors of Death are located.