Surtr, also spelt Surtur or Surt, is a primordial Jötunn (giant) present in the Norse mythology. He's considered the one who shall end the world during Ragnarök. He's designed to be the "supreme evil" of the old scandinavian myths, a position he shared with Loki.
The old norse manuscrits describe Surtr as the first of the fire giants to emerge from the flames of Muspelheim, the realm of fire. He's a primordial jötunn, half-brother of Ymir and Audhumbla, embodiment of chaos and fire. He's also represented as holding a immense fire sword.
In Ragnarök, Surtr will lead the army of fire jotnar from Muspelheim to invade Asgard by the Bifrost bridge and enter in the battle field.
Surtr is destined to fight with the god Freyr, who unfortunate is killed by the flame sword of the giant because he gifted his own sword to his friend Skírnir long before the battle.
In the end of the battle, Surtr will shake his weapon and spread the primordial Muspelheim fire to all the Yggdrasil, consuming the Nine Realms and killing practically all the living beings in the cosmos, including himself and his own species.
Surt don't have a consistent personality and acts as a true natural force of massive destruction, who only exists to spread chaos in his path. He cares nothing about other living beings, as stated that he'll kill his own species and himself in order to destroy all the worlds and kill every other being in fire.
In Popular Culture
As with other major figures in Norse Mythology, Surtr frequently appears in other media more than often as antagonistic/villainous character. Most notably his Marvel counterpart. He also appears to inspire Balrogs in Tolkien works.
Surtr is a representation of the primordial times in the Norse mythology before the creation of the universe, where the jötnar acted as true cosmic forces without personality. He's likely the last of his type still alive.