The world is currently in its Sixth Cycle , though it's not often called that by its denizens. Lumarian records and first-accounts tell that this most recent cycle's end related to a war that occurred between the gods as they fought to preserve the Divine Flame and their own power over the mortal earth- ironically, ending in the very thing they were afraid of. The gods of the Fifth first sent their followers to fight for them, then eventually made the earth itself their battlefield as they fought and destroyed one another.
This may be true- however, an event that transpired in a location (somewhere near modern-day Silkspire) known as The Vault is said to be the true cause of much of the devastation suffered by the population of the Fifth cycle, as well as the reason the Divine Flame was called back to the City of the Gods.
The Vault itself and its unsealing is considered by most Fifth Scholars to be "ground zero" for the devastation which leveled cities across the world. While accounts vary, a few things about the events of the Vault are certain- That the Vault had contained Aphelia, who was imprisoned there and therefore absent from the pantheon for most of the Fifth cycle, and that use of a human vessel was employed, through which Aphelia was able to break free. A moonless night fell for six days, during which a flood covered most of the earth until receding after a flash of white. The moon took her place in the sky again after this event, which has been dubbed by most as the Lunar Reprisal.
It's said that only about 6% of the world's population survived the Lunar Reprisal, and what few people did remain did not document well the difficulties they faced in the new world created by such a cataclysm. However, it's known that the Vault's unsealing created a great distortion in the fabric of magic across the world. Magic became exceptionally difficult to cast- in places, entirely impossible due to Ethyris vacuums said to be created as the Vault drew energy unto itself. Additionally, no gods remained at this time due to the final events of the Divine War of the Fifth. This made cleric's spellcasting impossible, including healing and protective wards- an especially detrimental side effect considering the esoteric plague that began infecting early Sixth populations, especially in those that were close to the Vault at the time of its unsealing.
The Vault Contagion was a sickness that spread from person to person, initially caused by a lingering energy that existed within the Vault. It was also believed to have been carried by the eldritch creatures that lived inside the Vault, angels of Aphelia which were her sworn protectors during her time inside her prison. These creatures were extremely volatile and powerful, killing mortals with little discretion after they were released alongside their god, and spreading the Vault Contagion at an alarming rate. The Vault Contagion also seemed to affect people who utilized magic in and around the Vault, or in places affected deeply by its unsealing.
TVC was marked by flu-like symptoms, followed by a period of coma. Typically, people did not wake from this coma- and those who did only did so after they had mutated and turned into horrible creatures not unlike the aforementioned Vault Guardians. The forms TVC would mutate victims into were extremely varied, and what few firsthand accounts we have of TVC and its effects seem to suggest that the form taken by victims had strong connections to personal struggles and traumas they had undergone.
Though the dangers presented by TVC are not entirely gone, the world has mended such that fears surrounding TVC are almost entirely put to rest. Cases of TVC are rare, and the apocalyptic state the world was once in are just the faintest of memories in dusty tomes in the libraries of scholars. The World of the Sixth, though it has had it's ups and downs, has seen a full recovery. The world was aided greatly in this by technologies preserved by the Sealed City, Lumaria, as well as knowledge that has been preserved by the extant dragons of cycle's past.