The Destruction of Asgard was the result of the Fire Demon Surtur being released by Loki in the achievement of the doomsday prophecy known as Ragnarök, which had been figured out by Thor as the only way to stop the Goddess of Death Hela before she could lay waste across the Nine Realms and beyond.
The God of Thunder Thor experienced several nightmares in which he saw the mighty Fire Demon Surtur lay waste on his homeland of Asgard, a doomsday event known as Ragnarök in Asgardian culture. In order to prevent such a cataclysm, Thor journeyed to Muspelheim. He was captured by Surtur but eventually managed to escape, killing Surtur in the process and bringing the demon's skull to Asgard so that it would be locked within Odin's Vault.
Thor soon had to deal with another threat as his sister, the Goddess of Death Hela, returned from her banishment thanks to Odin's death. She cast out both Thor and Loki to Sakaar and seized the throne of Asgard, slaughtering the Einherjar and replacing them with her Berserkers to tyranically reign over Asgard.
However, Thor and Loki eventually managed to return to Asgard, accompanied by the rest of the Revengers and the Sakaaran Rebellion. An intense battle erupted on the Rainbow Bridge, during which the Berserkers army was destroyed. However, Hela remained alive and proved to be invincible to Thor and the Revengers. Thor ultimately figured out that he had been mistaken all along: they were not supposed to stop Ragnarök and the destruction of Asgard ; instead they had to cause it so that Hela would be killed and the Asgardians could escape with the Statesman, the spaceship used by the Sakaaran Rebellion.
While Thor and Valkyrie fought against Hela to buy some time, Loki fled the Commodore to the Asgardian Palace. He made his way to Odin's Vault and reclaimed the Crown of Surtur which he put into the Eternal Flame. The process caused Surtur to be revived in his mightiest form, a Fire Demon towering over Asgard and its nearby mountains and wielding the powerful Twilight Sword.
As Loki escaped, Surtur erupted from the Asgardian Palace, destroying it in the process, and began laying waste to the deserted city before him. Meanwhile, Thor and Valkyrie managed to make Hela fall from the Rainbow Bridge into the water and watched in sorrow as Surtur destroyed their homeland. Although Hulk lept at Surtur to attack him, he was knocked back to the bridge, where Thor and Valkyrie convinced him to give up on fighting Surtur. Hulk reluctantly agreed and took them aboard the escaping Statesman.
Still alive, Hela attempted to continue to fight Surtur by hurling massive spears in his chest. However, Surtur remained unaffected by her attacks. As foretold in the Ragnarök prophecy, Surtur thrust his blade straight through Asgard to its core, annihilating Hela and the threat she posed to the Asgardians in the process. The Asgardians watched helpless as their homeworld exploded, making them a wandering people.
Following the destruction of their homeworld, the surviving Asgardians who had gathered aboard the Statesman had to relocate on a new home. Led by their new king Thor, who eventually accepted the position on the throne, they decided to head towards Earth. However, during their journey, their spaceship encountered the infamous Sanctuary II, Thanos' gigantic spacecraft.
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