Ascians

[Cilia]

Are the Twelfth X's all Lahabrea's, or are all the Swords and Staves? Or, indeed, only one of the two between Sword and Staff, with the other being a different Ascian lending his underling for a mission?

I haven't done the post-50 SMN stuff either, but...

I would theorize that the "Twelfth X" Ascians are all Lahabrea's underlings. This ties into the Zodiac theme the Ascians derive from the Ivalice mythos. Lahabrea, in terms of the Zodiac, represents Pisces, the twelfth Zodiac Sign, so all of his underlings would be "Twelfth X." By that same token, all of Nabriales' Black Masks would be "Eighth X" and Igeyorhm "Ninth X," but without anything to verify this it's just a theory...

EDIT
Beat to the punch...

On the other hand, the lesser Ascians seem to be named after the Minor Arcana. Not sure if anyone pointed this out yet.

So why isn't Lahabrea able to flee when Thordan attacks him?

Probably because we beat him to within an inch of his life beforehand? Ascians can teleport, but when Igeyorhm tries to do so she takes time to do it - suggesting that, when weakened, Ascians can't instantly teleport. While they do possess pseudo-immortality, the Ascians still have to act in real time; if they could instantly teleport at any time they would be completely invincible without a mortal vessel, and none of them could ever possibly be killed. Nabriales and Igeyorhm would just warp away when we throw the White Auracite at them, Lahabrea would just warp away when King Thordan is about to kill him, and we'd never be rid of the headache.

It's either that or for story purposes.

[CyrilLucifer]

I wouldn't necessarily say that we don't have anything to verify the theory that Xth represents the Ascian they serve. Chalice's dialogue has lots of watery puns, despite Lahabrea being fire-based in XIV. With higher servants not matching the master's element, but rather his "trigon", that absolutely puts 12th under Pisces, Cancer, or Scorpio as well for reasons I edited into my post up there. It doesn't completely cement it, but it does add to its defense.

Edit: And this is, of course, ignoring that they openly say they serve Lahabrea.

[Cilia]

I wouldn't necessarily say that we don't have anything to verify the theory that Xth represents the Ascian they serve. Chalice's dialogue has lots of watery puns, despite Lahabrea being fire-based. With the servants not matching the master's element, but rather his "trigon", that absolutely puts 12th under Pisces, Cancer, or Scorpio as well for reasons I edited into my post up there.

Since when has Lahabrea been fire-based?

In the Ivalice mythos, Mateus is the ice esper. (Water is a very related element.) During the battle with the Ascian Prime, it uses both Ice and Fire moves; you could say the Fire is from Igeyorhm (who has the poorly defined element of "soul" in Ivalice mythos), while the Ice fits right in with Lahabrea's connection to Mateus. Additionally, as you pointed out, Pisces is one of the three Water-elemental Zodiac Signs, so the Ascian of the Twelfth Chalice making water puns is to be expected. Additionally, given the Chalice is the Water-aligned suit of the Minor Arcana (which the lesser Ascians seem to take their names from), this only makes more sense.

[CyrilLucifer]

It seems to work differently in XIV than Ivalice. Gameplay-wise, Lahabrea was definitely, absolutely sticking to using Dark and Fire elemental magic in the Research Facility. Same with Igeyorhm using Dark and Ice and Nabriales with Dark and Earth in their battles. That was basically the entire point of the two working together was that their elements in-battle contrasted. That's why you attack Igeyorhm first: her freezing can be really, really annoying if you don't.

It was a really weird moment for me seeing him having that focus on fire because, logically based on Ivalice lore and astrology, he shouldn't have been. It also shows that we can't follow Ivalice completely, despite it being a backbone.