Alien Hunters means open season for your squad, unleashing three new 'Ruler' aliens to hunt: the Archon King, Viper King and Berserker Queen. They arrive alongside a narrative mission, new weapons and armour, and a 'Hunter's Lodge' update to the Avenger's Armory.
Once the new mission has been beaten, the big game will show up at random in campaign missions and employ unique tactics to make your life miserable. If you fail to bring them down in time, they'll flee and come back to haunt you later. A breakdown of all the new firepower is up on the XCOM blog.
Alien Hunters will be out May 12, and if you don't own the Reinforcement Pack, it'll cost £8/$10.