War of the Chosen introduces various enemies and possible allies, and the latest trailer is here to present some of them.
XCOM 2: War of the Chosen expansion is all about expanding (surprise, surprise) the core gameplay with mechanics and elements that extend the core campaign. Most of the changes will be the result of Factions and the titular Chosen being added to your story.
The video embedded below introduces the Lost, third group that will attack both your soldiers and aliens. It also gives a good look at the recruitment mission for some of the factions accessible through the new challenges. If the original campaign for XCOM 2 was hard, it gets even more interesting right now.