Ethereals are the last, and most fearsome, main alien species encountered in the classic game XCOM: UFO Defense. While not as physically powerful as some of the other alien species, such as mutons, ethereals have devastating psionic abilities. Their psionic abilities can cause your troops morale to drop making them drop their weapon and run, or start shooting randomly. They can also exercise mental dominance over your squad members, taking control of them and turning them against you.
With the danger they present comes great reward. Capture one alive and you can begin a path of research that will enable you to train your troops to flex their own psionic powers.