From CT Quest

- Xande died after founding the Allagan Empire,
- The Allagan Empire, at some point, built the Syrcus Tower as an energy source. That energy made their civilization flourish.
- Unfortunately, it also lead to an age of decadence.
- Amon, a technologist, decided to revive Xande, since he was convinced what they needed was a strong leader. to do so, he also creates clones of Xande descendants, Unei and Doga.
- Xande, reborn, decide to resume the conquest of the world. He managed to crush Meracydia, probably with the help of voidsent (we see clones of Unei and Doga controlled by voidsent, in an Echo vision)
- The resurrection process apparently left scars on Xande souls, and decided to destroy the world using the Cloud of Darkness.
- Xande realized he needed more energy to control the Darkness, and thus built Dalamud and sent it in the sky, to increase the energy output of Syrcus Tower.
- Some miscalculation (really.) lead to the an energy surge released in the ground, leading to tremendous earthquake destroying the whole civilization.
- Xande, along with Amon, decided to freeze time until another moment.
- The fall of Dalamud apparently reactivated CT, and Xande woke up, resuming his operation of summoning the Darkness in our world.


So this was neither internal strife nor natural disaster.

Something I don't quite understand though, is how it lines up with Bahamut. Did they needed it as a part of Dalamud, to make it work, or did they just thought "well, while we're at sending something in the sky, let's take care of that problem too" ?
Considering "Dalamud" apparently comes from "Bahamut", I'd tend for the first explanation, but... :<