There's no historic evidence of

not to mention that this film kinda lacked a lot of things. it's a uninspired retelling of The Most Dangerous Game with feral, drugged-out Vikings. when it ended, I was like "huh? that was it?"

Berserkers are pretty widely agreed to have existed. The part some historians disagree on is whether or not they took drugs before a battle to enhance performance. Some say yes, others no. But "berserkers" as a warrior class almost assuredly existed.