Ragnarök

"To wage war on those who would kill any who cannot defend themselves... To hold back the ever-growing tide of Ragnarök that comes to calm us all. This is not our duty. It is the reason for our very existence."

- Thor

Ragnarök is a major doomsday event in Asgardian culture.

Prophecy
The event of Ragnarök was prophecised by Asgardian culture, to be the end of Asgard. This event foretold of Surtr, empowered and rejuvenated by the Eternal Flame, unleashing his rage upon Asgard and consuming all things in his hellish fire. Asgard would be no more when Surtr sank his sword through the city.

Preventing Ragnarok
Odin sought peace and the longevity of Asgard during his reign as king. And in his duty as king, he decided to ward off Ragnarök by imprisoning Surtr and keeping him from ever touching the Eternal Flame. Odin confined Surtr to Muspelheim and prevented him from ever leaving the planet. He additionally locked the Eternal Flame underneath Asgard in his Vault, keeping Surtr from ever coming into contact with the powerful object.

Despite these efforts made against Surtr, it is believed that Ragnarök is ultimately unstoppable. While delivering an inspirational speech to his allies during the Marauders' War, Thor mentions "the ever-growing tide of Ragnarök that comes to claim us all".

Tactical Move
Many years 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 actualise Ragnarök and use Surtr's power to defeat and kill Hela. Knowing the Asgardian people were out of harms way, he ordered Loki to place Surtr's skull in the Eternal Flame. Surtr erupted from Asgard moments later, laying waste to the helpless city of Asgard before him. Surtr thrust his blade, as foretold in the prophecy, straight through Asgard, which incidentally killed Hela in the process.

Thor, Loki and the remaining Asgardians watched as their city burned and particulated into the void.

Asgard Lives On
Despite how the prophecy of Ragnarök outlined the fate of Asgard, it never foretold the fate of its Asgardians. Drifting through space with Thor to lead them, he aims to take the surviving Asgardian population to Earth.

Trivia
"Thor will defy you and wage war against the Jotuns from now until Ragnarök."

- Loki to Odin


 * In the non-canon video game Thor: God of Thunder, Odin uses the Odinforce to revive Sif after she is killed by a Frost Goliath, believing that it will prevent Thor from starting another war with the Frost Giants. Loki tells Odin that Thor will defy him and wage war against the Jotuns until Ragnarök.