Doors of Death

The Doors of Death are gateways out of the Underworld. During the events of The Heroes of Olympus, Gaea appears to be opening them allowing the dead to rise.

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 claims 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 the book hasn't been released, Rick Riordan mentions the Doors of Death during a preview video for The Son of Neptune.