The Heart of Sabik

[Cleretic]

I'd also point out that we actually know very little about the seven heavens so far (and even less about the seven hells, I'm not even sure we have proof they exist), so I feel like we can't completely rule out that they might've existed before the Sundering and the Twelve just moved in afterwards. Or maybe they were a theoretical/mythical thing that the Twelve took inspiration from, which would explain it being a cultural touchstone used by the Ancients while also not existing in the Ancients' time.

Both Lahabrea and Athena using it as a reference for us rather than them, as Anonymoose said, also makes sense. Lahabrea's watched the development of all this so he'd be familiar, after all. For Athena it's a bit of a bigger ask since she'd have had to actively learn it somewhere (and has a much smaller window to do so), but Athena seems like the type of person to have put extra effort into learning how to properly blaspheme against someone else's religion.

[Kesey]

The line "seven hells" is an allusion to Dante's Inferno. In it, Hell has 7 levels, each corresponding to the seven deadly sins. Then the saying "seven hells" irl is a colloquial term used to reference Dante's Inferno. So Athen saying it is just her cursing, most likely out of frustration with the situation fighting you has presented.

That being said, the 4 elemental fiends are also demons in Inferno.

And Kokytos boss of 9th circle also comes from Inferno, but is written as Cocytus which is the frozen lake found below the 7 levels of Hell where the devil and nonbelievers are frozen in despair. Cocytus means "lament". And if we do a little math, knowing that purgatory comes before you enter hell, plus the 7 levels, and then the frozen lake would also technically be on the 9th floor or circle if you will.