Amanita muscaria, a.k.a. the Fly Agaric. Found worldwide in temperate woodlands. Ingestion induces very variable effects per individual, often including vomiting, drooling, staggering ... but yes, also rumoured (disputed theory) to have been used by vikings to induce berserker rage; how they controlled the dosage to give a reliable effect is unknown, but further rumoured (more disputed theory) to have been achieved by filtering through the tribal shaman - he ate the mushrooms, then the warriors drank his urine. Are you sure this is tempting?
It sounds pretty easy to mod, just add an item that can be talked to, with an option only for Shamans to make it another item every class can use and when consumed it provides the effects of a Berkserker's rage. I'll look into it. If anyone wants to contribute to this silliness, gimme appropiate descriptions/names and dialogue (im bad at wrting u kno).