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 doomsday event in Asgardian culture.

A Fateful Prophecy
"The universe moves forever towards Ragnarök. This is its nature."

- Odin

For countless millennia, Asgardian culture has foretold of an event that would bring about the destruction of their homeworld. Known as Ragnarök, said event was destined to take place when the fire demon Surtur, empowered by the Eternal Flame, unleashed his rage upon Asgard and consumed it in fire.

In an attempt to prevent the fateful prophecy from being fulfilled, King Odin Borson confined Surtur to Muspelheim and locked the Eternal Flame in a vault beneath his palace.

Despite the Allfather's efforts, the people of Asgard believed that Ragnarök was inevitable, as made evident during a speech that would be delivered by Odin's son Thor towards the end of the Marauders' War.

A Prodigal Daughter
Unbeknownst to all, Odin possessed a secret no one else knew about. Long before Thor was born, the Allfather had a daughter named Hela, who fought alongside him as his Chief Executioner and the leader of the Einherjar.

Hela played a key role in the creation of the Asgardian Empire, helping Odin conquer the Nine Realms in a vicious and bloody campaign. But Odin gradually began to see her overambitious nature as a threat and, following a great battle, banished Hela from Asgard, imprisoning her in Hel for eternity.

Odin then went to great lengths to forget Hela ever existed, ordering that any mention or depiction of her role in Asgard's creation to be erased. Meanwhile, a resentful Hela started biding her time for the day when she would break free and exact vengeance against her father for his betrayal.

Ragnarök
To be added

Aftermath
To be added

Trivia

 * In the comics, Ragnarök was not a singular event, but an eternally recurring cycle of death that saw the cyclical destruction and subsequent revitalization of Asgard a countless number of times. The latest Ragnarök event was caused by a major battle between Asgard's people and an army of soldiers led by Loki utilizing copies of Mjølnir. The resulting battle obliterated Asgard, and the survivors settled on Earth in Broxton, Oklahoma.
 * 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.