The Destruction of Asgard was an attempt by the fire demonSurtur to initiate the doomsday prophecy known as Ragnarök.
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 due 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.