Berserkers are the result of a living human vessel that is inhabited by an animal spirit, the bear, and so they become like the bear. By incasing themselves in garments consisting of a bear's pelt or bones, the vessel channels the animal spirit and imbues himself with the bear's nature, strength and ferocity.
Berserkers are instinctively loyal to, can be conjured and commanded by and serve the supernatural being known as La Loba, or the Bone Woman.
In Season 4, Kate Argent has control of two Berserkers, having discovered them at La Iglesia that she uses as bodyguards and to do her bidding. These Berserkers are dressed in elaborate but crude armor constructed out of bone, wear skulls over their heads and faces, are armed with a detachable bone blade on their right arms, and outfitted claws on their left.
Later on Kate transforms Scott into a new Berserker, putting the werewolf under her control, to use him against his friends. Scott is able to fight off the animal spirit, overcoming the Berserker conversion.
In episode Echo House, Chris Argent relays to Derek Hale he had once encountered, fought a Berserker.
A few years before the series timeline, Chris learned of a group of people were messing around with a series of rituals. Eventually one particular young man became a Berserker, tapping into the animalistic mindset. Unfortunately, the young man's humanity and personality gradually deteriorated. He killed innocents, tearing their bodies apart, because unlike shapeshifters, Berserkers aren't tempered in the manner the werewolf is by the full moon and Chris was forced to kill the man.
In The Dark Moon, Scott and his Pack first encounter Kate's Berserkers in Mexico. One was stalking Scott and Braeden in the Aztec temple beneath La Iglesia where Derek Hale was being held. Scott scared it off with an Alpha's roar. The second demon had broken Stiles's Jeep down in the middle of the road with its blade. Malia catches sight of the monster and was wounded by it.
In 117, Peter Hale identified the Berserkers when one of them faced he, Scott and Malia. He advised they run, only for them to run into the second Berserker. Scott, Kira and Malia attempt to fight them off, but they were easily overpowered. Derek arrived and managed to hold
them off. The Berserkers heard a screeching werejaguar roar from Kate, a signal, and so they retreated.
In "Orphaned", the Berserkers under Kate's directions attack a number of assassins that were flowing into Beacon Hills, eventually attacking the
federal transport vehicle that was holding Violet, one of 'the Orphans', injuring Scott, Sheriff Stilinski and Rafael McCall, and killing Garrett, the other 'Orphan', in the process. Scott and Chris face off against Kate and the Berserkers at the abandoned Argent Arms International warehouse. Chris, armed with a machine gun opened fire on the Berserkers to no use. One Berserker was about to give Chris the killing blow, but Kate let out a werejaguar roar calling it off.
They possess superior strength, outclassing even werewolves. They growl like beasts and wear skulls and other bones as clothing.