Furious is a Seadragonus Giganticus Maximus and the main dragon antagonist in the How to Train Your Dragon series. He is the blood brother of both Hiccup Horrendous Haddock II and Hiccup Horrendous Haddock III. Furious led the Dragon Rebellion, and insurrection whose main goal was to exterminate the human race.
Furious is the younger dragon-brother of Hiccup Horrendous Haddock II who grew up with him with the Grimler Dragons. He appears in How to Break a Dragon's Heart and the following books. His fire is believed to be the darkest of any dragon's breath. It is explosive when it comes in contact to with an object. Before the events of the story, Furious was adopted by the two Grimler Dragons who adopted Hiccup the Second. They grew up together and were caught by a Hooligan hunting party. While growing up with his human father, Hiccup the Second branded his forehead with the Dragonmark to show his brotherly love for Furious. After Hiccup the Second reached manhood, he asked Furious to lead the Dragon Rebellion. When the petition arrived, Grimbeard the Ghastly thrust the Stormblade forward, but Furious jumped in front of Hiccup the Second, earning him the scar on his chest. Though Furious jupmed in front of Hiccup the Second, Grimbeard still successfully drove the Stormblade through Hiccup the Second's chest, killing him. Moments before Hiccup the Second's death and after Grimbeard realizing he was actually betrayed by Thugheart, Furious attempted to kill the king in revenge, but was stopped by Hiccup the Second who told Furious he didn't want his father's blood on his hands. Then Furious grabbed Hiccup the Second's corpse out of Grimbeard's arms and burried him on the Bay of the Broken Heart, shortly after Furious was caught by Grimbeard's men and imprisoned in Berserk.
100 years later, Furious was freed by Hiccup Horrendous Haddock III, but shortly after he betrayed him and started a dragon rebellion against humanity, telling Hiccup if he reached manhood it would be the end of dragons.
When Furious and his dragon army arrive, the war begins, but Hiccup has been exiled from his tribe because he has the Slavemark. Hiccup then seeks out to search for the Dragon Jewel, which is believed to put an end to the war.
In the next book, Furious sends a Triple-headed Deadly Shadow dragon to kill Hiccup. However, the dragon did not after learning the boy helped them reunite with Fishlegs, who is the son of their former rider. After the betrayal, Furious vows to kill Hiccup himself. Hiccup, realizing the danger, flees to a secret hideout the Deadly Shadow dragon shows them and hides there from Furious.
The Dragon Furious begins the book by searching all over the lands for Hiccup, destroying every place he searches. In the beginning, he nearly manages to catch Hiccup when they try to rescue Snotface Snotlout . Hiccup somehow escapes on the invisible Deadly Shadow, and Furious torches the land around their unground treehouse, in a rage to kill him. He stops, however, when he makes a deal with the Wodensfang to leave them be until Doomsday Eve, if the Wodensfang agrees to steal the Dragon Jewel from Hiccup and give it to Furious, so that they may engage in a duel to the death on Doomsday Eve. While striking the deal, Toothless is present, and he is revealed to be a Seadragonus Giganticus Maximus , much like the Wodensfang and Furious.
- The Green Death and Furious both share a few similarities with Alduin, a black dragon who is the main antagonist of the video game The Elder Scrolls V: Skyrim.
- In How to Break a Dragon's Heart, Furious is said to be green. In How to Seize a Dragon's Jewel, however, he is blue. During the 100 years he spent trapped, with very little movement, plant life covered his scales. Now that he has been free for two years, Furious' scales' color can be seen again.
- Furious is one of the only three known dragons who can speak Norse.
- Furious was tracking Hiccup by reading Toothless's thoughts, and the Wodensfang was blocking them, suggesting that Sea Dragons have telepathic powers as well as precognition.