The Sarkeen are a race of biologically constructed beings that are individually engineered for a specific purpose.
It is important to understand first one of the great problems of space exploration and interstellar military operations. The famed space explorer Zerbino Gaston once said that “God made most of the universe uninhabitable for our own sake.” The meaning was that when you go to a barren, lifeless world, there is nothing down there that can actively infect you, and allow you to spread an unknown contagion to your crew, friends and family, and your world. Every life-bearing world has dangers that go beyond the pure environmental. The Sarkeen have an interesting way around all this.
What they do long before they launch a general invasion of an inhabited world is to go there to evaluate the world, learn about its fauna and flora, identify the hazards, and then they take a sample of the dominant species, examine it, then apply its biology to their own. When it comes time to invade, they begin developing an adapted body-form that corresponds to the world(s) they are going to invade. The result is a Sarkeen trooper that is automatically resistant to most of the microbiological maladies of the world(s) they are invading, and they can breathe the air.
There are many kinds of Sarkeen, and almost all of them are humanoid in general appearance. The Spacefarer is a tall, thin humanoid with only four fingers on each hand (including an opposable thumb), large craniums, and large eyes with a highly diluted retina that can see in very low-light conditions (see picture above). Experts believe that these Spacefarers are more adapted to the rigors of outer space; captured (deceased) specimens have been found to have a greater resilience to radiation, and are quite agile in zero gravity.
The one adapted for Human worlds and habitats, known as the Groundpounders, have a thicker skin (some even have hair), tiny nostrils, and eyes that are a bit more expressive and adapted for brighter light. They have stronger frames that enable them to operate in Human normal gravitational conditions, and they can operate in dirtier conditions than the Spacefarers.
The Sarkeen are presumed to have some basic Psychic abilities, and there are reports of Sarkeen with greater Psychic ability. They were described with larger heads, and were able to move about telekinetically. These reports have yet to be confirmed.
Sarkeen hierarchy is, at present, unknown. The tendancy of the Spacefarer and Groundpounder varieties to fight fanatically, that they do not rout, retreat, or surrender without orders, suggests a morale and command structure that is unlike anything humanity has ever seen.
Their appearance has naturally led to a reevaluation of earlier UFO reports from as far back as the 20th Century, most especially the abduction stories of people who have claimed to have been abducted by creatures bearing their description.