Surt (Old Norse Surtr, “Black,” presumably a reference to his charred appearance) is a fire giant who leads his kin into battle against the Aesir and Vanir gods during Ragnarok, the destruction of the cosmos. His particular fate is to kill the god Freyr and to be slain by him in turn. He arrives from Muspelheim, the extreme southern region of heat and fire, bearing his weapon of choice of a burning sword, with which the world is razed before it sinks into the sea.
Historian Rudolf Simek has proposed that Surt is the supernatural force who corresponds to “the (volcanic) fire of the Underworld,” a personage who would surely have had a profound emotional resonance for any early Icelander, given the sheer amount of volcanic activity that characterizes that island.
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 de Vries, Jan. 2000. Altnordisches Etymologisches Wörterbuch. p. 562.
 Snorri Sturluson. The Prose Edda. Gylfaginning 4, 51.
 The Poetic Edda. Vafþrúðnismál, stanzas 50-51.
 The Poetic Edda. Völuspá, stanza 52.
 Simek, Rudolf. 1993. Dictionary of Northern Mythology. Translated by Angela Hall. p. 303-304.