House of Hades

The House of Hades is an underground temple in Epirus, Greece, dedicated to the god Hades and his wife Persephone. The mortal side of the Doors of Death is located here, while the other side of the portal is entered through Tartarus in the Underworld.

The Mark of Athena
Nico di Angelo explains that the other side of the Doors of Death are in Tartarus. Once Annabeth comes back from getting the Athena Parthenos, she is dragged in by a spider string leftover from her recent fight with Arachne. Percy attempts to save Annabeth, but he cannot pull them both up. Instead he says he won't leave her and tells Nico that they will meet all of them on the other side of the doorway. With Percy and Annabeth trapped in Tartarus, the remaining demigods plan on flying to the House of Hades aboard the Argo II and defeat the forces of Gaea to save Annabeth and Percy.