The Puncture

A mysterious cosmic event that caused Multiverse B to "pop" into Multiverse A, overwriting Multiverse A's metaphysics with its own while leaving its residents and history intact. The event can be understood only within the context of the Postulate of the Holosphere.

Origin
We don't yet know what caused the Puncture. The shadar-kai say that it was human magical experimentation in Kadderash, a world in Multiverse B, that caused the boundaries between multiverses to weaken and ultimately collapse. Both the accuracy of the shadar-kai version of events and the nature of the humans' experiments remain undetermined.

Effect
Apart from changing the multiverse's metaphysics, the Puncture added, subtracted, and rearranged some planes, hence collaterally robbing wizards of their elemental-derived magic. In the process the boundaries between planes weakened, turning the Astral Sea into a maelstrom as extraplanar beings streamed in. A number of these beings, including demons, angels, shadar-kai, and eladrin, have attacked various sites on Seth's Material Plane of Krynn in impromptu power grabs. Furthermore, the rearrangement of the planes robbed wizards of their accustomed power sources, resulting in a magical drought.

As a Deus
In game terms, the Puncture was of course a cover for the transition from 3.5e to 4e. But as Seth was just returning from the Lupine Wars and there was no obvious new hook for him to transition into, I turned the transition into a hook. Conveniently, the subsequent transition to 5e happened as wizards began to regain their power.