Each legion is headed by a general, or Legatus; they're modeled after the legions of the Roman Empire. Gaius led the XIVth Imperial Legion, again, because Final Fantasy XIV
Questions about the Garlean Empire
The reason it seems off to you just stems from a misconception about how the legions are built and numbered (as far as we know(?).
You seem to be interested in the big picture, though, so let's go over it! (I'll even include the relevant info you missed from 1.0).
The information we have so far suggests that there are currently fourteen Garlean Imperial Legions (because Final Fantasy XIV, the number just comes up a lot in this game, as do seven and twelve because they come up in real-world mythology a lot, too). Some official descriptions say the legions are "countless," so either SE is leaving the door open for mentions of legions that tie thematically into later games, or that's just an erroneous "Eorzean" assumption. As far as we know, all of the current legions existed even before the capture of Ala Mhigo, formed during the wars to unify Ilsabard and conquer Othard.
Each legion is headed by a general, or Legatus; they're modeled after the legions of the Roman Empire. Gaius led the XIVth Imperial Legion, again, because Final Fantasy XIV; he was the first emblematic antagonist of this game. Similarly, Nael van Darnus, Legatus of the VIIth Legion, shared some thematic similarities with Sephiroth of Final Fantasy VII. Furthermore, we just met Legatus Regula van Hydrus of the VIth Legion, because the whole Flying Continent / Warring Triad thing heavily ties into the themes explored in Final Fantasy VI.
In Version 1.0, we didn't know any of this, at first. It was just "The Empire is coming," and then we bumped into Gaius in Mor Dhona and only really got his name. The distinction between legions, if I remember correctly, didn't come up until Nael van Darnus showed up.
Gaius was famous for his successes - he'd slain so many people who conspired to take the crown from Solus zos Galvus (nine) that his blade was named Heirsbane. He was responsible for four cities falling to Imperial rule before he was tasked with commencing the wars in the Western Theatre by securing Ala Mhigo as a Garlean foothold in Eorzea. As far as we know, the XIVth spearheaded this invasion while the other legions were deployed elsewhere (the VIIth was in the Eastern Theatre, Othard, at the time). We don't know of legion numbers changing or leaders shifting around.
However, once he'd accomplished this, the homeland supplemented his forces and the Armada sailed on Silvertear Lake to secure the aetherial center of the realm in hopes of preventing primals from being summoned. The whole thing failed - Midgardsormr took out the flagship, dragons routed the Armada, and by the time damage control was performed, primals had begun to appear in Eorzea. Gaius was ordered to withdraw to rule and fortify Ala Mhigo until a solution could be found.
Nael van Darnus promised this answer, so the the Emperor ordered Gaius and the XIVth to support the VIIth, who led the 1572 invasion. Gaius wasn't very happy about it; not only did his pride take a beating, but he was the first to see that something was ... off ... about his new superior. The Emperor no longer trusted him to complete his tasks and didn't heed his advice. Gaius tried very hard to persuade Nael to abandon the Meteor project, which it was believed would annihilate Eorzea with a massive spacerock, because he thought it was profane - beneath them. He didn't like the idea of the Three Great Continents being united under Garlean rule by raising a flag over a barren land of ash and calling it victory. By the time he realized that Nael was misleading them all, it was too late to do anything about it within the Empire itself.
Instead, Gaius committed well-camouflaged acts treason. He shot down one of Nael's airships (which contained information relative to the Castrum being built in Mor Dhona as well as the equipment housed there which was being used to bring Dalamud down) and disguised it as an Eorzean Alliance success. Gaius' subtle sabotage of Nael's operation led to Darnus being slain. After the remnants of the VIIth Legion, who thought Nael truly was close to godliness by he end, entrenched themselves in Mor Dhona (to prevent Louisoix's hail-mary plan to stop the moon's fall), Gaius took what remained of the XIVth and withdrew to Ala Mhigo to left everyone to their fates. He even bid farewell to the adventurer who would one day be known as "The Warrior of Light" and wished them luck in the Battle of Cartenau, saying to "leave him something worth conquering."
Monks had superior Burst AoE even in Final Fantasy XIV ARR
Actually, Monks had superior Burst AoE even in Final Fantasy XIV ARR (thanks to PB), although their overall AoE was definitely inferior outside of the burst phase.
I try to take weapons and playstyle into account
Auron - Dark Knight: one handed sword, heavy attacks while also a good pool of HP. Hes also the first one to learn some "cover"-spell, which would make him a paladin... but... naaaah... he doesnt seem like one so I identify this as a tank-ability
Final Fantasy XIV:Could a seasoned Dark Knight help this novice out?
However, beyond that, starting out, I think you should make sure to use Fending armor and accessories until you are truly confident in your abilities as a Dark Knight.
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Final Fantasy XIV Leveling 48 Astrologian and need a bit of advice
As a disclaimer, I am only a level 48 Astrologian, and I do not have end-game healing experience, so I cannot say exactly how to prioritize everything.
I think game parser should be added to FFXIV
Let me start off by saying that all adding an official parser would do is put console players on more equal footing with PC players, most of whom are already parsing.