Something strange about the house of atreus albums is that its actually a fanfiction/reimagined version of the story. David DeFeis said that he imagined it as a prequel to marriage of heaven and hell.
You might notice lyrical references to them, like in this song that got posted:
>>126728820 There are also melodies references to them.
"G Minor Invention (Descent into Death's Twilight Kingdom)" uses the main riff from Emalaith at parts.
"In Triumph or Tragedy" uses a lick thats used in a few parts from "Sword of the Gods"
Separate from that, some songs are actually completely reused from other projects (with minute changes):
The Fire God was originally written by defeis as a song for Piledriver, and sung by someone else.
"The Fires of Ecstacy" was originally a song called "The Exorcist". That song was for an extreme metal band called "Excorcist", on an concept album about witch trials called Nightmare Theatre. This band also had David singing, but it was in a style closer to Death Metal or Black Metal.
In an interview, he said he got the idea for the albums after a theatre director he worked asked him to write stuff for a rock opera. This was after Marriage and Invictus were completed, and the director had already composed a play inspired by them.
Given all that, I have to wonder how much the content on the album was originally intended to be songs about the house of atreus.
And it almost seems like the connection to the marriage albums was a retcon.