The Destabilization

The metaphysical bomb coinciding with the end of the Brother's War, the Destabilization was a magical (not literal) shockwave which resonated across the magical fields, reshuffling and redefining their relations. Many forms of magic were weakened, some strengthened, all changed irreparably.
In an hour, thousands of pieces of golden-age infrastructure was rendered useless, untrustworthy, or even dangerous. Many flying fortresses and temples either crashed to the ground or flew off course forever. Teleportation and magical communication was disrupted, summons were released from binding, and quotidian artifacts simply disenchanted. Even divine magic became unmodulated: some archbishops could not summon even a mote of light, while unfortunate novices self-immolated with a casual blessing.
The war, of course, was immediately sidelined for larger issues. A period of political instability, hunger, and general chaos reigned, shattering the mighty empires of the golden age into feudal states.
The good news is that magic was not permanently fucked. Within a few decades, many fields were stable enough to reliably use, though none were as powerful or predictable as before. Slowly, the new metaphysical order has begun to reassert itself.