The Apocalypse was an event that occurred a millennium before the Etrian Odyssey games. The details of the apocalypse are scattered across the games' plots, but what is known is that it had resulted in the world becoming largely uninhabitable for humans, forcing them to initiate the Yggdrasil Project before it was deemed completely safe to use.
With the seven Yggdrasil Trees planted across the world, the miasma and pollution that had resulted from the apocalypse had been absorbed and cleansed from the earth, bringing about the civilizations we see in the games at present.
However, ever since the event, humanity's technology has greatly deteriorated. For example, the use of fingerprints in forensics has been a lost knowledge until it was rediscovered by Detective Austin. The Drive Blades were lost technologies outside the Cloudy Stronghold until Tharsis and the Explorer's Guild encountered the Yggdrasil Empire. It is possible that firearms were also a lost technology until the reawakening of Frederica Irving. Other technologies lost until rediscovered include plastic bottles (one was found in the Claret Hollows) and drywall (which is mined from the Shinjuku Towers).
Survivors of the Apocalypse:
- Frederica Irving
- Black Guardian
- Hollow Queen (possibly)
- Xiuan, the Medium (possibly)
- The setting of Etrian Odyssey 3 (which has nothing to do with the Yggdrasil Project) has its own version of the Apocalypse, called the Calamity.
- It is unknown what, exactly, caused the Apocalypse to occur, but in Etrian Odyssey IV, life is said to have still existed (just not suitable for human life), which implies that it wasn't nuclear weaponry.
- Sometime after the Apocalypse, the Indonesian city of Jakarta (which is the Trade City of Batavia) reverted back to its Colonial Dutch name.