Reconnaissance scouts, or recon scouts, were specialised Daleks sent out from Skaro to scout out new planets in preparation for invasion. Unlike other Daleks, Reconnaissance Scout Daleks possessed a number of abilities normal Dalek soldiers lacked. The Kaled mutant could possess other lifeforms like humans by sticking to the back, piercing the skin and using its tentacles to intergrate itself into the central nervous system of the creature and form a mental link basically essentially a parasite with control over its host. If it was cut up and divided into pieces then the mutant could be restored to life by ultra violet energy and teleport the pieces back. The reconnaissance Daleks were also capable of disrupting The Doctor’s TARDIS’ circuitry and disabling their sonic screwdriver.
One such Dalek arrived on 9th century Earth. It was defeated by an army of humans on British soil in the Battle of Hope Valley. The mutant was divided into three pieces by the Three Custodians, while its casing was destroyed. Some parts of the casing remained, and the third Custodian did not complete his journey to the burial site in Yorkshire.
This part, recovered by archaeologists in Sheffield, was exposed to ultraviolet light and the energy restored it. The mutant was able to possess Lin and forced her to rebuild its casing using what was left from the 9th century. It continued its mission in preparing Earth for an invasion from a Dalek fleet.