Berserkers are Viking styled troops armed with a Labryc axe. They will deal out a lot of damage to enemy units, but have very poor defence capabilities. These units are ideal for taking out troops who are not armoured, such as Archers or Spearmen.
They can also easily take out Crossbowmen in enough numbers, if they can get close enough, that is. These troops are fairly expensive to replace, so be very careful to not act impetuous taking them over man traps or other similar defence systems that could cause unnecessary damage to them.
Berserkers work well in conjunction with Axe throwers, as the AI Lord Olaf already uses. Both units compliment each other nicely, because Axe Throwers have the ranged capacity that Berserkers do not while Berserkers have the strong offensive capabilities that the Axe Throwers do not.
Berserkers are fairly weak to missile fire though however, and it is recommended that you at least attempt to remove any ranged troops before sending them in, so your Berserkers can deal the most amount of damage. Berserkers are best used for harrassing enemy units, capturing enemy estates and destroying enemy buildings and/or workers.
You can recruit Berserkers from the rank of Royal Champion. You will need 80 gold as well as 1 peasant from the campfire before you can recruit a Berserker.