Berserker pendants were talismans created by Berserkers which gave the bearer the ability to order people to do as they wished.
A pendant would slowly change the person using it into a Berserker. Each time it was used, the pendant would slightly change the bearer both physically and mentally until the bearer desired to complete the transformation into a Berserker. During transformation the user would develop prominent veins over the skin's surface. The final form had the appearance of a man wearing animal skins. The only cure, if done early enough, was to remind the affected person who they were so that the memories would reassert their identity.
According to UNIT archives, a Berserker pendant surfaced in a barracks in Ealing in 1940, but was lost during a German bombing raid. This same pendant was rediscovered by Jacob West, who used it for a time until it began to transform him, at which point he abandoned it. Rani Chandra took it and used it for a while until she realised its dangerous properties when her father offered to die in response to an idle comment made by her, so she placed it in Sarah Jane Smith's attic. It was discovered by Paul Langer, who stole it and used it to control his son, Clyde. Mr Langer was almost completely possessed by the power of the pendant until his wife and son reminded him who he really was. Clyde used the pendant to erase his mother's memories of the incident, then disposed of it in the sea. (TV: The Mark of the Berserker)