I feel this may be incorrect. From what I could gather, we don't know exactly when Bahamut was sealed away. We only know that he was sealed away by the ancient Allag. If there is solid confirmation that can be provided, please point me towards it as I can't seem to find anything confirming that.
As a point of reference, if the release of Bahamut from Dalamud was the tipping point that threw us into the Seventh Umbral Era, wouldn't you think his first appearance was enough to throw the realm into an Umbral Era as well? That being said, we know that the Allagans prospered and flourished throughout the Third Astral Era. If Bahamut were to appear, the realm would have likely fell into an Umbral Era, bringing about the Fourth Umbral Era which we know would be around the time the Allagan Empire fell. Of course, until we can get confirmation as to when, this is all subjective. However, it doesn't make sense to me to have a Elder Primal flying around the skies wrecking havoc on the realm forcing the Allagans to seal him away and then the people still walking around like it was another day in an Astral Era.
There are actually several sources which tell us how the 5th Astral Era ended and how the 6th Umbral Era came to rise.
In the quest, Of Archons and Muses, Tyago has this to say.
The great flood she speaks of is known as the Great Deluge, but what caused the Great Deluge?Tyago Moui: According to the loremasters' books, there was an age of shadow and calamity some fifteen hundred-odd years ago, when a great flood drowned the world─the sixth Umbral Era, they called it. Believers say it was the twelve Archons what came to fight the darkness then.
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Tyago Moui: They say Ahldbhar was the mightiest of the twelve Archons. Rhalgr the Destroyer himself, taken form in Roegadyn flesh─a bloody mountain of a man, and a fearsome warrior, besides. Not unlike our friend at the Coliseum, eh?
Tyago Moui: You must've seen renderings of Rhalgr at some point─in paint or stone. Looks like a bag of angry muscles in a mage's robes, right? Aye, well, those are done in the likeness of Archon Ahldbhar.
Tyago Moui: Ahldbhar fought relentlessly against a clan of wicked sorcerers that sought only to wreak havoc on the world through their dark powers. But then, something happened... Something that threw Ahldbhar into deep despair. And in the end, he turned and took up with the enemy, though none can say as to why.
Tyago Moui: What came of him after his betrayal? Well, I confess I haven't heard the end of the tale myself. That bard's a wily bugger, see, and he only ever gets part way through before his throat dries out. Meaning I have to give him yet another bloody ale─free of charge, of course.
In the quest, The Price of Integrity, Walcher has this to say.
Y'shtola has this to say further along in the quest line.Walcher: Perhaps you, too, have heard the legend of the Sixth Umbral Era, and how the great deluge that ushered in the dark age was in fact conjured forth by a band of immortal mages who forged a blood pact with the demon lords of the seven hells to deliver a thousand thousand souls in exchange for sole dominion over Eorzea.
Walcher: I believe that the false prophet Urianger is one of those mages, awoken from fifteen centuries of slumber to complete the fell pact left unfinished so long ago.
Thancred goes on to tell us a bit more in the quest, Career Opportunities.Y'shtola: Good. Now, perhaps you would begin by telling me what it is that brought you here. Might it be that you have an interest in the Archon's prophecies? Curiosity is known to have killed the coeurl, yet it is the thirst for knowledge that drives Man to the truth. You have taken a great risk this day─but I would expect naught else from an adventurer such as yourself.
Y'shtola: Or could it be that you are merely afraid? Afraid of the surging darkness that nears our borders, even as we speak? You have no doubt heard the whisperings of the Sixth Umbral Era and the cabal of mages at its center.
Y'shtola: Yes, there are those who claim it was the mages and their unholy pact with the demon lords of the seven hells that triggered the cataclysm, but there are others who believe it was the mages who led the realm out of that dark age.
Y'shtola: Who now can say which is true? No man alive. Yet one thing is certain─the rise of a Seventh Umbral Era is inevitable.
Lastly, in the White Mage quest line, The Wheel of Disaster, Raya-O-Senna tells us this.Thancred: Ne'er till land consumes sun can sea bear moons,Heavens spew crimson flame, hells seep black dooms.
Thancred: You seem shocked. I am a bard, I'll have you know, and not utterly devoid of talent. The verses of Mezaya should pose no significant problem.
Thancred: I confess, I am glad to have journeyed so far in search of my inspiration and muse. For the noble display you have shown me this day, I shall repay you with a kindness. I shall tell you of the prophet.
Thancred: More than a thousand years past, a great calamity was visited upon this world. Across the sky, a terrible blackness spread till no light could be seen, and then descended, seeping into the very hearts of men. Thus was he sorely afflicted, and his ways perverted. Chaos was in all places all at once, and myriad creation was destroyed.
Thancred: Avarice reigned supreme, and civilization was thrown into a lawless struggle for wealth and power─the mighty taking as they saw fit with complete disregard for nature and fellow man alike. Evil begat evil and ripened unchecked, taking on the sickly sweetness of rot, near to bursting and rank with the promise of decay.
Thancred: It was then that the Twelve, grieved by the corruption and sin that had gripped their creation, decided to purge the world of its evils with a cleansing deluge, proclaiming: “Behold the power of pure water!” And thus began the sixth of the Umbral Eras.
Thancred: There is a moral in this story for those wise enough to find it. You see, it was we mortals who brought the great flood upon ourselves and our world, having left the gods no recourse. And so if our faults ushered in an Umbral Era in the past, then our strengths can prevent the coming of another in the future.
Thancred: So what say you, <Name>? Will you pledge yourself to the side of good and fight to save the world? Know that you cannot rely blindly on others in things such as this. The road to victory must be found within yourself─as it must in all of us.
Thancred: Gods, speaking to you has given me the most sublime idea for a poem! You are my muse, <Name>! Truly, you are!
Thancred: But I shall take up no more of your time─nor you of mine. Until our paths cross again, be good and be well.
With that, here's my interpretation and what I believe brought about the Sixth Umbral Era.Raya-O-Senna: I take from your silence that you are not. No matter, it shall be my pleasure to educate you. The Fifth Astral Era is said to have begun approximately three millennia ago.
Raya-O-Senna: The ice age that ushered in the Fifth Umbral Era made the land a barren and merciless place, and man was pushed to the limits of his resourcefulness in the struggle to survive. Yet survive he did, through the discovery of magic as we know it─an event which marked the dawning of the Fifth Astral Era.
Raya-O-Senna: At first, man was well pleased just to have the means to keep the cold at bay and compete with the other races. But man is nothing if not an ambitious beast. It was not long before he began to seek mightier magicks, hoping to win greater glory.
Raya-O-Senna: It was this desire that brought forth black magic, the arcane art of destruction, into the world. In order that this force of chaos be kept in check and balance preserved, at roughly the same point in history, white magic, the arcane art of succor, came into existence.
Raya-O-Senna: Emboldened by magic, man went on to reach the zenith of glory. But his hunger knew no bounds. Over time, even they who donned the white began perverting their powers for the sake of self-gain, and in this single-minded pursuit scrupled not to sully the sanctity of the Twelveswood.
Raya-O-Senna: In his pride and avarice, man brought down the wrath of the elementals upon himself.
Raya-O-Senna: A great deluge was sent to cleanse the land of his wicked presence, in the wake of which the forest rose to swallow up all that was not washed away. Thus did the Six Umbral Era begin...or so it is told.
In the Fifth Astral Era, a darkness began to spread across the realm, filling the hearts of men with darkness and corrupting them, causing them to pervert the powers they had gained. This corruption of man brought about chaos and destruction upon Eorzea, avarice reigning supreme and greed for power clouding the hearts of man. As the Twelve watched and grew sick of their creation, they sent upon man the great deluge, flooding the realm to purge it of its sins.
A cabal of immortal dark mages, said to have made a blood pact with the demon lords of the Seven Hells, soon rose to wreck havoc across the realm during this time of chaos in an attempt to deliver a million souls to their masters in exchange for full dominion of Eorzea.
The Archons and the Grand Companies rose to battle the threats that plagued the land, but through a course of events, Alhdbhar, the incarnation of Rhlagr, turned sides and joined the dark sorcerers.
The long and short of it is that I believe there was more to the Sixth Umbral Era than just the perversion of magic by man, though it most definitely played a role in the descent from the Fifth Astral Era.