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)

They are rumoured to return in The Waters of Mars as a creature with eyes that are similar to that of a berserker is seen in the next time trailer for The Waters of Mars.