Seventh Umbral Calamity
The Seventh Umbral Calamity, also known simply as The Calamity, was a cataclysmic event that brought great destruction and change upon the lands of Eorzea. This event marked the end of the Sixth Astral Era (and, in turn, Patch 1.23b) and the beginning of the Seventh Umbral Era (Patch 2.0). The events of the Calamity are visualized in "The End of an Era" and "Flames of Truth" cinematics. "The End of an Era" plays upon first beginning the game on a new Adventurer, while the "Flames of Truth" is part of the post 
The Final Coil of Bahamut - Turn 3 quest cutscenes.
Historically, "The End of an Era" cinematic played for all players logged in to Patch 1.23b in the minutes prior to the server switch-off, though the login servers collapsed and died multiple times under the weight of all the players attempting to log in for the shutdown event. The video was uploaded to Youtube after an official apology by producer-director Naoki Yoshida at the beginning of FINAL FANTASY XIV Letter from the Producer LIVE Part IV. It would be just under two years before players could see the "Flames of Truth" conclusion with the release of Patch 2.4. That video was uploaded to Youtube on Apr 11, 2015, about halfway between patch 2.55 and patch 2.56.
Lore
“The history of Eorzea is marked by a repeated cycle of devastation (Umbral Eras) and prosperity (Astral Eras). The first six Astral Eras were each preceded by a calamity corresponding to one of the six elements. Following on from the Calamity of Wind, which ushered in the First Astral Era, the Calamity of Lightning brought destructive thunderstorms, the Calamity of Fire withering droughts, the Calamity of Earth devastating earthquakes, the Calamity of Ice an endless winter, and the Calamity of Water a lands-consuming deluge.
With all the elements accounted for, many scholars convinced themselves that there could not be another calamity─that the Sixth Astral Era would last indefinitely. Alas, the Seventh Umbral Calamity would indeed befall the land, triggered when the Garlean legatus Nael van Darnus saw the Meteor project to fruition. Dalamud, Hydaelyn's lesser moon, was plucked from the heavens, and upon its arrival above central Eorzea, it split apart to reveal the elder primal Bahamut, who, in a fit of inchoate anger, laid waste to anything and everything around him. However, owing to the countermeasure enacted by the Archon Louisoix, the realm was spared complete annihilation, and after the ashes had settled, underwent a remarkable rebirth.
Five years on from that fateful day, the forces of the Eorzean Alliance succeeded in defeating Legatus Gaius van Baelsar and his occupying XIVth Imperial Legion. Though other threats still remained, this victory marked a turning point in the fortunes of the realm, and so it was proclaimed that, by the light of the Crystal, the Seventh Astral Era had come, and the realm was at last truly reborn.
The Seventh Umbral Calamity was the result of the Garlean Empire's "Project Meteor". The intent of the project was to bring the lesser moon Dalamud down onto Eorzea in an attempt to purge the land of primals once and for all, even if it meant untold destruction. When, after the defeat of Nael van Darnus at Rivenroad, the moon's descent continued, the people of Eorzea prepared for impact. The Archon Louisoix and the Circle of Knowing began prayers to The Twelve in earnest, while the Eorzean Alliance met with the remainder of the Garlean Empire's VIIth Legion in Carteneau Flats. It was there the event known as the Battle of Carteneau was waged, and there where Dalamud was set to impact. However, before impact could occur, the moon broke apart to reveal that it was not a moon but a prison for an Elder Primal: Bahamut. Eorzea's last line of defense was Louisoix and his imprisonment ritual, but that, too, failed. Before a flash of light engulfed the battlefield, Louisoix managed to send a small party of adventurers into the safety of an aetherial rift. What happened after—what events unfolded, and what became of those adventurers deemed the "Warriors of Light"—remains a mystery even to those who lived to tell the tale.