Berserker Tribe

In the Books
The Berserk Tribe were a tribe of insane Vikings living in the Uglithug Territories. Fishlegs is a descendant of the Berserks.The Berserkers all wear chains and padlocks, so to restrain them, just in case if they go into Berserker mode. They are led by the unnamed Chief of Berserk. The Berserks were known to perform human sacrifices, by offering victims to The Beast. They tried to feed Hiccup, Fishlegs, Humungously Hotshot, and ten other fiancés of Tantrum to the Beast, yet Hiccup struck a deal with the beasts, and commanded the Berserks to free everyone from the prison, under death threat. The Berserks did so, and fled when The Beast set fire to the island. The Berserks swiftly sail away from the island, never to be seen again.

In DreamWorks Dragons: Riders of Berk
The Berserkers make their first appearance in episode 15:"Twinsanity" The Berserkers visit Berk as it is time for the annual signing of the peace treaty. Stoick was expecting Oswald the Agreeable, the chief of the Berserker tribe, but to their great astonishment, Oswald's son Dagur (whom Hiccup describes as a lunatic) turns out to be the new chief of the Berserkers, and is now called 'Dagur the Deranged'. Dagur explains that his father has been "retired", and he took over the responsibility of being the new chief, though Dagur doesn't have any qualities of a good chief. After a few events in the episode, Dagur and the Berserkers were forced to flee, but told Gobber to consider the treaty signed.

THe Beserkers returned in The Night and the Fury, when they accompanied Dagur to Dragon Island.

In View to a Skrill Part I, the Beserkers found a frozen Skrill and tried to dig it out, but were stopped by the dragon riders. They fought off the riders agin when they were trying to capture the the dragon again. While they did not get it, the Beserkers formed an alliance with the Outcasts, who had the Skrill, to attack Berk. In the end, Dagur used the Skrill to eliminate Alvin, and become chief of both the Beserkers and the Outcasts. Their invasion was put on hold after Hiccup had freed and refroze the Skrill.

The Beserkers command a vast armada of ships. According to the stories told by Stoick's grandfather, the Beserkers had the ability to command the power of the Skrill, using tethered harnesses to force the dragon to attack with lightning. This was proven in View to a Skrill Part II.

Origins
Beserkers were actual Norse warriors  who  fought in a nearly uncontrollable, trance -like fury. Most historians  believe that berserkers worked themselves into a rage before battle, but some think that they might have consumed drugged  foods.

Trivia

 * The Berserkers's Crest is a Skrill.