Curse Your Sudden And Inevitable Betrayal. Basically you need a civ who you were friends/allies with to declare a surprise war i believe you can also go to war on a civ who used to be your friend (and still has the diplomacy bonus). After playing 327 hrs civ 6, it still hasn't popped up between the ai's and i've tried everything.
Hello, i'm having so much trouble with this achievement curse your sudden but inevitable betrayal!, in fact it's the last achievement i need to complete all civ 6 achievements. But i've been using twitter, which limits me to 140 characters. Not getting sick are you?” connix asked as she sat down across from poe in the mess hall.