Catastrophe (Final Fantasy XIII)

Catastrophe, also known as Day of Ragnarok from 500 AF onwards, is an important event that happened at the end of Final Fantasy XIII, and is a key event to its sequel, Final Fantasy XIII-2.

Datalog
Three years ago, mankind stared into the face of extinction when Cocoon almost crashed onto the surface of Pulse.

''The sphere's fall was halted by Lightning's companions, Fang and Vanille. Their desperate sacrifice summoned a massive pillar that cradled the plummeting Cocoon in its crystalline branches, averting an unimaginable disaster. The two l'Cie now sleep within the pillar in crystal form, and there are some who believe Lightning slumbers alongside them.''

A new age began from that day forth, with each subsequent year marked by the letters AF, or 'After the Fall.'

Final Fantasy XIII
When Lightning, Sazh Katzroy, Snow Villiers, Hope Estheim, Oerba Dia Vanille and Oerba Yun Fang defeated Orphan in its adult form, Cocoon lost its power source which kept it floating. Vanille and Fang decided to use the power of the beast, Ragnarok, to prevent Cocoon to crash land on Pulse, and turned into it. They managed to stop the fall Cocoon from falling by encasing Cocoon in crystal and placing it on the crystal pillar, but it also caused them to fall into the crystal statis within the pillar's core while Lightning, Sazh, Snow, and Hope were crystallized somewhere on the Archylte Steppe.

Then the latter four mysteriously wakes from crystal stasis and notices the crystal pillar supporting Cocoon. From the crystallized Cocoon, Serah Farron and Dajh Katzroy come down who were also in crystal stasis, but woke up as the rest. Lightning, with a smile on her face, gives her blessing to Serah and Snow's marriage.

Final Fantasy XIII-2
It is revealed that the goddess Etro intervened within the timeline by releasing Lightning, Serah, Sazh, Dajh, Snow, and Hope from crystal stasis and causing a time disturbance which also caused too quick death of the ancient seeress, Paddra Nsu-Yeul. Also, her intervention caused chaos to leak from Valhalla and sink into the soil of Gran Pulse, and so, some of the people who settled on Gran Pulse were able to use magic. Even the events of the catastrophe were altered as Lightning disappeared from history's pages only to be tasked with eternal fight against Caius Ballad and to be remembered by her sister, Serah.

Trivia

 * People of Cocoon, and Pulse migrants, adopted a new calendar after Cocoon's fall, aptly called "After the Fall" ,and which is abbrievated to "AF."