Morsus Adeptus was a warlord that is said to have flown from the north, over the wall, and proceeded to eridicate anyone in his way, with his army of thousand.
It took the collective power of what is now called Amestris to subdue him, and his army. Even so, the death toll was monstruous.
The plague that struck the land some hundred years later was named after him, having a similar effect of arriving from seemingly nowhere, and eradicating thousands.
The alchemists who studied the Morsus Adeptus, theorised that it prefered hosts of great magical affinity.
Due to this, much of the magically talented people, of either layer, diminished greatly.
Thanks to this, it was also discovered that all humans have the potential for magic, or at least, they possess those same natural energies. Where, it was previously believed that those with magical talent was a mutation. The difference was merely that those magically talented had a stronger connection with the natural energies surrounding everything.
Some still believe it to be a mutation.