Berserker

Berserkers were a warrior race who used pendants to control people. They visited Earth at some point during the Viking age, where they were mistaken for Human warriors wearing animal skins as armor that could, when angry, enter a berserk rage either naturally or with the aid of drugs. They left behind at least one Berserker Pendant, which fell into the hands of Jacob, and later, Paul Langer. (SJA: The Mark of the Berserker)

The pendant was actually the aliens way of expanding there army has the more an idavidual would use the pendant the more they would transform in to a Berserker untill finaly they where one of the species. Unit was able to capture the pendants last victim but the pendant it self was lost when the army base the victim was captured in was destroyed.

A person who uses the pendant starts out with a faint tattoo on there hands that becomes more solid the more they use the pendant. After that the tatto spreds up the body become solid blue veiny growths and there eyes and voice enventual change aswell givieing the victim a feral appearance. It should be noted that some people can resist and even rid themselves of the pendant like Rani and Jacob where able to both for diffrent reasons. Jacob who was affraid pulled the pendant off as it tried to trasform him while Rani got rid of it instantly when she found it could be used to kill sugesting that a person with a srong enough will and sense of right and wrong can resist the power of the aliens.