Centuries before Sarah and Harry's time, a damaged Zygon craft landed on Earth, where they intended to await rescue. But help never came, and years later the crew learned that their planet had been destroyed in stellar explosions, leaving them no other alternative but to inhabit the Earth.
The Zygons are slightly smaller in stature than the average male human, though solidly built. Their skin is mottled orange in color, and ridged, with rows of large sucker-like protuberances running from the forehead down the spine and more rows of smaller protuberances along the limbs. Zygons can injure or kill their enemies by grabbing them on the shoulders, which implies an inbuilt physiological weapon. They are a warlike race, led by a warlord. Zygons speak softly, in a sort of gurgling whisper, and their health can be monitored through use of the synchron response. When a Zygon dies, its body can be destroyed remotely by dispersal.
Zygon technology is based upon organic crystallography, which combines machine and organic components. Their ship looks very organic, lit by a phosphorescent glow and operated with fleshy controls, aside from the crystalline doors. The ship is hidden underwater, with a passage leading up to land that the Zygons can use. The Zygons also have the ability to adopt the bodyprint of humans. To do this, they must first capture a suitable human, who is put into an alcove with a cylinder over their head. When a Zygon wishes to use the bodyprint, its form blurs and glows red, then the glow fades to reveal the new body. This not only alters the body, but changes the voice as well, although the Zygon's mannerisms are not affected. This means that the friends of the person whose body the Zygon is using, and even strangers, may think that they are acting strangely. Once the Zygons have a human in custody, they must activate the bodyprint every few hours or the original will die and their bodyprint cannot be used again. They also like to plant listening and viewing devices in the area where the disguised Zygons are living, so that they can monitor the others who live there.
Zygons depend on the lactic fluid of the Skarasen for survival, and Skarasen embryos are brought with them on journeys. Although Earth weapons can injure Zygons, they have no effect on the Skarasen - even a nuclear missile would seem like little more than a pinprick. Like the mix of organic and machine seen in the rest of Zygon technology, the Skarasen is an armored cyborg. It is extremely powerful, and responds to commands. The Zygons can target a place or person for destruction by planting a trilanic activator, also called a target reciprocator, which serves as a signal device for the Skarasen. Also part organic, the device can attach itself to the target surface, making it almost impossible to remove. These were used to enable the Skarasen to destroy oil rigs, which it accomplished by chewing through the supports. Its approach is heralded first by radio blackout, then a loud noise. After its masters were destroyed and their ship blown up, the Skarasen retreated back to Loch Ness, where it still lives.
