Ethereal

Overview
The Ethereals are "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.

History
Despite their immense psionic power, the Ethereals discovered that the gradual atrophy of their bodies would eventually reach the point their psionics could no longer compensate and keep them alive. Desperate to find a cure for their condition, the Ethereals sought out other species in order to harvest their genetic material for this purpose. Each species they discovered were conquered and enslaved, forced to fight other races the Ethereals targeted after them. These other species, including the Sectoids, Mutons, Vipers and Chryssalids among others, each failed to give the Ethereals what they needed and so were retained as unwilling soldiers.

When the Ethereals discovered the Humans of Earth in 2015, they launched a full-scale invasion with intent to harvest Humanity next. Resistance came in the form of XCOM, a multinational military organisation dedicated to protecting Earth from extra-terrestrial threats. XCOM gradually gained the advantage as they reverse engineered the aliens' technologies and used it against them, including the means of harnessing psionic power. After a series of victories over the invasion army, a psionic human in XCOM's ranks, hereafter referred to as the 'Volunteer' makes contact with the Ethereals via the psionic network through which they control their armies. The so-called 'Uber Ethereal' claimed the Ethereals to be benevolent, that their goal was to find a species that shared their psionic gift to "prepare them for what lies ahead", all while denouncing the other species under their control as failures. XCOM were unfazed by these claims and infiltrated the Ethereals' command ship, killed the Uber Ethereal and destroyed the vessel, at the cost of the Volunteer's life, seemingly halting the invasion.

Despite XCOM's efforts, the Ethereals intensified their invasion and eventually defeated XCOM, capturing their commander and conquering Earth. The Ethereals dissolved Earth's governments and replaced them with a puppet authority known as the ADVENT Coalition, containing the remnants of Humanity within new city centres and presenting themselves as benevolent idols commonly referred to as Elders. The Ethereals soon discovered that Human genetics were the key to curing their condition, meeting both the physical requirements and psionic potential they needed. To maintain subtlety over their dominion of Earth, the Ethereals reserved their alien slaves to more specialised roles in supporting the occupation, while the bulk of ADVENT's forces were represented by Hybrid clones disguised as regular Humans. XCOM, now an underground resistance movement, recovered their Commander from ADVENT custody, prompting a guerrilla war between the two factions.