The crazy part is Berserkers where a real thing, Viking Berserkers to be exact. There are many historical accounts of them flying into such an insane rage during battle that they continued to fight even after suffering injuries that would completely debilitate or out right kill someone. People where so scared of them that large groups of soldiers would avoid a conflict with them, even if the group of Berserkers was small in number.
Well, quasi-historical accounts, anyway. Most of the actual references to Norse warriors with superhuman powers come from poetic sources, which is kinda like taking the
Iliad as proof that gods existed and killers from the Aegean littoral could go toe-to-toe with them. The reality of the
berserkr is a bit hard to suss out; Norse warriors who dressed up in bear-skins almost certainly existed, and they were probably associated with a 'fought like madmen' style, but going beyond that is hard, and ascribing magical powers of death and survival to them is exceedingly silly. It's not even clear to me that they were any better than 'normal' warriors.