Quick Overview XCOM: Enemy Within is an expansion pack for XCOM: Enemy Unknown, a turn-based tactical strategy game. The expansion adds the ability to gene mod soldiers to add to their powers as well as the ability to create MECs, powerful robotic soldiers. These new features are created by collecting a resource called Meld on the tactical battlefield. A number of new maps were also added to the tactical game, as well as multiplayer. The PC version of Enemy Within is an add-on that requires the base game Enemy Unknown.