Each time it was used, the pendant slightly changed the bearer both physically and mentally until the bearer desired to complete the transformation into a Berserker. During transformation, the user would develop prominent blue veins over the skin's surface, and their irises would turn icy blue. 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.
The pendant had the ability to give the user control over others, controlling their body, will and altering emotions. However, the pendant did not always give the user the ability to take away free will. It was also able to to control the electrical impulses in the brain, as shown by Jacob West as he ordered Steve Wallace, Mr Cunningham and his classmates to be quiet; it caused their brains to stop sending signals to the vocal chords, causing them to loose their ability to speak as well as stopping them from moving even with their free will intact. As shown by Paul Langer, it was also able of suppressing someone's memories.
There were three known ways the pendant could take back every order of those who used it. When Rani Chandra used it on Haresh Chandra, she then told him to forget everything she told him, turning him back to normal state of mind. As Jacob began tranforming into a Berserker, he resisted and begged the pendant to take back every order he gave when he was using it. After Paul Langer was freed from the pendant's influence, everyone that he had used it on had were returned to normal. (TV: The Mark of the Berserker)
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, abusing it's power to get attention by tormenting those who oppressed him. During Mr Cunningham's detention because classmate Steve Wallace offended him Jacob retaliates by using the pendant on the entire class taking their voices and stopping them from moving on his orders. As he began Transforming he went to the toilet to try and reassert his control over it convinced it to take back every order he gave everyone in detention throwing it away afterwards and in the process everyone in detention ran out in fear of Jacob.
Rani Chandra took it after witnessing what's Jacob has done to the other students 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 with the intent of analysing once Sarah Jane got back due to Mr Smith being disabled in her absence. It was discovered by Paul Langer, who was introduced by what Clyde Langer does in the Attic after realising what it does when he accidentally commands Haresh Chandra he used to it to control his son and others to buy whatever he and his son wanted and to show him the world in the hope of winning is son's love.
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 but still remains active. (TV: The Mark of the Berserker)