The Ethereals is "The Master Race" that fought against X-COM during the later days of the First Alien War. The Ethereals are physically fragile race, its body's muscles are severely atrophied, the internal organs appear to be under-developed and the sensory organs, including the eyes, do not appear to function at all.
However, the Ethereal brain is very well developed and draws on a high proportion of the body's blood supply. It possesses awesome mental powers which allow for telepathic communication and telekinetic abilities. These telekinetic powers defy the laws of physics known by humans. Utilising said power, this physically retarded being that seems incapable of sustaining any life functions turn out to be the most dangerous species that X-COM forces faced during the First Alien War. Similar to high rank Sectoids, Ethereals can use Psi Attacks to demoralise soldiers in combat, or even take control of their minds.
The Ethereals are alway seen wearing billowing orange robes to cover their twig-thin bodies. Unlike Sectoids, they prefer to send other races (especially Mutons) to carry out missions on their behalf. It isn't until late in war that the Ethereals take matters into their own hands. The Ethereals use the devastating robot juggernaut known as the Sectopod as their preferred Terror Unit.