Nopony knows exactly, and it's not like there's a lot of info - just that some stuffed-collar in Stalliongrad thought it'd taken hold of their king and thought Pale Pony-induced nukes - the real one, as far as I can tell, not Cálice - was the way to clear it out.

Considering the ties to the elements of Harmony, it might be the same thing that turned Luna into Nightmare Moon. No way of tellin'.