The Etydions were a humanoid race, with green skin, white hair, and horns on their chins and the sides and tops of their heads. They had long arms ending in flippers. Their DNA was capable of infecting and overwhelming that of others, slowly transforming them into Etydions. Their telepathic abilities allowed them to act as transmitters and receivers of powerful emotional thoughts.
The Etydions were once a well-known race of powerful telepaths. They would offer their services to interplanetary military forces, using their telepathic abilities to instil great fear in opposing forces, leaving them unable to fight back. Eventually, the survivors of such attacks banded together and destroyed the Etydion homeworld in revenge. The Etydion race was thought to be extinct and data pertaining to their kind was no longer included in even the most detailed of alien databases, such as K9.
However, one Etydion survived, making his way to Earth in the year 2050. On the verge of death, he offered himself to The Department, which kept him alive and implanted his DNA in VR headsets, distributing them across the British Isles disguised as a 6D video game called Little Green Men. The Etydion used his abilities to make children all over the world obedient. However, the children soon began mutating, as the Etydion had decided to revive his race through them. His telepathic abilities prevented Starkey, Thorne, or even K9 from doing anything to stop it. In the end, it was June's sorrow over what was happening to her daughter that convinced the Etydion to stop. Reversing what he had done to the children, the Etydion seemed to die, an orange energy leaving his body soon after. (TV: The Custodians)