Surtur

Surtur is a mythological figure in Norse Mythology, mentioned once in the Dragons: Riders of Berk Comics.

Mythology
Surtur is a Jotunn - a mythical race of giants equivalent to the Gods. He is mentioned in the Poetic Edda and Prose Edda as figuring prominently in Ragnarok (loosely, the end of the world). He is foretold to fight against Odin and the Gods wielding a flaming sword. Specifically he will battle the God Freyr and defeat him. Then the flame from Surtur's sword will burn the world.

"Underworld"
While a Rogue Submaripper attacks Stoick's ship, the Hooligan sailors aboard dash away in a frenzy and some jump into the water to escape the angry dragon. One exclaims, "By the flaming horns of Surtur! Submaripper off the starboard bow!"