Banished by Odin
For countless millennia, Asgardian culture has foretold of an event that would bring about the destruction of their homeworld. Known as Ragnarök, said the event is destined to take place when the fire demon Surtur, empowered by the Eternal Flame, unleashes his rage upon the Asgard and consume it in the fire.
Beginning of the End
- "Thor, son of Odin."
"Surtur! Son of a bitch! How are you?"
- ―Surtur and Thor
Two years after the Battle of Sokovia, Thor had been unsuccessfully searching for the Infinity Stones, and was now imprisoned by the fire demon Surtur. Surtur reveals that Thor's father Odin is no longer on Asgard, and that the realm will soon be destroyed in the prophesied Ragnarök, once Surtur unites his crown with the Eternal Flame that burns beneath the city. Thor killed Surtur and claimed his crown, believing he had prevented Ragnarök.
Destruction of Asgard
Some time later when Thor came to reclaim Asgard from the clutches of Hela, he found it too difficult a task to defeat her alone. With the surviving population of Asgardians disembarking from Asgard on a large lifeboat, Thor made the decision to actualize Ragnarök and use Surtur's power to defeat and kill Hela. Knowing the Asgardian people were out of harm's way, he ordered Loki to place Surtur's crown in the Eternal Flame, thereby resurrecting him.
Surtur erupted from Asgard moments later, laying waste to the helpless city before him. Surtur thrust his blade, as foretold in the prophecy, straight through Asgard, which incidentally killed Hela. During the destruction, Hulk jumped towards Surtur's head in order to kill him, but he failed to do so. Eventually, Surtur killed himself in the process. Thor, Loki, and the remaining Asgardians watched as their realm burned and the remnants scattered into the void.
Although the prophecy of Ragnarök doomed Asgard, it never foretold of the fate of its Asgardians. Drifting through space with Thor to lead them, the new King of Asgard resolved to take the surviving Asgardian population to Earth. 
Surtur was a sinister, cold, emotionless, remorseless and diabolical individual who would not hesitate to kill anyone who stood in his way, be it friend or foe. He was the ultimate personification of destruction; he had used his mighty powers against his enemies, and had no fear to destroy everything in his path, indicating the fact that Ragnarok must happen and that only he can ensure it happens.
He is a seemingly indestructible individual; almost nothing can break or destroy him, not even Hela.
Despite his dark, vastly sinister and diabolical personality, Thor eventually decided to let the prophecy come to pass in order to destroy Hela once and for all. However, this does not mean that Surtur redeemed himself nor became an anti-hero. Rather, he stayed who he always was: a diabolical, destructive demon.
Though Hela succeeded in nearly wiping out the entire population of Asgard with her chaotic reign, Surtur was ultimately successful in accomplishing his goal: to destroy Asgard in its entirety and complete the prophesy of Ragnarök.
Powers and Abilities
- Fire Giant Physiology: Surtur was one of the most powerful beings in the Nine Realms. Upon being resurrected by the Eternal Flame, Surtur's powers were immeasurably increased, with him single-handedly bringing Ragnarök to Asgard and destroying the incredibly powerful Hela herself fairly easily.
- Fire Manipulation: As a Fire Giant, Surtur can generate massive waves of fire from his Twilight Sword to engulf his targets in flames.
- Superhuman Strength: Surtur's original level of strength was on par with that of Thor, but after being revived from the Eternal Flame, it was tremendously increased to incalculable superhuman levels, to the point that he effortlessly swatted away a leaping Hulk, and even fairly quickly destroyed Hela herself.
- Superhuman Durability: Surtur, after being revived from the Eternal Flame, became virtually invulnerable, only slightly stumbling from Hulk's leaping blow, and being barely affected by Hela piercing him with many oversized blades.
- Expert Swordsman: Surtur is skilled at combatively utilizing his massive flaming Twilight Sword, using it to swiftly slice apart Asgardian palaces, and to destroy Hela.
- Twilight Sword: A large, burning blade seemingly made up of fire. It can shoot flames to engulf any enemy and grows with Surtur after he had been revived. It's tremendously strong, enough so to destroy Asgard entirely when Surtur brought about Ragnarök by plunging his blade into the ground, shattering Asgard and everything on it.
- Crown of Surtur: A burning crown that holds Surtur's being. When removed, it turns a dull black/grayish color and Surtur's body disappears. It is also Surtur's source of power. When combined with the Eternal Flame, Surtur becomes powerful enough to swiftly destroy a realm as powerful as Asgard.
- Eternal Flame: The Eternal Flame is an ever-burning fire which was originally kept in Asgard's weapons vault. After Hela uses it to revive her old warriors and wolf, Fenris, it was ultimately used by Loki to revive Surtur and bring about Ragnarök. By placing the Crown of Surtur in the Eternal Flame, the fire giant was able to destroy Asgard completely with his overwhelming power.
- Surtur is based on the Norse mythological figure Surtr. In the Marvel Comics, he is a recurring foe of Thor, and is responsible for the destruction of the Korbinites's home galaxy when creating his Twilight Sword.
- Surtur is mentioned by Fandral in the Thor: The Dark World novelization.
- Surtur, Helmut Zemo and Loki are the only villains whose plans were successful to any extent, as Surtur managed to fulfill his destiny of destroying Asgard and bringing about Ragnarök.