In Norse mythology, Surtr (Old Norse "black"[1] or more narrowly "swart",[2] Surtur in modern Icelandic), also sometimes written Surt in English,[3] is a jötunn; he is the greatest of the fire giants, who serves as the guardian of Muspelheim which is along with Niflheim, the only two realms to exist before the beginning of time.[4] Surtr is attested in the Poetic Edda, compiled in the 13th century from earlier traditional sources, and the Prose Edda, written in the 13th century by Snorri Sturluson. In both sources, Surtr is foretold as being a major figure during the events of Ragnarök; carrying his bright sword, he will go to battle against the Æsir, he will battle the major god Freyr, and afterward the flames that he brings forth will engulf the Earth.
In a book from the Prose Edda additional information is given about Surtr, including that he is stationed guarding the frontier of the fiery realm Múspell, that he will lead "Múspell's sons" to Ragnarök, and that he will defeat Freyr. Surtr has been the subject of place names and artistic depictions, and scholars have proposed theories regarding elements of Surtr's descriptions and his potential origins.
Surt is the most powerful of the fire-giants, and he rules over the realm known as Muspelheim.