Long has it been whispered around Berk that one day a huge monster will rise from the sea and the world will end… And when a spate of local seaquakes start erupting, agitating dragons, things aren’t looking good… What can scare a dragon?? Hiccup and the gang investigate and they do indeed meet up with something huge… Could this be the end of Berk? Meanwhile, in a very special back-up story, Astrid is stranded on an isolated island and is hunted by a group of Smothering Smokebreaths… And to make matters worse, she can’t find her battle-axe.
— "The Legend of Ragnarok" Back Cover
Legend has it that an ancient dragon–a giant Seadragonus Giganticus Maximus –will rise from the ocean one day and end the world-and this is known as Ragnarok. As Berk experiences minor earthquakes and earthquakes–which are disturbing the dragons’ behavior–Chief Stoick orders all of Berk's sea patrols to remain at anchor. Hiccup, meanwhile, is trying to train a Changewing he later names Phantom. But every time he makes progress it responds by hypnotizing him with its mesmeric gaze (as it does its prey)–with comedic results. Meanwhile, Alvin the Treacherous plans to take advantage of the earthquakes and attack Berk. However, something huge from the sea destroys his ships, and the Seadragonus Giganticus Maximus approaches Berk and it is seemingly unstoppable! Even Alvin tries to help the citizens of Berk. Fishlegs identifies this colossal dragon as a Purple Death. After futile attempts to stop the beast, Hiccup leashes Phantom and takes her to the Purple Death. She uses her hypnotic gaze to subdue the Purple Death and send him back into the sea, to presumably sleep another few hundred years.
Ragnarok is an event in Norse Mythology that is prophesied to take place in the future. It is a prediction of the end of the world and the death of a number of Norse Gods, followed by the rebirth of the World.