What are the "Seven Hells"?

I don't know if anyone has asked this but, do we have any substantial info on what are the Seven Hells, often mentioned almost as a curse word throughout the game? I'm a little bit out of touch with what was mentioned in the AST quest line and the Seven Heavens, also featured in the Myths of the Realm raid series, but I can't help but think that these two ideas are related. Still, I haven't seen any piece of relevant info about the Seven Hells...

My guess is that seeing the upcoming expansion will be about Norse mythology, are they going to incorporate them in a kind of Nine Realms raid or anything like that? I want to know moar