The Siege of Asgard was an attempt by the Asgardian goddess Hela to invade Asgard.
Millennia ago, King Odin Borson and his chief executioner Hela, who was also his oldest child, conquered the Nine Realms in a bloody centuries-long campaign that led to the creation of the Asgardian Empire.
Odin, however, began to see his daughter's ambitious nature as a threat and was forced to fight her in a battle that ended with Hela being defeated, banished and imprisoned in Hel.
Immediately afterwards, Odin decreed that any mention or depiction of Hela's involvement in Asgard's creation was to be erased, even replacing a fresco depicting her conquests with inaccurate images.
A Family Reunion
In 2017, two years after the Battle of Sokovia, Odin's son Thor was captured by the fire demon Surtur while on an unsuccessful quest to search for the Infinity Stones. While listening to Surtur taunt him about the impending Ragnarök, an event that would bring about the end of Asgard, the God of Thunder learned his father wasn't on Asgard and promptly escaped, but not before stealing the demon's crown.
Returning to Asgard, Thor discovered that his adopted brother Loki had been posing as Odin since the Second Dark Elf Conflict and, after exposing the God of Mischief's deception to the populace, forced him to take him to Odin. The two made their way to Earth and with the reluctant help of Stephen Strange, managed to track Odin down to Norway.
Their reunion, however, was short-lived, as Odin revealed that he was about to die and that his death would allow Hela to break free. True to the Allfather's word, he passed away and went to join his late wife Frigga in Valhalla. The moment he did so, a portal to Hel opened and Hela triumphantly emerged, demanding that her younger brothers acknowledge her as Asgard's rightful Queen.
Thor, however, refused and hurled Mjølnir at the former chief executioner only to watch in shock as she caught the hammer with one hand and used her powers to destroy it. The two brothers then attempted to escape via the Bifrost Bridge only for Hela to follow and knock them into space before making her way to her old home.