Near the end of the XCOM 2 'Welcome to the Avenger' trailer that surfaced in August, we caught a brief glimpse of something called "Dark Events." The narrator described them as "enemy black ops projects" and made it very clear that they're all different kinds of bad news. He wasn't kidding.
Dark Events, as Firaxis explained today, are "counter-operations" undertaken by Advent—that'd be the human organization that's collaborating with the alien invaders in this alt-universe XCOM—that will begin to occur once your team starts reclaiming territory. Each Dark Event that's successfully executed will inject a random modifier to the gameplay, some of which will have a real impact on the action.
"[Dark Events] run the gamut from something as small as improving Advent soldiers’ gear in the field to more dramatic gameplay changes such as deploying a UFO that will hunt the Avenger," Firaxis wrote. "Advent can also negatively affect XCOM supply drops and, in a far more chilling Dark Event, hide extra Faceless amongst the populace of each mission. Rapid Response poses another massive threat to XCOM, guaranteeing Advent reinforcements will be deployed during all Guerrilla Ops missions. We’ve also heard reports of Advent employing Viper Rounds—poisonous ammunition made from Viper venom."
That's apparently just a small sampling of the Dark Events that Advent will be able to roll out as it loses ground. Players will be able to prevent them from taking effect, however, by launching Guerrilla Ops against them. All in all, it strikes me as quite similar to the randomized missions in XCOM: Enemy Unknown, but with far more dire consequences for failure. I think that sounds pretty cool.
XCOM 2 comes out on February 5, 2016.