The Cataclysm: technically a planetary extinction event occurring over thousands of years, but the date schoolchildren learn for the Cataclysm is when Earth reached one hundred percent uninhabitable status for carbon-based life.
It wasn’t actually a fast process. There was, in fact, warning; that warning is part of what led to six of the Seven Peoples fleeing the Ever-Dying home-world and creating their own parallel worlds: the Silver Dawning for the Fey; The Plane of Dreams for the the Dream; Umbra for the Darkness; Zenith for the Sun.
Within those four worlds, all the magical peoples eventually managed to create strongholds (or at least make mutually beneficial deals). However, this balance then threatened when it became clear Earth was dying, and multiple billion humans all desperately needed help.
Not all. Not by a long shot. There simply wasn’t room – or so goes the claim.
In the end, the Ever-Dying humans were divided into three groups: the Redeemed, who were taken into other worlds in various states from guests to slavery; the Forgotten, who were left to die; and the Pure, who were chosen by the Association to fly away from Earth and try to repopulate elsewhere – preferably someplace without any magic.