I see what you're saying...MUST FIX! Ummm...This is a lot harder than I originally thought it would be...Wait! I think I have the explanation! Maybe...Anyway, the civilization in question had attempted to invade and conquer a lot of the more powerful empires as well as some weaker ones before the assassination and was severely weakened by the time the Yuan-ti started the war. Let's also say they weren't completely wiped out but came very close through stealthy Yuan-ti operations that no one else in the war was aware of.