Skishtari gene egg

A  was an egg laid by the Skishtari which which was used to conquer reproduce. (AUDIO: Survivors in Space)

Biology
The egg was laid in an unsuspecting population, whereupon it eventually hatched, and the infant Skishtari consumed the entire population around it. Then, the Skishtari parents arrive to start a family. (AUDIO: Survivors in Space)

History
A Skishtari egg was once placed into the heir to the Robotov Empire, Alex, which gave Cyborg health difficulties. The Fourth Doctor removed the egg, and attempted to take Alex, the egg, and Boolin back to Nest Cottage, via wormhole, for protection from the Skishtari. (AUDIO: Tsar Wars) However this wasn't entire successful, as they were separated during transit, and Alex and the egg ended up in Hexford in the 1850s, while the Doctor and Mrs Wibbsey ended up in 1861.

By 1861, the egg had begun to influence Alex, now calling himself Andrew, and he began to trap people from Hexford inside the egg. He eventually used it to create a dragon. (AUDIO: The Broken Crown) The four of them, The Doctor, Mrs Wibbsey, Alex, and Boolin, ended up inside the egg, where they were forced to play out the tale of Aladdin. They eventually escaped and buried the egg underneath where Nest Cottage would be. (AUDIO: Aladdin Time)

In 2012, the Skishtari came to retrieve their egg, dragging all of Hexford through a wormhole to do so, however they left the egg behind in the hands of the Doctor. (AUDIO: The Hexford Invasion) The egg was then brought by the Doctor to Hexford, now on Rebel Moon, while he was trying to save the people there. The egg then began to hatch, however it's bond with Alex, now Robotov Emperor, lead it to follow his orders to kill its parents. (AUDIO: Survivors in Space)