Spoilers: Theory for Pandaemonium Story

[Kesey]

The new years poem referenced the underground, so I guess there is the possibility that this raid series will be the 14 version of hell which is accessed underground.

Also let's remember Lahabrea was in this cover art too:

I'm just guessing, but I think this raid will deal with the final plans set in motion by Lahabrea, his remaining creations, and the remaining Ascians. As evidence I think the current Eden series was probably masterminded by Lahabrea and 14's hell/Pandemonium will just where all his other schemes are resolved.

[LineageRazor]

But what else does "Pandaemonium/Pandemonium" mean? According to Merriam-Webster, pandemonium can mean "a wild uproar (as because of anger or excitement in a crowd of people)" or "a chaotic situation" [Link]. It is also a synonym for hell.

I like this theory! I had been focused on the idea of Pandaemonium being Hell, since that's pretty much what it was in FFII - but it had completely slipped my notice that Pandaemonium is also a SOUND. Given the game's ongoing references to audible things as things of importance (the Echo, the Resonance, the Sound), perhaps its this other definition we should be paying more attention to! Indeed, the Sound that triggered the End of Days in the time of the Ancients pretty much could be described as Pandaemonium all on its own.

They may live long if they don't suffer any serious injuries or diseases that may kill them but one still has to wonder where does the souls of those who die from the original world goes to if the cycle of rebirth was not established yet.

While we don't have a lot of details, we do know from Emet-Selch's Tale from the Shadows that even in the time of the Ancients there was an Underworld that souls went to on death. Whether that Underworld is the same as the Lifestream, was transformed into the Lifestream by Zodiark, or exists alongside it even now is unknown. It was apparently, though, the natural and expected destination for the deceased at the time of the Ancients.

Tying into the above - if the Sound originated from the Underworld, then it ties Hell, Pandaemonium, and the Sound all together pretty neatly. Of course, we still need to learn WHY the Underworld started producing this Sound, if that's indeed what happened...

[Hank_Hotspur]

It is similar to the lower part of the Lahabrea/Mateus symbol if it's flipped yes, but at the same time both are similar in profile to Zodiark from XII.

All the Ascians have inverted "face sigils" bearing their corresponding esper from FF12. So while it comparatively looks similar to Zodiark, it is more likely a reference to Lahabrea himself rather than Zodiark. I also doubt they are going to have a raid series about Zodiark after 6.0 is supposed to be wrapping all that up.

[Rannie]

Tying into the above - if the Sound originated from the Underworld, then it ties Hell, Pandaemonium, and the Sound all together pretty neatly. Of course, we still need to learn WHY the Underworld started producing this Sound, if that's indeed what happened...

Too many souls trapped in there without release or too many souls fighting against their own death as when the one soul forged itself to pheonix in that first Tales from the Shadow about Emet. Basically like a pressure cooker lid put on wrong and exploding?

[WhiteArchmage]

Too many souls trapped in there without release or too many souls fighting against their own death as when the one soul forged itself to pheonix in that first Tales from the Shadow about Emet. Basically like a pressure cooker lid put on wrong and exploding?

"Hell is empty and all devils are here!" From Shakespeare's The Tempest.

Note that Pandemonium etymologically means "All The Demons" (Granted, it could alternatively mean "ALL ANGELS" because the greek word δαιμόνιον [daimonion] means both "little devil" and "little angel" but has generally evolved to mean "demon"), and that's why it's usually a synonim for "Hell".

Pandemonium as the birthing place of...idk, corruption? (because "demons" in Hydaelin could mean voidsent, but with the revelation that voidsent and sin eaters are pretty much two sides of the same coin...) isn't that far-fetched.

[Shyron]

What if Lahabrea isn't controlling this demonic place, but is instead a prisoner? What if we have to rescue him for some other purpose? It might be rather difficult to suddenly turn Lahabrea into a credible menace after how low he fell in 2.0 and 3.0. So perhaps this isn't that. Maybe this is about turning him into a friend.

[Enkidoh]

What if Lahabrea isn't controlling this demonic place, but is instead a prisoner? What if we have to rescue him for some other purpose? It might be rather difficult to suddenly turn Lahabrea into a credible menace after how low he fell in 2.0 and 3.0. So perhaps this isn't that. Maybe this is about turning him into a friend.

I sure hope not! Mostly because I just found him insufferable throughout ARR and HW, but also because I've... kinda made up a headcanon of him back in Amaurot that he was actually nothing but a hack with a silver tongue that just used to steal other's ideas and passed them off as his own, especially Azem (or rather, 'my' Azem) - given Azem was not usually in Amaurot but out in the world doing what they still do thousands of years later (fighting monsters, saving innocents and well, being pretty much a big hero ), it's not hard to see how he was able to get away with it without blowback. Of course there is zero canon-proof that was ever a thing but I still like to think it is!

Yeah, I really, really dislike Lahabrea and I always felt cheated I didn't get to kill him myself back in the Singularity Reactor (thanks a lot Thordan ><), so I'm kind of looking forward to Pandemonium just for the hope I actually get to kill him now - I sincerely hope I do!

[Kesey]

I sure hope not! Mostly because I just found him insufferable throughout ARR and HW, but also because I've... kinda made up a headcanon of him back in Amaurot that he was actually nothing but a hack with a silver tongue that just used to steal other's ideas and passed them off as his own, especially Azem (or rather, 'my' Azem) - given Azem was not usually in Amaurot but out in the world doing what they still do thousands of years later (fighting monsters, saving innocents and well, being pretty much a big hero ), it's not hard to see how he was able to get away with it without blowback. Of course there is zero canon-proof that was ever a thing but I still like to think it is!

Yeah, I really, really dislike Lahabrea and I always felt cheated I didn't get to kill him myself back in the Singularity Reactor (thanks a lot Thordan ><), so I'm kind of looking forward to Pandemonium just for the hope I actually get to kill him now - I sincerely hope I do!

Although I want to agree with your sentiments, I really hope he is just dead and this raid is the mechanism he established before permanently dying as a plan B for Ascians if Zodiark is ever destroyed.

[Nezerius]

Mostly because I just found him insufferable throughout ARR and HW

But how much of that was due to bad writing? Ascians were a very one-dimensional type of antagonists during ARR/HW/SB, with the only exceptions being Elidibus (who mostly worked in the shadows), and Emet-Selch, close to the end of Stormblood.
If Lahabrea had been written with the same depth as Emet-Selch, I'd imagine he would've been a far less insufferable character. Especially when looking at some of the stuff that was brought up in Amaurot.

[Nightsword]

Pandaemonium is likely the place where Zodiark sealed the beasts and monsters created prior to his summoning with his reordering of the world.