House of Hades

The House of Hades is an underground temple in Epirus, Greece, dedicated to Hades and Persephone. The other side of the portal is entered through Tartarus (The Doors of Death).

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 the will meet all of them on the other side with Annabeth. Now the six demigods aboard the Argo II must fly to Epirus to find The House of Hades to save Annabeth and Percy.