The "major Chaos invasion" means to win against a Chaos Incursion, i.e., the daemonic subjugation CB given to all daemon princes. AI basically never become daemon princes because it's hard as fuck (though I have seen one AI daemon prince of Khorne), so that's very unlikely to happen, and even if they did become a daemon prince, that AI would have to decide to specifically declare their daemonic subjugation on you, which seems unlikely.
The only real way to get that "win against invasion" flag is to successfully beat (and not just white peace, but enforce demands) Vlad after he declares war on you using the undead subjugation CB you get from the Carstein ring. If someone else now has the Carstein ring, take note of it because that's what enables that CB. There's also the possibilities of winning against an Orc WAAAGH! or a Cathayan subjugation (the last one is very unlikely since that requires a Cathayan ruler to successfully eliminate the renegade Jade). The Orc WAAAGH! is probably the easiest way other than beating Vlad since they actually happen, unlike incursions or Cathayan subjugation.
All that said, it's far easier to unite the Empire just by subjugating it the old fashioned way and declaring yourself emperor when you have the 160 provinces you need.