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. It would be just under two years before players could see the "Flames of Truth" conclusion with the release of Patch 2.4.
Lore
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.