So, The House of Hades is an underground temple in Epirus, Greece. That's where the Doors of Death are, in the mortal world, right?
Anyway, I have done some pointless research. So here you go:

I found out that the temple is called The Necromanteion, which means Oracle of Death. It's a temple of necromancy devoted to Hades and Persephone.
The site was believed to be the Doors to Hades, which lead to the realm of the dead (kinda like the Doors of Death).
People used to come here to call upon their dead ancestors and talk to them. O.o
According to tradition, the temple was located on the banks of the River Acheron.
The site is also at the meeting point of the Acheron, Pyriphlegethon and Cocytus rivers.

Let's learn about Acheron! :D
Back when Greece was the centre of power, the newly-dead would be ferried across this river by Charon, in order to enter the Underworld. The souls that were neutral, sat on its banks.
The River Acheron is one of the five rivers of the Underworld and was known as the river of pain.
According to some scriptures, it was a place of healing, where souls were cleansed of sin.
Before Acheron became a river, he was the son of Helios and Demeter, or Helios and Gaia. During a contest between the Titans and Zeus, Acheron refreshed the Titans with drinks, so he got turned into a river of the Underworld. Fun.
Acheron's son, Ascalaphus, was the orchardist of Hades. He told the other gods that Persephone had eaten a pomegranate in the Underworld. He was punished by being changed into a screech owl, becoming the familiar bird of Hades.

What about the River Pyriphlegethon?
I'm gonna call it Phlegethon from now on. -.-'
Phlegethon is a river of fire or boiling blood, which coils around the Earth and flows into the depths of Tartarus.
It is parallel to the River Styx.
It is said that the goddess Styx fell in love with Phlegethon, but she was consumed by his flames and sent to the Underworld. Eventually, when Hades allowed her river to flow through the Underworld, Styx was reunited with Phlegethon.
(I'm not sure how true that is, since she was married to Pallas and bore him children, but stranger things have happened).
The people who committed crimes against their fellow men are punished in this river, along with tyrants and murderers. Their punishment is to be sunk in the flowing, boiling blood because they caused hot blood to flow when they committed violent deeds.
The river is guarded by centaurs, who force souls to remain at their level.

Anyway, back to the Necromanteion.
In Homer's Odyssey, the House of Hades was also described as the entrance by which Odysseus made his nekyia (a rite by which ghosts were called upon and questioned about the future).

When people wanted to speak to the dead here, they ate a meal of broad beans, barley bread, oysters and a narcotic compound (like heroin). D:
Then they participated in a cleansing ceremony and sacrificed sheep.
After this, they would descend through a series of underground corridors, leaving offerings as they passed through a number of iron gates.
The priest/death oracle would then pose a series of questions and chant prayers. The people would then see him rise from the floor and begin to fly around the temple, by the use of theatrical cranes. Weirdos.
I have no idea what happened next. He would probably come back down and tell them he has been possessed by the spirit they were calling and the spirit would answer their questions via him.

The site was burned down by the Romans, but its remains can still be seen.

Thank you for reading this article of pointless information. If you would like to make a theory article for HoH based on this article's information, go for it, because I can't be bothered. xD

All information is from various Wikipedia articles and has been collected and put together by me, Phoenix. xP

NOW SPECULATE BELOW. ^(^O^)^

(This all might not have anything to do with the next book. I just thought I'd give you all an Ancient History lesson). xP