Latest publication from "Your Living City. Stockholm" by Monika Lipanovich.
"Also Viking berserkers may have used it to induce a trance-like state. But historians know very little about the berserkers apart from scattered Old Norse myths and epic sagas. One intriguing hypothesis as to the source of their behaviour is that the berserkers ingested a specific kind of mushroom with psychoactive properties. Some of the more hotly contested hypotheses suggest that the berserkers ingested a hallucinogenic mushroom just before battle to feel immortal."