Introduction
Surtr is a fire giant, ruler of Muspelheim, the primordial realm of fire, described in the Poetic Edda's Voluspa as sitting at its border wielding a flaming sword. He is one of the most powerful and destructive figures in Norse eschatology, fated to lead the forces of Muspelheim against the gods at Ragnarok and ultimately to engulf the whole world in flame.
This article follows Old Norse mythological tradition, chiefly the Poetic Edda and Snorri Sturluson's Prose Edda, noting where the two sources differ.
Guardian of Muspelheim
Surtr is present from the very beginning of Norse cosmology, since Muspelheim, one of the two primordial realms (with Niflheim), existed before the created world and was the source of the fire whose meeting with Niflheim's ice formed Ymir. Surtr's flaming sword is said to shine brighter than the sun itself.
Ragnarok
At the onset of Ragnarok, Surtr leads the fire giants of Muspelheim across the rainbow bridge Bifrost, which shatters under their weight, into the final battle against the gods. He is fated to kill Freyr, who by then has given away his magic sword and fights only with an antler. After the battle, Surtr casts fire over the whole world, consuming it entirely in the flames that mark the world's end, before its eventual rebirth.
Meaning
Surtr's fire complements the ice and cold so pervasive elsewhere in Norse cosmology (Niflheim, the frost giants, the long winter Fimbulwinter that precedes Ragnarok), giving the mythology's final destruction a dual character of both freezing and burning, appropriate to the harsh climate that shaped the original Scandinavian tradition.