The Massacre of the Valkyrie was the result of a deadly clash between the Valkyries and Hela, who sought to escape her banishment in Hel. The battle resulted in the death of all but one of the Valkyrie.
Millennia ago, Odin Borson of Asgard and his army embarked on an ambitious and violent campaign to conquer the Nine Realms. With the help of his oldest child and Chief Executioner Hela, the Allfather succeeded in his goal to create the Asgardian Empire.
It was shortly afterward, however, Odin realized Hela's ambitious nature exceeded his and was a threat to Asgard. He engaged his oldest child in battle and, after defeating her, banished her to Hel for eternity.
The Allfather then proceeded to write his oldest daughter out of Asgard's history, ordering that any mention or depiction of her involvement in the Empire's creation be erased.
Riding through a portal into Hel, the Valkyrie charged towards Hela on their steeds and threw their spears at the former Chief Executioner, prompting her to unleash her powers of weapon manifestation and throw innumerable Necroswords into the air. Several of the warriors and their horses were killed instantaneously while others were knocked off their horses and fell to the ground.
Hela then proceeded to engage the remaining Valkyrie, overpowering and killing them and their steeds. The only exception was the leader Brunnhilde, who survived after a Valkyrie leaped into the path of an oncoming Necrosword.
Sometime later, Odin, who was convinced the Valkyrie perished, intervened personally, engaging Hela in battle and overpowering the Goddess of Death before reimprisoning her.
With her comrades defeated and killed, Brunnhilde abandoned her title and went into exile. She eventually found herself on the planet Sakaar, where she began working for the Grandmaster as one of his bounty hunters.