The God War

At its height, the Great Kingdom of Urcia was so powerful as to challenge the gods themselves—literally. They traveled the planes in steampunk/space jockey/Spelljammer helm kind of planar travel suit-pod things, killing gods one by one. This included gods from all the various D&D settings, Earth pantheons, etc. Eventually the remaining gods got together to put an end to this, which they did by sending the gargantua to destroy Midavel.
The gargantua smashed most of Urcia and were considered unstoppable. In desperation, Urcia's best wizards got together and decided to depower the gargantua completely by eliminating magic from the world. They did this by imbuing the essence of magic into a young man, then sacrificing him (kind of a combination of Jesus being the Word Made Flesh and dying for our sins and the murder of Mystra breaking the weave). This was the Weird Made Flesh, and his death ended magic.
The Urcians buried the inert gargantua deep in the earth along with the corpse of the Weird Made Flesh, then did their best to erase their own history and knowledge of magic so that no one would repeat their mistakes.