Gamers will remember the shadowy human organization ADVENT from the XCOM: Enemy Unknown expansion Enemy Within. The group was a Ninth Column for the alien invasion, collaborating with them to disrupt XCOM's operations, even invading the player's base at one point. In XCOM 2, their role is changed. Since the aliens have successfully taken over the Earth, the group serves as the aliens' counter-terrorism division, with your XCOM team as "terrorists".
ADVENT will be responsible for what developer Firaxis describes as "Dark Events". Dark Events are missions that pop up that cause random modifiers to the game if they are successfully completed by the alien occupation force.
They can cause minor effects such as giving ADVENT soldiers better gear like venomous ammunition or give them major resources such as deploying UFO's that will attack the player's Avenger aircraft, or placing extra Faceless, tree sized monsters that somehow can disguise themselves as ordinary humans, among the human population.
Players can stop the Dark Events from coming to fruition by going on special missions, but, of course, that means using precious manpower and time on what are essentially side quests. Thus, players must weight the benefits and cost before deciding to commit to them.
XCOM 2 is due to be released on 5 February 2016. The original XCOM was released 9 October 2012 and earned a 9.0/10 score in our. The expansion, Enemy Within, also earned a 9.0/10 score.