have you heard of the late bronze age collapse?
https://en.wikipedia.org/wiki/Late_Bronze_Age_collapse
you could implement it as a cool end-game event. most historians don't actually know what caused the late bronze age collapse so you can make many explanations in the game as to what caused it, like a non-human race attempting to conquer the world and genocide all other races. this could be a nice explanations for why there are no other non-human races alive today, or maybe magical daemons spawning and causing havoc.