In Norse mythology, Surtr is the fire giant who leads the attack on the gods at Ragnarök.
Surtr is a powerful being associated with Muspelheim, the fiery region of the Norse cosmos. At Ragnarök, he advances with the forces of fire and attacks the divine order. The sources describe him carrying a flaming sword or a weapon bright enough to illuminate the world, although translations and manuscript details vary.
Surtr kills Freyr during the final battle. He then sets the world ablaze, an event that contributes to the destruction of the existing cosmos before the world is renewed. His role makes fire, rather than only warfare between gods and giants, a central part of Ragnarök.
Surtr is sometimes confused with Ymir, the primordial giant whose body helps form the world. Ymir belongs to the creation story, while Surtr belongs primarily to the apocalypse. He is also distinct from Loki, who leads or accompanies other enemies of the gods in Ragnarök.