Description
Berserkers have long been known in the northern kingdoms for their absolute, unflinching ferocity. Some have suggested that their berserk rages are the result of chewing a particularly potent form of mushroom.
This axe seems to provide some of the same effect, though whether that is simply in the mind of the bearer, or the result of mushroom spores soaked into the haft of the weapon, is unknown.
Source
The Battle-axe of the Berserkers can be found in 
Legendary Chests.
Repair
This item can be repaired with a Legendary Weapon Repair Kit.