Surtr was the son of Múspell, the glowing and burning southern region of the first world. He sat at the land's end i.e. the border, to defend the land [1,2].
At the end of the World (Ragnarok) the sons of Múspell with Surtr as the first shall cross the rainbow bridge Bifrost and the bridge will break. The fight will take place on the field Vígríðr, which is a hundred leagues wide each way and start with Heimdall rising up and blow mightily in the Gjallar-Horn to awaken all the gods. In the following fight the gods will die along with the Fenris Wolf and the Midgard Serpent. In the fight, Surtr is the one to kill Freyr. The last gods standing will be Loki and Heimdallr who will be the slayers of each other. Then straightway shall Surtr cast fire over the earth and burn all the world [1,2].
Surtr has a flaming sword with sufficient magic to kill gods.
Folk tales: Viking age (800-1100 CE)
Written form: The Poetic Edda (around 1250 CE)
Country of origin:
In Wagner's opera Götterdämmerung
Surtr was given a wife named Sinmore, but as this is an opera performed the first time at Bayreuther Festspielhaus, 17. August 1876 i.e. the story was not a part of the original mythology, and Sinmore should not be considered canon in the Norse mythology.
1: The Poetic Edda
2: The Prose Edda