You may first see this creature at the end of the tutorial mission. It is an electric yellow female-humanoid being, seemingly made of electricity and circuitry. You will first fight it after you attempt to Brainjack a ADVENT Captain: you may want to make sure that Captain is otherwise the last enemy on the map. It has high health, a high dodge rate (so even point-blank shots may 'graze' it for partial damage), and high accuracy. It's abilities include teleportation, an AOE rift-like attack that disables weapons for XCOM agents caught in it, then explodes for significant damage if you leave a soldier in the effect. Sometimes after damaging it, it may also clone itself: both clones will have the same amount of damage. For the storyline requirement, the best time to fight it is when there's no turn counter or anything keeping pressure on you.