Surtur was an enormous fiery demon native to the dimension of Asgard. Possessed of evil intelligence and vast power, Surtur was one of the major elemental forces of evil that the Asgardians feared. He was a mystical being whose existence predated that of Odin himself. Over 1,000 feet tall and wielding a huge fiery sword, he was once imprisoned by Odin at the Earth's core. Along with Ymir the Frost Giant and the Midgard Serpent, Surtur was the most powerful of Asgard's native enemies.
Surtur has been freed from imprisonment a few times over the years. The first time the Asgardian god of mischief Loki set Surtur free to wreak havok on Earth. The second time Surtur was set free by Marduk of the Sons of Satannish, a cult of human demon-worshippers. Surtur was banished to some unnamed netherworld at the end of that encounter. He appeared, yet again, in what was to be one of the Asgardians greatest battles against Surtur. Surtur hatched a plan to lure Asgard's armies to Earth leaving the city vulnerable to attack. He crossed Bifrost with his newly forged Twilight blade believing Asgard to be defenseless; Odin, Thor and Loki were there to make a last stand to defeat Surtur. Their victory did not come easy though, as Odin fell along with Surtur into a dimensional rift.
Surtur apparently died along with the Asgardians during Ragnarok.