Mysteries of Mirkwater
The Varyags speak of some of their warriors who work themselves to a frenzy, eating certain mushrooms that make them ignore pain. When they are in their rage, they throw themselves on their enemy with reckless abandon.
For a Berserker, use the Barbarian class with the Berserker path, with the following exceptions:
Berserkers may only wear bearskins (AC4) as armour. They can only rage if they work themselves into a frenzy eating mushrooms for 10 minutes.