Whenever a berserker takes damage, they roll an openended 3d6. If the result is less than their morale, they become berserk.
A unit that has gone berserk will appear to gain a red pulsing glow. It will not cast spells or follow orders: it will instead run forward and attack the nearest enemy. It will not retreat, not even due to the turn timer, and its morale is set to 99. The berserk state is cancelled if its fatigue reaches 100, though this does not prevent a unit berserking again once its fatigue falls. A unit that has gone berserk also gains fatigue per round (in a similar mechanism to exhaustion) equal to the (1 + berserk value)/2, rounded down.
The value of the berserk ability are the modifiers applied during berserking, which are:
The spells rout.and give berserk level 0, meaning the only benefit is being unable to
Mounted units may not berserk, except for the special hero Cu Chulainn of MA Eriu. The tag "mountedbeserk", which only Cu Chulainn has, is required for a mounted unit to berserk. However, they're still affected by items which directly apply the "goneberserk" tag such as Lycanthropos' Amulet.