The Greys
On June 24th 1947 an object crashed into the desert outside the small town of Roswell, New Mexico. Agents of the US government who had previously held Delta Green security clearance were reactivated and sent in to investigate. The investigators recovered the craft, the wreckage of an extraterrestrial 'flying disk', and when they entered they discovered three dead and one living alien creatures. These creature's are the Greys, and their discovery prompted President Truman into creating an off-the-books unit tasked with analysing the wreckage and covering up the evidence of the crash known as MAJESTIC-12.
Physiology
Grey's are physically unassuming creatures, an average specimen stands approximately four feet (122cm) tall. They have enlarged craniums, thin lips and almond-shaped eyes that have no whites. A Grey's hand has four fingers, with the 'pinky' being the longest, and an opposable thumb. They lack any kind of reproductive organs and their digestive system is withered and atrophied. The Grey's are photosynthetic; their skin collects energy through sunlight and so lack any need to consume solid foods. A chlorophyll-like substance analogous to blood circulates through their body distributing this energy and collecting waste products, which are excreted through the skin. This excretion gives the Greys a 'burning cardboard' odour.
If deprived of sunlight for roughly eight hours, a Grey will fall into a 'biological stasis' that very closely simulates death, incapacitating them until they are exposed to sunlight again. They can also enter this stasis voluntarily when exposed to harsh conditions.
Activities on Earth
The Grey's interfere with life on Earth only to a minimal degree. Their primary method of interaction, excepting their communications with MJ-12, is the abduction of random humans. These abductions are frequently traumatising experiences, though the vast majority of abductees are returned to Earth unharmed. Most return with their memories of their abduction erased, but through therapy these repressed memories can be brought to the surface. The hallmarks of a Grey abduction are bizarre questions and experiments. Abductees may be exposed to messianic messages of prophecy, false stories about imminent disasters or even claims that humans are required for the Greys to mate. None of what the Greys tell their captives is true, and the purpose of these experiments is not yet known.
Behaviour
The behaviour of the Grey's is hive-like, according to MJ-12 observation. Grey's have very little understanding of the individual, every one of them sounding and acting exactly alike. Multiple Grey's in the same area will often move in lockstep, and speak in identical sentences if asked the same question. Grey's seem to know and have memories of every other thing that another Grey has seen or heard
History
In the course of MJ-12's investigation into the Greys, they assembled a history of the creatures. The living Grey recovered in the Roswell crash claimed to be from a planet that orbited the star Zeta-Reticula III in the M-31 star cluster. Their planet was Mars-like, being covered in endless deserts. The Grey claimed the civilisation established there was utopia-like with its people living in harmony with nature and each other, technological progress having solved all material problems. This civilisation sought to expand across the stars, but their physical forms were unsuited for interstellar travel. Thus the subspecies that became the Greys was created, being optimised for space travel in every way - their photosynthesis allowed them to travel incredible distances without requiring supplies and their lack of solid waste products took much stress off of survival systems. Concerned with the possibility of their manufactured space-caste breeding out of control, the Master Species sterilised the Greys at a biological level.
As part of their plan to spread their civilisation across the stars the master species of Zeta-Reticula embarked on the construction of colossal 'worldships', capable of ferrying vast numbers to new worlds. However during the construction of the worldships an unknown object, massive in size, collided with their sun at near relativistic speeds and triggered a supernova like explosion. All of the planets orbiting Zeta-Reticula were destroyed, including the Grey's homeworld. None of the Master Species survived, leaving the Greys alone in the universe. The only two worldships to survive this apocalypse set out in opposite directions hoping to find a world suitable for settlement where the Greys could find a way to undo their sterilisation and rebuild their civilisation.
In approximately 2500 B.C.E. one of the worldships found Earth and determined that humanity's genetics were compatible enough with their own that human biological samples might hold the key to reversing their sterilisation.
Language
The Grey language is pictographic in nature, including symbols such as simple mathematical shapes as well as human religious symbols such as the cross, the ankh and the all-seeing eye. Analysis into written samples of the Grey language concluded that it was little more than fragmented 'sentences' without grammatical connections linking them together. MJ-12 believes that this is because of the Grey's hive-like tendencies having rendered written language obsolete and left current written samples of Grey language ceremonial rather than functional in purpose. Attempts by MJ-12 to understand the Grey's 'true language', which appears to be a form of telepathy, ran into a dead end as the Greys expressed confusion whenever questioned on the topic. It is hypothesised that this telepathic link is so fundamental to the Greys experience of life that they aren't aware of it in a way that can be explained.
Technology
Grey technology is incredibly advanced, bordering on the impossible. The function and operation of Grey tools can seem bizarre and miraculous, but they give the impression of operating in a way that makes sense to the human mind. The flying disk recovered by MJ-12 from Roswell, code-named 'The Bucket', exemplifies this technological advantage - the entire exterior is made of a lead-like material that is entirely frictionless and has proved impenetrable to any force that MJ-12 could throw at it. The interior has a source of artificial gravity that keeps the inhabitants feeling an even 1G, even as the ship accelerates to over 20,000 miles per hour (32,000 km/h) or performs death-defying feats of deceleration. The mechanisms by which such technological feats are achieved is unknown.
The Greys are at times willing to share parts of their technology with MJ-12, and it is for this reason that the Accord was written. The most important information given to MJ-12's Steering Committee by the Greys are The Cookbook and The Report.
The Truth
The Greys however are not what they seem. In truth the Grey's are little more than automatons of the Mi-Go, created in order to serve as a proxy for their nefarious goals on Earth. A Grey is not a sentient creature but is instead directly controlled by a Mi-Go overseer, allowing for the Mi-Go to study humans without revealing themselves. This is the reason behind the Grey's almost-human appearance as well, being deliberately crafted to appeal to humans, as well as their hive-minded behaviour; each 'expedition' of Greys is likely controlled by a single Mi-Go intelligence. The Grey language is entirely artificial, deliberately designed to throw MAJESTIC investigators off balance. Their technology is based off that of their Mi-Go controllers, deliberately designed to be as 'understandable' to human minds as possible, minimising the truly alien, multi-dimensional aspects of true Mi-Go technology.
References
Delta Green: Eyes Only
Delta Green: Handlers Guide