Dagur The Deranged is the son of Oswald the Agreeable.
Riders of Berk
Dagur first appears in Season 1 of the TV series, Episode 15: "Twinsanity".
The Hairy Hooligans tribe are expecting an annual visit from the chief of the Berserkers, Oswald the Agreeble. But then when the Berserker ships arrive at the island, it is shown that Oswald the Agreeable's son Dagur has become the new chief of the Berserkers, and is now given the new title Dagur the Deranged.
He came to Berk to renew the peace treaty between the Berserkers and the Hooligans. He spent most of the time insulting his father, and looking for dragons to kill. Later in the episode, when Barf and Belch bursts into the Great Hall, he led the Berserkers in catching them. Barf and Belch were brought to the arena so that Dagur and Stoick could kill them. Dagur was stopped by an "attack" staged by the other dragons, directed by their riders via hand signals. Dagur was forced to flee, but told Gobber the Belch to consider the treaty signed.
Defenders of Berk
- He is said to have overthrown his father, and pursues a mission to bring the Berserkers back to their old ways.
*Dagur will play an important role after being introduced in season 1.
*It is speculated that he may have killed his father when he says, "Put down that axe, Dagur!"
*Given how many details he knows of Hiccup's dragon-training practices and his victory over the Red Death — and particularly his unwillingness to divulge his 'sources of information' — it's speculated that Dagur is also secretly allied with Alvin and the Outcasts.
*It's interesting to note that Berk feels it necessary to have a peace treaty that's renewed yearly with the Berserkers and hide that they train dragons (which is a good thing as Dagur threatens to attack if he finds the "army of dragons" rumor to be true), while in Dragon Flower, Stoick is perfectly fine with openly flying Thornado to meet the Chief of the Shivering Shores by himself. This indicates that Berk knows who will honor a treaty and who can be trusted, and that the two concepts are not mutual.
*Due to his actions and obsession with killing, it is implied (very strongly) that he may have Bipolar and mentally unstable.