Berserkers were a warrior race who were the inspiration for Norseberserkers. They visited Earth at some point. They encountered Vikings and were mistaken for human warriors wearing animal skins.
The Berserkers created special pendants, which they made to expand their armies. The pendants would allow people to control other people. With every person they controlled, they would become more like a Berserker. At least one Berserker pendant existed on Earth. It fell into the hands of Jacob West and, later, Paul Langer.