XCOM 2

You will gain access to the Alien Hunters DLC mission early in the main campaign. You will know this happens when a new icon appears on the Geoscape. You can scan the icon at any time (it never disappears) but you should not begin the mission until you are fully prepared. The mission itself isn't difficult, as most of the enemies are weak (Vipers and lesser forms of Vipers) but completing this mission allows the alien rulers to show up in your game at any time. There are only three of them but they are extremely powerful and have a much bigger health pool than anything you have seen in the main game. They will attempt to flee from battle after a few turns or after taking enough damage. From there, they can show up in later missions randomly but any damage you do is carried over to the next encounter. The first alien ruler appears at the end of this mission (see Viper Vanquisher).